Core Analysis Manual

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Report EP 94 - 1130 April 1995

CONFIDENTIAL

CORE ANALYSIS MANUAL by H.H. Yuan SIPM EPD/222 and B.A. Schipper KSEPL RR/37 Contributions by R.M.M. Smits KSEPL RR/37 J.G. Maas KSEPL RR/44

PETROPHYSICS AND RESERVOIR ENGINEERING This document is confidential. Neither the whole nor any part of this document may be disclosed to any party without the prior consent of Shell Internationale Petroleum Maatschappij B.V., The Hague, the Netherlands. The copyright of this document is vested in Shell Internationale Petroleum Maatschappij B.V., The Hague, the Netherlands. All rights reserved. Neither the whole nor any part of this document can be reproduced, stored in any retrieval system or transmitted in any form or by any means (electronic, mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner. SHELL INTERNATIONALE PETROLEUM MAATSCHAPPIJ B.V., THE HAGUE EXPLORATION AND PRODUCTION

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The authors appreciate the comments and review of the following people: P.van Ditzhyijzen, SIPM- EPD/22 A.J.T. Grimberg, SIPM- EPD/21 A.B. Graper, SIPM- EPD/21 H.Niko, SIPM- EPD/221 P.R.A. Betts, SIPM- HTRH/52 P.M.T.M. Schutjens, KSEPL- RR/37 K.A. Heller, SIPM- EPD/21 J.P. van Hasselt, EPX/43 E.C. Thomas, SOC F.R. Bradburn, SOC Many petrophysicists at PDO, EXPRO, NAM, SSB, SVEN

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SUMMARY Core analysis is the acquisition of experimental data measured on core material for determining parameters used for developing and managing a hydrocarbon reservoir from initial discovery to mature field development. There are two main reasons for core analysis. Firstly, core analysis data are used by petrophysicists to calibrate wireline logs in the determination of hydrocarbon reserves. Such data include routine core analyses as well as special core analyses such as measurement of electrical parameters for resistivity log interpretation. Secondly, reservoir engineers use core analysis measurements such as relative permeability and pore volume compressibility to provide input parameters for reservoir computer simulation. Core analysis data are also used by other disciplines such as for production technologists to determine injectivity and well performance and for explorationists in quantifying acoustic rock properties. Geological core analysis (the subject of a manual in preparation) is done to establish the geological framework of a reservoir. Careful planning of a core analysis programme requires the involvement of an integrated team of petrophysicsts, geologists, reservoir and production engineers and explorationalists to ensure that core measurements meet critical data needs. Since optimum analysis programmes require multi-disciplinary input, the manual is prepared in such a way to assist teams of petroleum engineers to develop core analysis programmes. The contribution of each PE discipline is highlighted. An appendix on application of value of information concepts as applied to core analysis is given to provide a clear method for evaluating and justifying core analysis projects. The various parameters which can be obtained from the analysis of core material are discussed briefly. The available measurement techniques are detailed and discussed briefly. The available measurements techniques are detailed and recommendations are made concerning the reliability of the techniques and how best to obtain quality results. Core sampling guidelines that allow easier application of core data, proper core preparation procedures, core screening methods for obtaining representative cores, wettability considerations and ancillary measurements that ensure quality and data applicability are described in detail.

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GENERAL RECOMMENDATIONS •

A core analysis programme should be assembled with input from all PE disciplines to ensure that the right data are measured with the proper procedures on appropriate core material. The core analysis programme allows proper planning in the multi-disciplinary environment and assists in the management of the core analysis programme.

•

Value of information concepts should be used for core analysis programmes and in programmes justification.

•

High quality data can be obtained with careful core selection, core screening and core preparation steps. Proper core screening is necessary to obtain relevant core data.

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Extensive special core analysis programmes can be discussed with SIPM EPD/22 and/or KSEPL RR/37 to assist in decisions as to where work should be carried out.

•

SIPM recommends that special core analyses be carried out in-house using facilities at KSEPL. Bellaire Technology Center, Houston, and Calgary Research Center can be used as alternatives with sufficient prior planning and available capacity. If Shell E&P laboratories are not available, core contractors approved by SIPM can be used.

•

If contractor laboratories are used it is recommended that KSEPL be requested to carry out duplicate special core analysis measurements on a small number of samples in order to verify the performance of the contractor. A review of any extensive core analysis programme is recommended and will be provided by SIPM upon request.

•

To date quality assessments have been made on the techniques used at Core Laboratories, Simon Petroleum Technology, Poroperm-Geochem, GAPS Geological consultants, and Corex (Aberdeen). SIPM recommends regular quality assessment of any core analysis contractor involved in Shell work.

•

All measured data, procedures and equations used should be requested from the analysis laboratory, including any data used in calculating final results such as raw data.

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Recommendations for multi-disciplinary core analysis planning A core analysis programme should address all issues in core analysis from justification, core acquisition, well-site handling, core preparation to core measurements and data application. Planning should include the following: •

Justification (see Chapter 2 and Appendix 1) and clearly stated objectives of the core analysis programme (Chapters 3 and 4)

•

Core acquisition considerations including type of coring bit, core barrel, overbalance, drilling fluids, well-site handling (see Core Handling Manual); core transport and fluid sampling considerations

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Multi- disciplinary input ensuring proper utilisation of core material and representatives of the samples to be used in the core analysis programme

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Core analysis considerations including types and scope of the core analysis, numbers of samples, core sample screening methodology, core preparation methodology especially cleaning, experimental conditions (confining pressure, temperature, pore pressure, fluids to be used, experimental duration, etc), wettability conditions and so on

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Fluid analysis considerations focusing on types of fluid analyses that can be used to support interpretation of core data. It is a frequently overlooked aspect of formation evaluation

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Detailed measurement sequence defining expected measurements on each core sample. Scheduling is important so that the performing organisation can meet the deadlines required for core analysis data

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Costs and value of information concepts used in programme justification

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Finalised core analysis programme allowing each discipline to contribute and to agree to the goals and methods and allows for better project management.

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Acknowledgements Many people have contributed to the preparation of this manual. Staff at SIPM and KSEPL have extremely reviewed the manual and helped in ways too numerous to detail. Interest and review from Opcos has also encouraged us to make the manual as useful as possible. The information contained in this manual has been collated from a number of previous SIPM and KSEPL publications. Especially useful in the writing of this report were: EPD/22/23 SIPM Coring Series Bulletin l: Core Justification EP 88-1465 EPD/22/23 SIPM Coring Series Bulletin IIl: Core Analysis EP 89-0105 Rock Characteristics Research – Special Core Analysis KSEPL, brochure 1991. B.A. Schipper, R.J. van den Oord, and S.J. Adams Petrophysical Core Analysis Contractors - Procedures and Quality Assessment . EP 92-1355 S.J. Adams and R.J. van den Oord Capillary Pressure and Saturation Height Functions EP 93-0001 This manual completes a series of three manuals dealing with aspects of coring, core handling and core analysis. The first two manuals are: J.A. Okkerman and L.C. van Geuns Core Handling Manual EP 93-2200 L.C. van Geuns and J.A. Okkerman (in preparation) Geological Core Analysis

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Contents SUMMARY GENERAL RECOMMENDATIONS Recommendations for multi-disciplinary core analysis planning Acknowledgements List of Figures (p) denotes photo List of Tables 1.

I II III IV X XIV

Introduction About this manual Problems solved by core analysis SIPM/KSEPL recommendation on core analysis Availability of other Shell E&P Laboratories for core analysis Quality in core analysis Literature

1 2 6 8 9 10 12

Economics of core analysis 2.1 Value of Information (VOl) 2.1.1 VOl Nomenclature 2.1.2 Value of prospect screening - Summary 2.1.3 Value of project optimisation - Summary 2.1.4 Value of correct core analysis data - Summary 2.2 Value of information examples as applied to core analysis projects 2.2.1 Example 1 - Prospect Screening (Unconsolidated Sandstone) 2.2.2 Example 2 - Ekofisk 2.2.3 Example 3 - Project optimisation 2.2.4 Example 4 - An Opco VOl Example 2.3 Core analysis aspects of VOl 2.4 Literature

13 14 15 16 17 18 19 20 22 24 26 28 30

1.1 1.2 1.3 1.4 1.5 1.6 2.

3.

Planning a core analysis programme 3.1 Planning in an integrated PE team 3.1.1 Core analysis programme development 3.2 The core analysis programme 3.2.1 An example of a core analysis programme 3.3 Considerations for major lithologies 3.4 Where to perform the core analysis programme 3.5 Literature

31 34 35 36 39 43 46 47

4.

Core and fluid analysis considerations 4.1 Scope of a core analysis programme 4.2 Multi- disciplinary considerations 4.2.1 Petrophysics 4.2.2 Geology 4.2.3 Reservoir engineering 4.2.4 Other disciplines 4.3 Core measurements 4.3.1 Basic core analysis 4.3.2 Special core analysis 4.4 Coring considerations and well-site planning 4.5 Core handling 4.5.1 At the well-site 4.5.2 Upon arrival at the Laboratory 4.6 Core screening

48 49 50 50 51 53 54 55 55 58 61 64 64 65 66

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4.7 Core sampling 4.7.1 Sampling for basic core analysis 4.7.2 Sampling for special core analysis 4.7.3 Sampling considerations 4.8 Core sample preparation 4.9 Core sample screening for special core analysis 4.10 Core preservation 4.11 Fluid measurements 4.11.1 Brine measurements 4.11.2 Oil measurements 4.12 Fluid handling considerations 4.13 Sequencing and scheduling 4.14 Costs 4.15 Economic impact and justification 4.16 Project reporting 4.17 Project review 4.18 Literature

67 67 68 69 70 71 75 76 76 77 78 79 79 79 80 83 85

5.

Core preparation 5.1 Plug drilling 5.1.1 Drilling consolidated samples 5.1.2 Drilling unconsolidated samples 5.2 Core cleaning 5.2.1 Cleaning consolidated samples 5.2.2 Cleaning unconsolidated samples 5.3 Core drying 5.3.1 Oven drying 5.3.2 Critical Point Drying (CPD) 5.3.3 Humidity controlled drying 5.4 Review of some contractor preparation procedures 5.5 Literature

86 87 87 88 90 90 91 93 93 94 97 98 101

6.

Basic core analysis 6.1 Porosity and grain density 6.1.1 Bulk volume by buoyancy in mercury 6.1.2 Bulk volume by mercury displacement 6.1.3 Bulk volume by caliper 6.1.4 Pore volume by liquid saturation 6.1.5 Grain density by pycnometer 6.1.6 Grain volume by buoyancy 6.1.7 Grain volume by Boyle's law porosimetry 6.2 Steady- state gas permeability 6.2.1 Air permeability 6.2.2 Probe permeability 6.3 Fluid saturations 6.3.1 Fluid saturations by Dean-Stark extraction 6.3.2 Retort method or summation of fluids 6.4 Literature

102 103 103 104 106 107 108 110 112 114 114 118 120 120 122 123

7.

Porosity and permeability at stress and whole core analysis 7.1 Stressed Porosity 7.1.1 Stressed pore volume by liquid saturation 7.1.2 Stressed pore volume by Boyle's Law porosimetry 7.2 Stressed permeability 7.2.1 Stressed steady-state permeability 7.2.2 Pulse decay permeability

124 125 125 126 127 127 129

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7.3 Whole core analysis 7.3.1 Whole core porosity by Boyle's Law porosimetry 7.3.2 Whole core steady-state gas permeability 7.3.3 Other whole core measurements 7.4 Literature

131 132 133 135 137

8.

Capillary pressure 8.1 Mercury/air capillary pressure 8.1.1 Mercury/air capillary pressure by high pressure injection - Autopore 9200, 9220 8.1.2 Mercury/air capillary pressure by pressure equilibrium 8.1.3 Stressed mercury/air capillary pressure 8.2 Oil/water capillary pressure 8.2.1 Oil/water capillary pressure by centrifuge 8.2.2 Oil/water capillary pressure by pressure equilibrium 8.3 Gas/liquid capillary pressure 8.3.1 Gas/liquid capillary pressure by centrifuge 8.3.2 Gas/liquid capillary pressure by porous plate vessel 8.4 Literature

138 141 142 145 147 149 150 152 154 155 156 157

9.

Electrical properties 9.1 Formation Resistivity Factor, FRF, and cementation exponent, m 9.2 Resistivity index, I, and saturation exponent, n 9.2.1 Resistivity index by pressure equilibrium 9.2.2 Resistivity index by continuous injection 9.2.3 Resistivity index by porous plate vessel 9.2.4 Resistivity index by rapid desaturation 9.3 Cation Exchange Capacity (CEC) and Qv 9.3.1 Qve by membrane potential 9.3.2 Qv by multiple salinity measurements, Co-Cw 9.3.3 CEC by conductometric titration 9.3.4 CEC by absorbed water correlation 9.4 Literature

158 159 161 163 165 168 169 170 171 174 176 178 179

10. Wettability and interfacial tension 10.1 Wettability 10.1.1 Cleaned-state samples 10.1.2 Restored-state samples (aging) 10.1.3 Fresh-state samples 10.1.4 Preserved-state samples 10.1.5 Pressure-retained core samples 10.1.6 Restored state vs native state 10.2 Wettability determination 10.2.1 Amott 10.2.2 United States Bureau of Mines method (USBM) 10.2.3 Other wettability determination methods 10.3 Interfacial tension 10.3.1 Interfacial tension by 'Pendant Drop' 10.3.2 Surface tension by 'du Nouy balance' 10.3.3 Interfacial tension by spinning drop tensiometer 10.4 Literature

180 181 183 184 185 186 186 187 189 190 192 193 194 195 197 199 200

11. Relative permeability 11.1 Steady-state measurement 11.1.1 Relative permeability by steady-state 11.2 Centrifuge measurement 11.2.1 Oil/water relative permeability by centifuge

202 204 204 207 207

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11.2.2 Gas/liquid relative permeability by centrifuge 11.3 Unsteady-state measurement 11.3.1 Oil/water relative permeability by unsteady state displacement 11.3.2 Gas/liquid relative permeability by unsteady-state displacement 11.4 Relative permeability at reservoir conditions 11.4.1 Restored-state (see section 10.1.2) 11.4.2 Native-state (see section 10.1.3, 10.1.4, 10.1.5) 11.5 Literature

211 213 213 216 219 219 219 220

12. Mechanical rock properties 12.1 Compressibility 12.1.1 Uniaxial compaction 12.1.2 Hydrostatic compaction 12.1.3 Oedometer compaction test 12.2 Rock strength parameters 12.2.1 Rock strength by triaxial testing 12.2.2 Brinell Hardness Number (BHN) 12.2.3 Thick-Walled-Cylinder strength test (TWC) 12.2.4 Unconfined Compressive Strength test (UCS) 12.3 Acoustic properties 12.3.1 Acoustic Travel Time (ATT) 12.4 Literature

222 223 224 230 231 232 233 234 236 239 240 240 241

13. Supplementary tests 13.1 Rock analyses 13.1.1 Grain size by laser diffraction 13.1.2 Grain size by sieve analysis 13.1.3 Grain size by image analysis 13.1.4 Source rock analysis 13.1.5 Cap rock/seal analysis 13.2 Fluid analyses 13.2.1 Counter Current Imbibition (CCI) 13.2.2 Oil and gas analyses 13.2.3 Formation water and core water analysis 13.3 Rock-fluid compatibility 13.3.1 Compatibility flood 13.4 Miscellaneous tests 13.4.1 Acid response test 13.4.2 Solvent flushing - for wax removal 13.5 Literature

244 245 247 249 250 251 253 254 254 256 258 260 260 263 263 266 267

Core analysis research activities 14. 14.1 Rock characteristics 14.1.1 Ultrasonic Velocity Cell (UVC) 14.1.2 Acoustic transmission anisotropy 14.1.3 Apparatus for Pore Examination (APEX) 14.1.4 Resistivity 14.2 Fluid flow 14.2.1 Capillary Pressure and Resistivity Index by Continuous Injection, CAPRICI 14.2.2 Relative Permeability at Reservoir Conditions (3-phase), REPARC-3 14.2.3 Critical gas saturation 14.3 Supplementary 14.3.1 Nuclear Magnetic Resonance, NMR 14.4 Literature

268 269 269 271 273 275 277 277 279 281 283 283 287

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APPENDIX 1 Value of Information A1.1 Value of prospect screening A1.2 Value of project optimisation (reducing uncertainty) A1.3 Value of correct core analysis data (Shell EP Laboratories vs contractors).

289 290 293 297

APPENDIX 2 Core screening techniques A2.1 X-Ray Computer Tomography scanning (CT) A2.2 Core gamma ray A2.3 X-ray fluoroscopy A2.4 Coreslab inlarging A2.5 Literature

301 302 306 308 309 311

APPENDIX 3 Petrophysical data from geological analysis 312 A3.1 Microstructure/Petrography 313 A3.1.1 Petrography from Scanning Electron Microscopy (SEM) and Enhanced Image Analysis (IA) 314 A3.1.2 Petrographic image analysis from thin sections 319 A3.2 Mineralogy 321 A3.2.1 X-ray diffraction 322 A3.2.2 Energy Dispersive X-ray analysis (EDX) 323 A3.2.3 Mineralog 324 A3.3 Literature 325 APPENDIX 4 Core analysis on small cores, sidewall samples and cuttings A4.1 Small core samples from slim holes A4.1.1 Analysis of a 13/4" diameter core A4.1.2 Analysis of a 25/8" diameter core A4.1.3 Further slim hole core analysis A4.2 Sidewall samples A4.2.1 Rotary drilled samples A4.2.2 Percussion sidewall samples A4.2.3 Sidewall sample measurement techniques A4.3 Cuttings A4.3.1 Collection/sampling A4.3.2 Measurement techniques used in cuttings analysis A4.4 Literature

326 327 328 329 330 331 332 333 334 335 335 337 339

5 Sponge core analysis APPENDIX A5.1 Oil-Wet sponge analysis A5.1.1 Sponge analysis by gas chromatography A5.1.2 Other oil-wet sponge analysis techniques A5.2 Water-wet sponge analysis A5.3 Literature Appendix 6 Conversion from hydrostatic to uniaxial strain conditions Points Index

340 341 341 342 343 344 345 348 349

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List of Figures (p) denotes photo 2.1 2.2 2.3 2.4

Economic choices for example 1 - prospect screening Economic choices for example 2 - Ekofisk Economic choices for example 3 - project optimisation Economic choices in Opco example

3.1 Flow diagram for core analysis planning 4.1 4.2 4.3 4.4

Recommended flow diagram for basic core analysis Recommended flow diagram for special core analysis (p) Longitudinal CT-scans (tomograms) of a core plug (p) Flow diagram highlighting core analysis data review

5.1 5.2 5.3 5.4 5.5 5.6

Drilling plugs with liquid nitrogen (p) Unconsolidated sample cleaning apparatus at KSEPL (p) A conventional drying oven (p) Illustration of the principle of critical point drying A sample after CPD (p) A sample after air drying (p)

6.1 Bulk volume by buoyancy in mercury at KSEPL (p) 6.2 Pycnometer at KSEPL (p) 6.3 An automated pycnometer (p) 6.4 Grain volume by buoyancy at KSEPL (p) 6.5 Schematic of a typical Boyle's law porosimeter 6.6 A typical Hassler-type core holder 6.7 Capability for permeability anisotropy and air permeability measurements at KSEPL (p) 6.8 Air permeameter at KSEPL (p) 6.9 Schematic of a probe permeameter 6.10 Dean-Stark apparatus at KSEPL (p) 7.1 7.2 7.3 7.4 7.5

Schematic of stressed brine permeability Schematic of pulse decay permeameter Schematic of flow paths in whole core horizontal permeability measurements Schematic of flow paths in whole core vertical permeability measurements Whole core stressed porosity and FRF at CRC

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Capillary pressure curve parameters An Autopore 9220 (p) Penetrometers for an Autopore 9200/9220 Schematic of mercury injection apparatus Schematic of stressed mercury/air capillary pressure apparatus Schematic of coreholder for high-speed centrifuge Schematic of pressure equilibrium cell

9.1 9.2 9.3 9.4

Schematic of formation resistivity factor cell Typical I-Sw relationships Hysteresis in the I-Sw relationship View of the cell for resistivity index by pressure equilibrium method, and oil/water capillary pressure curves (p) 9.5 Schematic of resistivity index by continuous injection 9.6 Multiple resistivity index by continuous injection cells at KSEPL (p) 9.7 Schematic of Qv by membrane potential 9.8 Membrane potential measurement at KSEPL (p) 9.9 Co as a function of Cw for a shaly sandstone 9.10 CEC by conductometric titration at KSEPL (p) 10.1 Wettability concepts 10.2 Initial water saturation on primary and secondary drainage -water-wet system 10.3 Diagram showing the difference between initial water saturation on primary vs secondary drainage 10.4 Amott and USBM wettability indices 10.5 Interfacial tension by pendant-drop apparatus (p) 10.6 De Nuoy balance at KSEPL (p) 11.1 Relative permeability curves 11.2 Effect of wettability on relative permeability 11.3 Schematic of steady-state apparatus at KSEPL 11.4 Schematic of core holder for centrifuge relative permeability measurements 11.5 Interior of centrifuge apparatus for relative permeability at KSEPL (p) 11.6 Comparison of centrifuge and steady-state method (first drainage with n-decane/nitrogen, Berea sandstone) 11.7 Schematic of unsteady-state apparatus at KSEPL (p) 11.8 Comparison of steady-state and unsteady-state (Welge) methods

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12.1 12.2 12.3 12.4

Schematic of first triaxial compaction apparatus at KSEPL Schematic of second triaxial compaction apparatus at KSEPL Radial displacement transducer Typical compaction curve showing axial displacement as a function of pore fluid pressure during uniaxial compaction under pore pressure depletion conditions. 12.5 Brinell Hardness equipment at KSEPL (p) 12.6 Pressure cell for thick-waIled-cylinder (TWC) strength test 12.7 Samples after thick-waIled-cylinder (TWC) testing (p) 12.8 Unconfined Compressive Strength (UCS) at KSEPL 13.1 13.2 13.3 13.4 13.5 13.6 13.7

A typical grain size analysis report Grain size by laser diffraction (p) Sample is immersed in toluene Weight change with time indicates residual saturation Typical residual-initial curve from counter current imbibition measurements Automated compatibility flooding set-up Typical acid response curve

14.1 Schematic of UVC cell at KSEPL 14.2 Acoustic transmission anisotropy 14.3 A representation of APEX data 14.4 Schematic of EMPRESS at KSEPL 14.5 Schematic of CAPRICI 14.6 CAPRICI at KSEPL (p) 14.7 REPARC-3 equipment at KSEPL (p) 14.8 Critical gas saturation experiment at KSEPL (p) 14.9 NMR spectrum from a rock sample 14.10 NMR spectrometer at KSEPL (p)

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A1.1 Economic choices for prospect screening core analysis A1.2 Economic choices for optimisation core analysis A1.3 Economic choices in selecting core analysis laboratory A2.l CT-scanner at KSEPL (p) A2.2 Scanning of core material using CT-scanning A2.3 Natural core gamma ray scanner (p) A2.4 A coreslab image A3.l An SEM secondary electron (SE) image A3.2 An SEM back scattered (BSE) image A3.3 An SEM cathodoluminescence (CL) image A3.4 Quantitative analysis from SEM A3.5 A thin-section image A3.6 Analysis of the thin-section image A4.l Autopore penetrometers used for drill cuttings analysis

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List of Tables 4.1 Information from geological evaluation 4.2 Basic core analysis parameters and their uses 4.3 Information derived from core preparation 5.1 Laboratory comparison for core preparation 5.2 Summary of cost and timing in core preparation 10.1 Definitions of wettability 12.1 Brinell hardness number suggested loading schemes A3.1 Image Analysis Regression Statistics

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Introduction

Core analysis encompasses techniques used to derive formation properties from core material taken from the well-bore. The techniques generally involve measurement on plug samples of the core material. In most cases, the sample should be maintained in or restored to a state that would be representative of the state of the material in the formation and may, for example, necessitate the application of appropriate stresses and/or temperature. In other cases, measurements are made on the matrix material itself without regard to representative state. Measurements range from the simplest determinations of porosity to the most complicated measurement such as three phase relative permeability measurement at reservoir conditions. Core analysis measurements are of interest to a wide range of disciplines in EP from petrophysics, geology and reservoir engineering to drilling, production and exploration. Because core analysis has so many customers, it must be a focus of the integrated efforts of PE teams throughout Shell.

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About this manual

The core analysis manual presents a strategy and plan for obtaining the highest quality data and maximising the value from core analysis measurements. All too often core analysis fails to get the engineering attention that it deserves. Frequently, core analysis planning is done poorly, if at all, and the results of such efforts in terms of data acquired can often be confused and contradictory. Yet core analysis remains an important source of critical information for quantifying reservoir models and calibrating formation evaluation tools like wireline logging. As the construction of reservoir models becomes more sophisticated, the demand on acquiring properly measured formation properties using core analysis becomes that much more important. This manual is about the many facets of core analysis, but the authors have taken the approach that business processes involving core analysis need to be "re-engineered" to reflect the business needs of Shell EP companies in the '90's. Two important business processes begin this manual and they are: •

evaluating economic impact of core analysis and

•

planning a core analysis programme.

Chapters 1 to 4 address economic and planning issues of core analysis. While core analysis can be expensive, the value of core analysis is generally much greater. Obtaining proper value for a given expenditure is key to hydrocarbon resource management. The economics of core analysis is the subject of Chapter 2 and is based on value of information concepts. Several examples are given to allow PE staff to realise the value of their own core analysis projects. Once core analysis economics are assessed, a detailed core analysis programme should be assembled. The subject of Chapter 3 addresses the needs of core analysis planning in an integrated multi-disciplinary environment. As part of the planning process consultation with core analysis experts at SIPM/KSEPL is recommended. Proper planning leads to the delivery of highquality core analysis data for the development of the hydrocarbon resource.

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Chapter 4 discusses in more detail the issues to be addressed in getting started with a core analysis project. Chapter 4 should be used as a guideline for those who are starting afresh with a core analysis project. Every issue mentioned in Chapter 4 merits attention although the critical items deserve the most attention. The remainder of the manual describes the most commonly used core analysis techniques performed today. The methods are included to ensure that PE staff are acquainted with general methodology of core analysis. The method descriptions are aimed to be sufficiently detailed to assist with planning, to allow optimisation of core measurements and to provide a means of ascertaining quality and consistency. Beginning with Chapter 5, a reasonably comprehensive survey of core analysis preparation techniques is given. From Chapter 6 onwards the progression from basic analysis to aspects of special core analysis is presented. Chapter 13 addresses supplementary analyses and chapter 14 presents an overview of research activities that represent a glimpse into the probable future of core analysis activities. By necessity, brevity has been imposed in order to allow the manual to be of reasonable length. Each measurement technique is described by the following scheme: •

principle - brief description of how the measurement is made

•

points - remarks that highlight critical aspects of the measurement. An assessment of every measurement is made whether the technique is recommended, acceptable or not recommended. The not recommended assessment is not to say that the technique always produces incorrect data but that the technique has inherent tendencies which make obtaining reliable data more difficult. General issues such as limitations, advantages and disadvantages are also addressed, including possible data handling issues.

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•

precision/accuracy - an estimate of uncertainty in measurement accuracy. The estimate is either expressed in measurement precision, repeatability or uncertainty in true value of a measured parameter.

•

price/timing/number of samples - is designed to understand approximate costs, how long an experiment can take and an appropriate number of samples balancing cost and time involved. Prices are meant to be approximate and can vary significantly (by up to 40%) between regions depending on market competition and local factors. Timing addresses the length of time a measurement takes which should assist in the scheduling and timing of a core analysis project. Recommended number of samples is the recommended minimum number to ensure reasonable characterisation of a rock type in a core analysis programme. More samples should be considered if the justification through Value of Information shows the measurement value to be very great, as might happen with any special core analysis measurement.

•

peripheral measurements - necessary ancillary measurements which quantify measurement consistency, check quality and assist in data interpretation. Without peripheral measurements, core analysis data are difficult to apply. Peripheral measurements deserve considerable attention and are the key to establishing a quality core analysis programme.

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The Appendices contain information that may be useful relating to CT-scanning and CT-scanning interpretation. While the manual addresses measurements made from samples taken from whole core, core analysis can also be made on other material sources such as sidewall cuttings and drill cuttings. There are many specialised core analysis techniques which are not included in this manual such as coal bed methane analysis. The authors have not attempted to provide an exhaustive manual on core analysis but to address the core analyses used in day-to-day Shell operations. This manual completes a sequence of manuals on core, which are Core Handling, EP 93-2200, and Geological Core Analysis, (in preparation).

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Problems solved by core analysis

Core analysis can be used to answer many questions related to the production of hydrocarbons from the subsurface. Besides providing the opportunity to directly see and describe the rock formations of interest, core analysis also provides: •

Detailed description of geological environment and setting including variation of lithology, rock composition and rock type along the length of the core.

•

Important petrographic information can be obtained from microscopic examination of the core material using techniques such as thin sections, scanning electron microscopy, X-ray diffraction.

•

Non-destructive imaging using the CT-scanner or core gamma scanner.

•

Values of routine petrophysical formation properties as a function of depth: - porosity; - permeability; - grain density; - oil and water saturations.

•

Log interpretation parameter values: - Archies lithologic exponent, m; - Archies saturation exponent, n; - Waxman-Smits parameters, clay conductivity, m* and n*; - Grain and fluid densities.

•

Distribution of fluids within the hydrocarbon column from capillary pressure measurements.

•

Grain size distribution data for engineering application in well completion programmes and for geological application in assessing heterogeneity and depositional environment.

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•

Values of special formation properties for reservoir engineering such as: - wettability; - relative permeability: - effective permeability to oil, - effective permeability to water; - initial water saturation; - residual oil saturation.

•

Values of exploration formation properties: - shear and compressional acoustic velocity; - acoustic impedance.

•

Values of rock mechanical parameters used in production engineering and platform design: - rock strength; - compressibility; - compaction; - waterflood sensitivity.

•

Tests for non-reservoir rock, seal analysis and source rock analysis.

•

Fluid measurements are also important to provide a complete picture of the downhole environment. Brine properties such as composition and conductivity, oil properties such as viscosity, acid and base number and identification of gas/oil using High Pressure Liquid Chromatography (see HPLC manual).

•

Measurement of the interfacial tension measurement between oil and water used in scaling mercury/air capillary pressure curves to oil/water systems.

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SIPM/KSEPL recommendation on core analysis

Core analysis remains one of the critical sources of formation data necessary for proper development planning and field management. Careful multi-disciplinary core analysis planning in the integrated PE environment is necessary for acquisition of high-quality core analysis data, avoiding sub-optimal data acquisition and improving data application. Proper economic analysis, including "Value of Information" concepts, are important in appreciating the role core analysis plays in field development and hydrocarbon resource management. SIPM recommends the application of Shell core analysis technology which has been developed at Shell E&P laboratories world-wide. Accordingly, critical special core analysis measurements should be performed at KSEPL, Rijswijk, where possible, or at a core contractor recommended by SIPM/KSEPL. At the very least, major core analysis programmes should involve SIPM/KSEPL, who will develop and maintain a strategy of core contractor quality assurance. Consultation with SIPM EPD/22 and KSEPL RR/37 (who will act as focal point for KSEPL) is recommended for any core analysis questions including core data interpretation.

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1.4

Availability of other Shell E&P Laboratories for core analysis

1.4.1

Bellaire Technology Center (BTC), Houston, Texas

SIPM has reached an agreement with SOC which allows Bellaire Technology Center (formerly known as Bellaire Research Center) to provide core analysis services for SIPM when mutually convenient. Should KSEPL be unavailable to perform any critical core analysis, it is now possible to arrange, via SIPM, for work to be performed at Bellaire Technology Center, Houston, when facilities there are available. Bellaire Technology Center has a long history of excellence in core analysis and has developed numerous Shell standard techniques over the years. SIPM regards BTC as a source of high quality core analysis data which incorporates Shell technology. Arrangements should be made through EPD/222. Consult with EPD/222 for further information. 1.4.2

Calgary Research Center (CRC), Calgary, Alberta

Calgary Research Center has developed considerable expertise in whole core analysis (see chapter 7). These capabilities can be used for Group whole core analysis when mutually convenient and should be arranged by SIPM EPD/222.

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Quality in core analysis

The organising principle behind this manual is the central issue of quality in core analysis. Quality is a critical issue. How often does a user of core analysis ponder the question:

How valid are these core analysis data? The process of formulating an answer to this question begins with the recognition that core material, by nature, is heterogeneous. This fact should be incorporated into every aspect of the acquisition of core analysis data from inception to final data delivery. Without incorporating quality into the performance of the core analysis project, data quality assessment is very difficult to perform. This manual endeavours to build quality into the process of core analysis measurement through prior planning and making the correct suite of core analysis measurements. These steps are summarised as follows (and more details are provided in the appropriate sections in the manual): •

core analysis planning - much attention is focused on this activity because data assessment is a multi-disciplinary activity. For example, rock sample selection must involve a geologist for proper attention to rock type.

•

sample screening - this step is frequently omitted but all too often core analysis measurements are performed on samples that are unfit for measurement even though they may appear as perfectly formed cylinders while they may contain internal heterogeneities invisible from the outside.

•

peripheral measurements - this manual emphasises measurements that can and should be performed to determine data consistency, to check data quality or to improve data application. Certain parameters are critical in the determination of core analysis quantities. The most important parameter in much of core analysis is the pore volume because it is the fundamental determinant of the porosity and all saturation measurements are normalised to the pore volume. Thus any error in pore volume is translated directly into errors in saturation.

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•

core analysis contractors - much of the core analysis performed for Shell companies is done by contractors. Their results suffer from a high degree of variability in quality. Experience shows that it is important not only to use a reputable contractor laboratory but to use the most reputable personnel within those organisations. It is therefore advisable to pay attention to the individuals performing the core analysis within contractor labs and, where possible, to identify the key personnel who can provide quality data. It is insufficient to only trust the manager or sales representative. In general it is preferable to select a core analysis vendor that is capable of carrying out the bulk of the core analysis programme to reduce handling and transportation that degrades core.

•

old core analysis data - many of the techniques presented in this manual can be applied to the examination of old core analysis data. The quality of old core analysis data is difficult to determine when quality assessment planning has been omitted. But the thinking and planning for a quality core analysis programme can assist in determining how old core analysis data may be assessed. Unfortunately, determining the quality of old core analysis is, at best, uncertain.

•

application of core data - this manual is designed to allow core analysis measurements to be performed in which quality is assured so that core analysis data can be more easily applied. If core analysis is done with proper regard to geological setting, rock typing and sample screening incorporating proper wettability considerations, then the core data should be suitable for application in formation evaluation or reservoir simulation.

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Literature

SIPM EPD/22/3 SIPM Coring and Core Analysis Series Bulletin 1: Core Justification. EP 88-1465, June 1988. Advances in EP research 1990-1 SIRM research brief. The Log Analyst Special Issue - Core Analysis 1991 European Symposia Abstracts - SCA and Formation Evaluation, September-October 1991 van der Grijp KH. and van den Oord R.J. Well-Site Hydrocarbon Differentation using High Performance Liquid Chromatography (HPLC) EP 93-0550. Keelan, D.K., Core analysis for aid in reservoir description JPT November 1982. Maas, J., Boutkan, V., Ligthelm, D., Fit-for-purpose basic reservoir data Production newsletter, February 1993. Skopec, R.A, Recent advances in rock characterization The Log Analyst, May-June 1992, p 270. Tannemaat, R., Core analysis methods EP 59259, BSP, April 1983. Haeringen, A. van, Results of a conventional core analysis contractor comparison exercise. EP 89-0234. Schipper, B.A., Aperen, A.E. van, Looyestijn, W.J., Quality assessment of core analysis procedures of Core Laboratories Aberdeen. EP 90-1886. Schipper, B.A., Aperen, A.E. van, Looyestijn, W.J., Quality assessment of core analysis procedures of Poroperm-Geochem Limited, Chester. EP 90-1901 Schipper, B.A., Hofman, J.P., Quality assessment of core analysis procedures of Corex Services Ltd, Aberdeen. RKTR.93.052, May 1993 (EP 93-1296). Schipper, B.A, Oord, R.J. van den, Adams, S.A., Quality core analysis - essential to our business! Production Newsletter July/August 1992. Schipper, B.A. Quality Assessment of the Core Analysis Services of Simon Petroleum Testing, Aberdeen. RKTR.94.089, May 1994 (EP 94-0974)

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Economics of core analysis

Core analysis projects are subject to the necessity and rigor of economic justification. It is important that each project be judged by its economic impact and not just solely on cost. Careful economic justification generally shows that core analysis projects have a far greater economic value than their cost. By recognising the economic benefits of core analysis, core analysis projects can be developed that are optimised both technically and economically. Such justification applies to all aspects of coring and core analysis and to data acquisition in general. The economics of core analysis is driven largely by its role in reducing uncertainty in formation properties, particularly hydrocarbon volume and hydrocarbon saturation. While accurate hydrocarbon volume determination depends on a number of variables such as geological architecture and reservoir distribution, a critical initial step is the determination of hydrocarbon saturation. Hydrocarbon saturation is mostly obtained from wireline resistivity logs. However, the actual relationship between hydrocarbon saturation and log resistivity is extremely variable, making calibration with core measurements an essential step. One Opco has estimated that an accurate understanding of their resistivity index-saturation (I-Sw) relations is worth at least US $12.5 million annually in effective economic benefit, which is far more than the expenditure on core analysis. Other core analysis parameters are critical to decisions in managing a hydrocarbon resource. For example, sizing waterflood or water handling facilities can depend critically on relative permeability parameters, such as water endpoint relative permeability. Again economic impact studies show the value of the core analysis project usually far exceeds project cost. To determine the economic impact of a core analysis programme, Value of Information (VOI) concepts will be used which are detailed in Appendix 1. A summary of value of information concepts is given in section 2.1 and examples of how value of information is applied are given for a number of cases in section 2.2. Further discussion as to applications of VOI calculations are provided there.

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Value of Information (VOl)

In making a decision whether to gather information, it is possible to determine the economic value of the information itself. Value of Information (VOl) concepts rationalise the decision as an act of choosing between alternatives, each of which have an economic impact. One alternative involves the gathering of the required (core analysis) information and the economic impact is calculated based upon the information provided. The other alternative is the economic impact calculated in the absence of the information. The difference in economic impact between the two alternatives is the value of information. At this point the value of information does not include the cost of the information gathering itself. Economic impact is calculated as Net Present Value (NPV). Once the value of information is calculated, the decision whether to proceed or not is then based on the value of the information versus the cost of the information gathering. If the value of information is larger (see section 2.3 on Justification) than the cost of information gathering, then it is economically justified to obtain the information. The difference between the value of information and the cost of the information gathering is called the Value of Appraisal. For core analysis, the economic alternatives are simply whether to proceed or not with the core analysis project. The fundamental choice of whether to proceed with a core analysis project is addressed in two different ways: • prospect screening where there is very large uncertainty typically associated with exploration appraisal; • project optimisation, where quantifying rock and fluid properties can narrow design criteria in development planning. The basic theory behind the VOl calculation is presented in Appendix 1 (Appendix 1.1 for prospect screening and Appendix 1.2 for optimisation). In the next sections, the VOl nomenclature and a summary of the concepts is presented so that the example presented in section 2.2 can be better understood but the reader is referred to Appendix 1 for complete details.

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VOl Nomenclature

The terms used in VOl calculations in this manual are defined here. NPV

Net Present Value - economic impact of a development is expressed in terms of net present value. (-NPV denotes negative value.)

NPVi

Net Present Value of ith branch

P(high)

Probability of having high reserves; taken to be 0.33 because P(high), P(medium) and P(low) are taken to have equal probability of occurring.

P(low)

Probability of having low reserves; taken to be 0.33.

P(medium)

Probability of having medium reserves; taken to be 0.33.

POCM

Probability of correct measurement in core analysis. Value is very high for Shell E & P laboratories and about 0.75 for core contractor laboratories. (0.75 is a very conservative number based on estimates from Shell core analysis experts, some of whom feel that it is much lower especially for many special core analysis services such as resistivity index, relative permeability and compressibility.)

POS .

Probability of success.

VOA

Value of Appraisal- value of information (VOl) minus the cost of gathering the information.

VOl

Value of Information - value of information itself without any regard to the cost of gathering the information.

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Value of prospect screening - Summary

In prospect screening discussed in Appendix 1.1, the VOl equation shows that information has a high value and that information gathering is economically justified. The justification stems mainly from the fact that expenditure for data acquisition is done to eliminate the possibility of making an unprofitable investment. Data acquisition is done to save on the loss of NPV if the field turns out to have insufficient hydrocarbon reserves. The VOl calculation covers all aspects of data acquisition from seismic data acquisition, wireline logging, production testing and core analysis. The VOl does not distinguish the value of the core analysis by itself and some method must by realised to assign benefit to the core analysis. In fact, the VOl of acquiring data is so high that it is easy to justify core analysis in exploration appraisal situations. Factors that must be known to properly evaluate the VOl for prospect screening are: •

probability of success, POS;

•

the economic impact of the development if reserves are not present, -NPV3, which denotes a loss;

•

a method of assigning the benefit of the VOl calculation to core analysis as part of overall data acquisition.

Using these factors in equation (A1.4) determines VOl for prospect screening.

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Value of project optimisation - Summary

In optimisation discussed in Appendix 1.2, the VOI is calculated to assess the economic impact of reducing uncertainty by performing the core analysis project. The VOI calculation is based on reservoir simulation results which are aimed at determining the NPV impact when the uncertainty in a critical parameter is examined. The sensitivity analysis for this critical parameter is done by using the maximum and minimum parameter values. In optimisation calculations, the total benefit of VOI calculation is due to the core analysis project. A VOI optimisation calculation requires the economic impact of 4 scenarios to be performed usually by reservoir simulation. For a given rock parameter, such as relative permeability, 4 cases are considered as follows: •

high reserves, optimised high with economic impact, NPV1, in this case, the most optimistic scenarios are used which translates into using a high oil relative permeability curve and a low water relative permeability curve and optimistic endpoint saturations;

•

low reserves, optimised low with economic impact, NPV3, in this case, the most pessimistic scenario is used e.g. a low oil relative permeability curve and a high water relative permeability curve and pessimistic endpoint saturations;

•

high reserves, base case with economic impact, NPV 4, is the case where an average case is used which might be using average relative permeability curves with optimistic endpoint saturations;

•

low reserves, base case with economic impact, NPV6, is the case where an average case is used which might be using average relative permeability curves with pessimistic endpoint saturations.

The results of these cases are used in equation (A1.8) to determine VOI for project optimisation. As described in Appendix 1.2, NPV2 and NPV5 do not enter in the calculation of VOI.

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Value of correct core analysis data - Summary

VOI methods can be used to evaluate any economic decision. Section 2.3 describes other aspects of VOI calculations pertinent to core analysis but an important example is the value of correctly measured data. Basing decisions on incorrectly measured data carries risk and this is discussed in Appendix 1.3. It turns out that using incorrect data puts at risk the primary benefit of the development and it is in fact, worse to use incorrect data than to have no data. These rough calculations can assess the risk of inaccurately measured data. Such a calculation requires: •

probability of success, POS;

•

the economic impact of the development case, NPV1;

•

the economic impact of the development if reserves are not proven, NPV3.

Equation (A1.12) is used to determine VOI of correctly measured data. It is not always possible to use KSEPL or another Shell E&P laboratory due to the demands of accessibility, regional preferences or the availability of Shell E&P laboratories. It is nevertheless true that improperly measured data carries a significant and generally underestimated risk.

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Value of information examples as applied to core analysis projects

In this section, a number of VOI examples will be shown to demonstrate how VOI techniques can be applied to justify core analysis projects. Both VOI and VOA are calculated. One of the examples (Example 4 - Section 2.2.4) is an Opco VOI example used recently to justify a core analysis programme. NOTE: For figures in this section, a red shaded rectangle denotes a human decision while a yellow shaded ellipse represents the consequences of measurement, which are various outcomes each with a given probability of occurring. Positive economic impact is given as NPVi. A negative impact is shown as -NPVi.

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Example 1 - Prospect Screening (Unconsolidated Sandstone)

The details of prospect screening are given in Appendix 1.1. Economic options are shown in Figure 2.1. The option of performing core analysis is clearly indicated by "Yes". However, the results of core analysis are to indicate whether or not to proceed with development given with a weighting by the probability of success. Net present value figures for each option are shown on the right hand side which are used to quantify the value of core analysis. Without core analysis, the "No" option, proceeding with development carries the risk of attempting to develop insufficient reserves. Avoiding the loss in an unprofitable development is the value of the core analysis.

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An unconsolidated sandstone play has an estimated probability of success of 0.5. A suite of rock measurements for the evaluation of this play is given in section 3.2.1. The core and core evaluation cost is DFL 5 mIn. The entire data acquisition programme including seismic and well testing is US $15 mIn. Proposed development costs include a platform and facilities at a cost of US $400 mIn. Economic impact if the reserves are inadequate is a loss of US $200 mIn. Here in example 1,

POS NPV3

= 0.5 = - US $200 mIn.

Equation (A1.4) yields

VOl

= US $100 mln

The value of information is US $100 mln and the value of appraisal is obtained by subtracting a cost of US $15 mln to obtain: VOA

= VOI - Cost = US $85 mln

Note that the value of information here reduces the uncertainty to zero i.e. the probability of success is now unity, which is the combined effect of all the data gathered and not just core analysis. The benefit assigned to core analysis, is the amount by which the core analysis increases the probability of success. In prospect screening, core analysis is easily justified because the economic benefits are so large. The reader is urged to consider section 2.3 on other aspects of VOI calculations for core analysis.

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Example 2 - Ekofisk

Ekofisk has experienced surface subsidence which has cost a lot of money for remediation. This problem might have been avoided or its impact reduced had sufficient special core analysis, particularly compaction measurements, been performed to obtain critical data necessary for field development. Hindsight is able to provide the economic impact of the various decisions made in field development. Economic choices are shown in Figure 2.2. The probability of success is estimated at 0.75. Net present value is presented on the right hand side of the figure. Remediation is estimated conservatively at US $500 million.

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A coring and core analysis programme would have cost about US $2 million. But the cost of repairing the Ekofisk structure is about US $500 million. Figure 2.2 summarises the economic choices. POS is estimated as 0.75. Here, in example 2,

POS NPV3

= 0.75 = - US $500 mIn.

Equation (A1.4) yields

VOl

= US $125 mIn.

The value of information is US $125 mln and the value of appraisal is obtained by subtracting a cost of US $2 mln to obtain: VOA

= US $123 mIn.

The value of core analysis is far greater than the cost of the core analysis project. With hindsight, it is reasonable to assign most of the value of acquisition entirely to core analysis. However, in a prospect screening situation, it is not reasonable to do this. Even if the probability of success were 0.90, then the value of information becomes US $50 million and the value of appraisal becomes US $48 million, which indicate significant value in core analysis.

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2.2.3 Example 3 - Project optimisation Much special core analysis is performed for field development where core data allows project optimisation. The details of optimisation economics is given in Appendix 1.2. The economic choices are shown in Figure 2.3. Net present value figures are shown on the right hand side and should be obtained from reservoir engineering computer simulation as well as incorporating costs estimated from production engineering. The value in core analysis lies in being able to ensure that the project is properly sized. Project optimisation occurs because of reduction in uncertainty of a key parameter determined from simulation of the process under study.

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A water-flood development project depends on detailed knowledge of relative permeability and capillary pressure. Reservoir simulation shows that the economic impact for the optimised high reserves case is US $20 mln, for the high reserves but unoptimised is US $10 mln and for the low reserves case optimised is US $8 mln and that for the unoptimised low reserves case is US $3 mIn. Core is to be taken with special precautions for preserving wettability and is to cost about US $200,000 including collection of appropriate fluids, well-site core handling and transportation. The total cost for taking core and performing core analysis is US $0.5 mIn. Equation (A1.8) yields:

VOl

= 0.33 * { 20 + 8 - 10 - 3 } = US $5 mIn.

The value of information calculation here results in a value of information of US $5 mIn. The cost of the core and core analysis programme is US $1 mIn. Therefore, the value of appraisal, VOA, is VOA

= VOI - cost = US $4.5 mIn.

Here the entire value of information benefit can be assigned entirely to the core analysis project.

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Example 4 - An Opco VOl Example

In this example, a well is being drilled. A core analysis, project can quantify the permeability development below the gas-water contact and reduce the uncertainty in predicting aquifer behaviour before the field is put on production. With proper characterisation of the aquifer, the possibility exists that aquifer influx may be limited which would avoid the drilling of an extra well. Without core analysis the drilling of an extra well is necessarily included in the development plan. Net present value figures were supplied from the Opco. Economic choices (simplified) are shown in Figure 2.4.

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The cost of the core analysis project including coring is estimated at DFL 500,000 including DFL 130,000 for core analysis. The core analysis project called for the acquisition of 4 coring runs below the GWC for a total of 72 m. In fact, the original Opco analysis included the option of continuous coring for 150 m which was found to have a slightly lower value of information and is not included here to maintain simplicity. The values of the top branch, Vyes' and bottom branch, Vno' are given by: Vyes Vno

= 0.5 * 0.4 * -12.36 + 0.5 * 0.6 * 12.36 = - 6.18 mln = DFL -12.36 mln VOl

= Vyes - Vno = DFL 6.18 mln

and thus the value of appraisal is obtained by subtracting cost: VOA

= VOI - cost

= DFL 5.68 mIn.

A value of appraisal of DFL 5.68 mln is calculated because of the probability that the drilling of an extra well can be avoided.

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Core analysis aspects of VOl

This section deals with issues that arise in using VOI calculations for core analysis justification. In some ways these issues can be considered consequences or corollaries to the examples shown in the previous section. •

Justification - VOI calculations should show a value of information, VOI, that is many times programme cost, i.e. at least three times. Thus, the appraisal value, VOA, should be at least twice the cost of the project. VOI calculations which show less value than this are not justified economically. This is recommended as a reasonable guide to using VOI calculations for core analysis projects.

•

Need for additional core - examples in section 2.2.1 and 2.2.2 discuss the situation of justifying initial core in an appraisal situation. VOl methods can be used equally well to justify the need for additional core even if there exists core from wells. Perhaps the previous cores were also justified by value of information techniques which indicated large positive value in core analysis. However, if the need for additional core is present then it must be due to some "failure" of previous cores. By "failure" we include the following: - previous cores missed an important target zone; - poor recovery in target zone (perhaps due to poor planning which allowed poor coring techniques and overlooked poor core handling. If the target zone is inherently difficult to core, then you must demonstrate that new insights into the coring process have increased the probability of success); - incomplete planning as to core analysis needs (there now exists previously unanticipated needs for core material such as to measure a critical parameter like compressibility); - insufficient material (not enough material available from previous cores); - poorly preserved core which indicates that the remaining core is unsuitable for measurement.

The justification for additional core carries with it the need to evaluate previous VOI calculations.

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•

What if coring risks the well? If the coring programme carries with it an identifiable risk of failure such as damaging an expensive well, such contingencies can be incorporated into the value of information calculation which is done by adding more branches allowing the possibility of failure. This is similar to the calculation in the Appendix 1.3 which shows the problem of incorrectly measuring core analysis parameters. There, the extra risk is quantified by the parameter POCM, probability of correct measurement. In the same manner, the risk to a well is carried out by quantifying the risk that coring has on the well. Once that risk is quantified, then the VOl calculation will show a diminished value because the risk of losing the well is incorporated. If the VOl calculation is value neutral indicating that coring may be too risky, then additional scenarios can be evaluated such as by coring on by-pass. In coring on bypass, after a target zone has been drilled and logged and identified, the well is sidetracked above the target zone and then cored through the target zone. This technique has been employed in drilling the deepwater Gulf of Mexico turbidites and has the advantage that the coring point is clearly known and the amount of core required is clearly identified.

•

VOl Lookback - it is a good idea to maintain the examples of VOl calculations to ensure that the assumptions made for the VOl calculation were reasonable. VOl calculations assume that the information will be successfully gathered. However, core analysis programmes can fail such as through bad coring practices, bad core handling techniques, or even poor core measurement techniques. In the case of such failures, it is likely that the full VOl was not obtained and this should be reviewed for future VOl calculations.

•

Value and planning - after VOl has shown that the core analysis programme is of significant value, the task at hand is to ensure proper planning to achieve the programme objectives. We hope that recognising the value of a core analysis programme provides inspiration for carrying out the critical aspect of core analysis, namely planning the analysis programme in an integrated PE environment. Planning is the subject of the next two chapters.

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Literature

Demirmen, F., Subsurface Appraisal Justification: The Value of Information June, 1994 EP 94-0585 E&P Economic Guidelines Report EP 93-2150, October, 1993.

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Planning a core analysis programme

Group experience indicates that coring and/or core analysis is usually poorly planned, is left to a late stage, and results in under utilisation and poor application of expensive core material and core data. SIPM recommends core analysis project planning in order that the proper multidisciplinary attention is paid to core analysis. Petrophysics, geology, reservoir engineering and other disciplines have significant input into the development of an effective core analysis programme. The core analysis programme allows for consensus building in the multi-disciplinary environment and assists in the finalisation of programme goals. With proper planning core analysis projects are better able to deliver required data in a timely fashion. Core analysis planning is shown in a flow diagram in Figure 3.1. Within the guidelines established for each Opco, coring and core requirements are defined for the prospect/field with input from each discipline. Once the design of the core analysis programme has reached consensus, the implementation of the programme can then take place. The recommendation for core analysis planning is to follow the outline given in section 3.2. The programme outline covers all items that impact the core analysis programme including the core acquisition itself. The list given in section 3.2 is extensive but is done in such a way as to maximise the information that can be obtained from core analysis and to assist in data interpretation. Experience has shown that all aspects of the core analysis programme can be critically important for subsequent data application. Of course, some items may not be necessary for any given application but it is nevertheless worthwhile to consider the impact each item can have on programme implementation. Each item addressed in the outline represents a separate section in chapter 4, where appropriate detail is found. Chapter 4 aims to present easy options for core analysis planners. Many of the coring and core handling considerations are covered in the "Core Handling Manual", EP 93-2200, but are included here in summary form for completeness.

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Core analysis project planning provides the following benefits: •

maximisation of quality in core analysis measurement through planning and selection of the appropriate measurement suite;

•

obtaining input and consensus from a multidisciplinary EP team so that the majority of the core analysis needs can be anticipated and incorporated and reduce the need for later remeasurement;

•

definition of clear timing constraints in order to properly impact appraisal or field development decisions.

•

optimisation of core analysis programme potentially eliminating the need for future supplementary measurements by clearly defining expectations of the core analysis measurements. This is especially valuable if additional coring can be avoided;

•

roles and responsibilities of each team member within a core analysis project schedule are clearly defined. Timely advice from each team member assists in on-time data delivery.

•

careful core selection and proper core screening increase the likelihood of measurement on the most representative samples;

•

an overall core analysis programme allows the core analysts to appreciate the entire scope of the project and to meet mutually agreed deadlines. Better planning and scheduling usually result;

•

core analysis projects can be more easily justified by using value of information concepts and proper consideration of economic benefits;

•

greater confidence and better utilisation of core analysis data is achieved. Improved application core analysis data in field development and resource management leads to improved field development planning;

•

better project management which includes better continuity during staff changes because expectations and scheduling are clearly noted;

•

easier presentation of core analysis plans and results to partners and other stakeholders in the hydrocarbon resource.

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Planning in an integrated PE team

The multi-disciplinary planning of core analysis produces a better core analysis program with input from each PE discipline. In order to encourage such involvement this section addresses the roles and responsibilities of each PE discipline involved: namely petrophysics, geology and reservoir engineering. Other disciplines such as production technology and exploration functions can also have needs that can be addressed through core analysis. The value of the integrated team approach to core analysis is the synergy that can be obtained when each PE discipline contributes to the planning of the core analysis programme. More details of the requirements of the integrated PE team are given in section 4.2. It has been customary for the petrophysicist and geologist to arrange coring and core analysis programmes. In the integrated PE environment, the petrophysicist remains likely to be the focal point for core handling and core analysis. However, over time it is expected that any member of the multi-disciplinary PE team can be focal point for a core analysis programme with the assistance of this manual.

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3.1.1 Core analysis programme development With the structure as outlined in the next section it is possible to develop core analysis programmes that are agreed to by the PE team relatively quickly. At least one meeting where the integrated team assembles to review and finalise the programme is recommended. However, much of the programme development can be accomplished by discussing relevant aspects with appropriate exploration and PE staff. Important steps in putting together a core analysis programme are as follows: •

appoint the core analysis programme focal point whose responsibility is to develop the core analysis programme - this is usually the petrophysicist;

•

create a trial or "strawman" core analysis programme by inputting as much of outline as possible;

•

review and finalise the core analysis programme with the integrated team. This allows synergy and detailed discussion to take place;

•

programme justification - management review is usually necessary to allow programme implementation;

•

meet with drilling and well-site personnel (preferably at the well-site) just prior to coring to review well-site activities regarding coring, drilling parameters, and well-site core handling and transportation;

•

implement programme. The use of spreadsheets and appropriate software are of considerable aid in tracking the progress of a project and are of great assistance in subsequent project review;

•

review project and core analysis data application.

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The core analysis programme

Scope and objectives of core analysis programme See section 4.1. Summarise measurement objectives of the core analysis programme. Multi-disciplinary considerations See sections 3.1 and 4.2. Responsibilities and required input from petrophysics, geology, reservoir engineering as well as other EP disciplines e.g. drilling should be specified. Core measurements See section 4.3. Summarise the types and number of measurements. Coring considerations and well-site planning See section 4.4. Address coring parameters and operational coring considerations. Consultation with the drilling department is necessary. If core analysis programme involves old core, note any comments that were recorded during original coring operation. Core handling See section 4.5. There are two aspects to consider: At the well-site See section 4.5.1. Note any well-site handling considerations. Upon arrival of core at Laboratory See section 4.5.2. Describe actions to be done immediately at the laboratory such as scanning, photography, slabbing, core description, etc. This facilitates subsequent measurements and minimises core handling and delay.

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Core screening See section 4.6 and Appendix 2. Specify additional procedures for screening the whole core. Core sampling See section 4.7. Specify a method for selecting the most representative samples. Consult with a geologist and reservoir engineer in special core analysis projects. Core sample preparation See section 4.8. Core sample preparation considerations should be specified with special emphasis on cleaning requirements so that core handling can be minimised. Core sample screening for special core analysis See section 4.9. Specify screening measurements and screening criteria to ensure measurement samples are the most representative. Core preservation See section 4.10. Core preservation is important to allow for future work. Fluid measurements See section 4.11. Specify the types of fluid measurements. Fluid handling considerations See section 4.12. In some core analysis programmes, careful consideration must be paid to the fluids used in the core analysis such as relative permeability measurements. This section should also summarise well-site fluid acquisition, transfer methods, transport and fluid preservation.

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Sequencing and scheduling See section 4.13. Summarise the sequence of core measurements, which is important so that data delivery deadlines can be met. Costs See section 4.14. Outline costs as part of justification process. Justification - economic impact - value of informationSee Chapter 2 and section 4.15. A value of information calculation determines economic benefits of the core analysis programme. See chapter 2 for examples. Project reporting See section 4.16 on data reporting formats. Project review See sections 4.17. Include specific details about a project review with appropriate timing which is important for data application.

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An example of a core analysis programme

Scope and objectives of core analysis programme To obtain core data for log calibration and determination of input into reservoir simulation for an unconsolidated deepwater sandstone. Multi-disciplinary considerations Geologist to select facies, describe core and thin sections. Reservoir engineering to provide input as to types of displacement process of interest. Geochemical input into hydrocarbon typing. Coring and core analysis to be coordinated by petrophysics. Drilling to provide advice on mud types and additives and potential coring-bit effects. Petrophysics and reservoir engineering to determine suite of rock and fluid measurements and appropriate value of initial water saturation. Core measurements Basic measurements should be made every foot for porosity, permeability and grain density. 6-10 facies should be selected for special core analysis measurement of capillary pressure, resistivity index, porosity and permeability as a function of stress, relative permeability and compressibility. Coring considerations and well-site planning Conduct organisational meeting at well-site; ensure equipment and supplies are ready: eg core cradle, core cutter, thermometer, dry ice for freezing. Use Christensen's CoreGard RC412 bit, with extended pilot shoe. Coring rate of 10-20 m/hr is recommended. Weighted salt/PHPA/polymer mud is to be used with starch-based fluid loss additive. 30 foot core barrels - occasional 60' core barrels if at end of coring.

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A total of 180' is targeted. Overbalance should be no more than 500 psi. Core should be brought to surface very slowly. Core handling At the well site: Stabilise core by freezing with dry ice. Allow 3-4 hours for core to freeze before cutting or until temperature reaches -50°C. Mark orientation and depths on liner. Cut core into 3 foot sections and prepare for transport. Ensure that core is not bent, flexed, jarred or rotated during handling and laying down. Use core cradle. Ensure transportation arranged. Upon arrival at the lab: Check labelling and orientation. CT-scan each 3 foot section, slab frozen core; describe core. Basic core plugs should be taken every foot. Plugs to be drilled frozen. Photograph with normal and UV light. Preserve 2/3 slab for later use. Core screening Core gamma scan in addition to above steps. Core sampling For special core analysis work, at each facies longer than about 2 feet (60cm) 10 horizontal (with respect to bedding plane) plugs and 6 vertical plugs should be drilled. Each plug should be CTscanned and thin sections taken (ideally one from each end) will assist in guaranteeing uniformity. Core sample preparation Sample cleaning should be done under stress with flow through cleaning using chloroform/methanol. Several pore volumes of tetrahydrofuran should be flown through samples for relative permeability. Relative permeability samples should then be vacuum saturated with brine and then brought to initial water saturation, Swi, and then aged at reservoir temperature for 4 weeks at Swi.

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Core sample screening Special core analysis samples to be screened based on CT -scans and thin sections. Post test analysis should include sieve analysis and thin sections. Core preservation Samples should be preserved for at least 10 years so that unanticipated needs can be addressed. Fluid measurements Brine composition using ion chromatography, inductively coupled plasma and X-ray fluoresence and brine resistivity. Oil measurements should include PVT, gravity, % sulphur, cloud point, acid and base number, and viscosity. Fluid handling considerations Brine and oil samples should be obtained if possible from RFT or MDT samples. The logging tool should be heated to 130°F prior to any fluid transfer. Fluid pressure should be raised to 100 psi above reservoir pressure. During fluid transfer logging tool should be rocked or agitated. Sequencing and scheduling A deadline for preliminary core analysis data has been established for 6 months after core has arrived at the laboratory in order to meet unitisation discussion meetings to be held after 9 months. 3 months has been allowed to perform reservoir simulation to meet planning objectives.

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Costs Core preparation, slabbing, photography, CT-scans etc

US $20k

Basic core analysis (150 samples)

US $15k

Capillary pressure (20 samples)

US $20k

Resistivity index (20 samples)

US $60k

Porosity and permeability vs stress (10 samples)

US $50k

Relative permeability measurements (10 samples)

US $50k

Compressibility measurements (10 samples)

US $50k Total

US $265k

Justification - economic impact - value of information Core analysis project is designed to avoid unprofitable investment by ensuring the presence of commercial hydrocarbons. Project reporting 20 copies of all data (including raw and interpreted) data are required. Project review Preliminary review is recommended after completion of the geological description and prior to selecting samples for SCAL work. Project review scheduled for 6 months after arrival of core.

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Considerations for major lithologies

Specifying the type of core analysis to be done also depends on the lithology to be studied. The major lithologies and related general considerations in a core analysis programme are given as follows: Clean (consolidated) sandstones • Coring is usually straight forward. • All core analysis methods can be applied. • Minimal Qv and CEC analyses are required. • Mineralogy can be limited to special core analysis samples. • Special precautions are not required for plugging or cleaning beyond usual care. • Drying at 105°C is acceptable. • Samples are not usually sensitive to fresh water. Unconsolidated sandstones • Requires special coring bits, core barrels and care in coring considerations (see Core Handling Manual) • Core stabilisation, such as freezing in dry ice, is required for transportation. • Core sample preparation is done at stress by solvent flushing and air drying. • Porosity from pore volume by liquid saturation and grain volume from dry weight and grain density. • Most measurements (including capillary pressure) should be done at stress in a core holder. • Compressibility measurements are usually important. • Core analysis planning is critical. Clay bearing and shaly sands • Coring fluids should not cause clay swelling. • Core handling should be aimed to prevent samples from drying out. • Plug drilling should be done using high salinity brine or refined oil. • Avoid high boiling point solvents during cleaning. • Samples should be dried at 95°C. • Qv and CEC measurements are recommended using membrane potential. • Capillary pressure can be done using mercury/air but air/brine or oil/brine capillary pressures are recommended to determine effects of clay-bound water.

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Resistivity-index vs water saturation curves may need correction for clay-conductivity effects. Rock-fluid compatibility floods may be important. Minerology and clay-typing is recommended.

Carbonates • Coring is usually straight-forward. • Samples often show variability between plugs and heterogeneity within the same plug. • Qv and CEC are usually not required; if required, only membrane potentials should be performed. • Carbonates exhibit variable stress sensitivity. • Compaction measurements may be important because carbonates can exhibit pore collapse at stress. • Not usually fresh water sensitivity. Vuggy carbonates • Coring can be difficult in vuggy carbonates. • Whole core methods may be important in obtaining representative properties. • Bulk volume by mercury should be done with a thin sleeve around the sample to prevent mercury invasion. • A high degree of mud invasion may occur. Low permeability formations • Low permeability samples are sometimes difficult to measure due to equipment limitations. • Core analysis planning is critical because all procedures take significantly more time. • Mercury/air capillary pressure curves should be measured with the Autopore 9220. Fractured reservoirs • Coring is often difficult. • Imaging techniques such as CT -scanning of the whole core can be used to assist in identifying heterogeneous sections. • Sampling may be biased due to fragile nature of the rock. • Whole core measurement may be recommended.

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Evaporites • Care must be exercised in selecting solvents for cleaning. Diatomite • Cleaning can take significantly longer than normal. • Samples typically exhibit very high porosity but low permeability. • Thin section and SEM are important. Coal • Degree of water absorption is critical in measuring producable methane. • Samples should be studied in an "as-received" state as well as oven-dried. Shale • Coring with oil base mud is recommended. • Use an anti-whirl bit • Cores should be cut with a fibreglass inner barrel. • Shale cores should be resin stabilised • Minimise mechanical impact during transportation • Gas-tight laminate bags should be used for storage. • Cut shale plugs with kerosene • Typical measurements can include Brinell Hardness, porosity, bulk density, moisture content, UCS, Vp, Vs and CEC.

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Where to perform the core analysis programme

The consideration of where to perform the core analysis programme, hinges on several issues: •

Programme Scope. Programmes which are predominantly for basic analyses can be done at core analysis contractors. Programmes with an extensive special core analysis component should be done at Shell E&P laboratories or approved core contractors.

•

Quality. To have the highest quality core analysis, it is always preferable to perform critical special core analysis at a Shell E&P laboratory. However, capacity at Shell E&P laboratories can be limited.

•

Regional. It is preferable to perform as much of the core analysis project as possible at the same place to minimise core transportation and core handling.

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Literature

Pink, M.J. Exploration and Appraisal Technology - Maximising Rewards by Integration Shell Selected Paper, January 1993 Van Ditzhuijzen, P. Petrophysics - In Touch With the Reservoir Shell Selected Paper, July 1994

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Core and fluid analysis considerations

This chapter is designed to provide more information in support of planning a core analysis programme as presented in section 3.2. Accordingly, it follows the same outline as presented in section 3.2. Each topic is expanded upon to present relevant considerations so that all aspects of a core analysis programme are included. Fluid analyses are also included here which are particularly important for performing analyses which involve wettability where using representative oil and brine samples are critical. Further information on fluid analyses is provided in Chapter 13.

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Scope of a core analysis programme

Specifying the scope of a core analysis programme is an important first step in planning and defines the measurement objectives of the programme. Proper scope and measurement objectives clarify expectations and allow each team member to understand and support the reasons for the core analysis programme. Moreover, a clearly defined project scope facilitates project approval as well as increases the probability of success. Clearly defined objectives enable the multi-disciplinary team to specify the necessary measurements and deadlines as well as assist the performing group to understand the critical steps that ensure measurement success. The scope of the project is defined by a set of objectives although each objective may entail a large number of measurements. Example measurement objectives are given as follows: •

basic core analysis properties for calibration of wireline logs;

•

capillary pressure and relative permeability of various rock types for input into reservoir simulation;

•

compressibility of a particular rock type or formation for compaction/subsidence predictions;

•

saturation model parameters to evaluate hydrocarbon reserves for a specific formation using resistivity logs;

•

impact of shaliness on conductivity measurements;

•

well injectivity in a particular formation;

•

residual hydrocarbon saturation;

•

impact of gas saturation on residual oil saturation;

•

impact of diagenesis on the hydrocarbon distribution.

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Multi- disciplinary considerations

This section is aimed at summarising the responsibilities and input that can be obtained from each member of an integrated team. It is placed early in programme development to encourage teamwork. 4.2.1

Petrophysics

The petrophysicist has two roles to perform: firstly as core analysis programme focal point and secondly, to co-ordinate petrophysical aspects of core analysis data acquisition. The petrophysicist has the nominal task of organization and implementation of the coring and core analysis programme. It is recommended that the petrophysicist put together a trial core analysis programme, i.e. a “strawman”. By passing the strawman core analysis programme to each PE team member and other disciplines a consensus can be reached. This may require one of more multi- disciplinary team meetings so that the synergy of the integrated team can be used. The petrophysicist must also ensure that the core analysis programme is consistent with other aspects of the petrophysical data acquisition programme such as the wireline log evaluation programme. The logging suite should be a guide to the core analysis programme. Resistivity logging, sonic logging, and density logging can all be calibrated over the cored interval by core analysis. Pay attention to potential mineralogy identification using spectral gamma ray logs, which can be better quantified with calibration from mineralogy obtained from core. The petrophysicist selects core analysis measurements relevant to the purpose of calibrating logs and determining input parameters into log interpretation models. For examples: • basic rock properties – porosity, permeability and fluid saturations; • capillary pressure measurement for saturation calculation; • stressed measurements if the formation is poorly considered or in anyway stress sensitive; • electrical properties for resistivity log interpretation along with cation exchange capacity; • acoustic rock properties for AVO calculations.

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Geology

The core analysis plan should be consistent and be closely interrelated with the geological core analysis plan. Some data are common to both analyses and measurement duplication should be avoided. Geological input is critical to guarantee that measurements are performed on samples that are most representative of the formation, especially for special core analysis. This may involve determining the number of significant rock types and also roughly estimating the reserves that may be contained in each rock type. The combination of rock typing, i.e. facies identification, is part of the process of geological core analysis. Table 4.1 reviews techniques and information typically obtained in geological evaluation. More detail is contained in the manual "Geological Core Analysis" by L. C. van Geuns and J.A. Okkerman (in preparation). Geological input should include some of the following: •

core descriptions;

•

facies analyses which is critical to the sampling for special core analysis measurements;

•

mineral identification (interaction with petrophysicist may ultimately yield mineral identification from logs);

•

investigating diagenesis as well as analysing the structure of the rock fabric (such variations can play an important part in interpreting core analysis data). Thin sections and grain size analyses are important;

•

lithology, depositional characteristics and age of the formations present for geological characterisation of the reservoir.

The geologist makes a preliminary static reservoir model. The ultimate quantification of the reservoir model is accomplished through the interaction of geologist, petrophysicist, reservoir engineer and other disciplines which should determine the goals of the special core analysis programme.

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Technique Macroscopic description • Slabbed core • Plugs/sidewall samples/cuttings

• • • •

Paleomagnetism Fracture analysis by 'goniometer' Visible light photography U.V. photography

Microscopic description • Thin section microscopy •

Scanning Electron Microscopy (SEM) with enhanced image analysis

Compositional analysis • Energy Dispersive X-Rays with SEM • X-Ray Diffraction • Mineralog from Core Laboratories •

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Information • • • • • • • • • • • •

Depositional environment Rock type/gross lithology Net/Gross Bedding/structure Degree of consolidation Grain size/sorting Fossils Hydrocarbon shows Core orientation Fracture orientation Visual record Hydrocarbon shows

• • •

Grain characteristics Porosity indication Microscopic distribution of minerals Quantification of microstructure and porosity

•

• • • •

Natural Gamma Ray Spectroscopy (NGS)

Geochemical analysis

•

Mineralogy (Clay) Mineralogy Mineralogy depth profile and approx. matrix density Quantitative determination of amounts of U, Th, and K. Used in spectral gamma ray log evaluation Chemical analysis of cap and source rock

Table 4.1 Information from geological evaluation

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Reservoir engineering

Reservoir engineering application of core analysis data is in providing input to computer simulation of reservoir performance. The required input is usually focussed on capillary pressure and relative permeability and the experimental conditions that are needed to ensure that the measured data are representative of in-situ conditions, particularly, wettability. However, the reservoir simulation itself should be used to determine the greatest sensitivities in core analysis data and so identify those parameters for which core analysis measurements are critical. This is accomplished by running sensitivities to various input parameters obtained from core analysis using simulation. Sensitivity analyses are performed to investigate the effects of variations in geology and possible recovery process options for reservoir development. Sensitivities might be run on any of the following parameters: •

relative permeability parameters including endpoint saturations and the shape of the relative permeability curve;

•

capillary pressure;

•

critical gas saturation;

•

pore volume compressibility;

•

flooding tests (such as hot water or steam flooding).

While it is unreasonable to run all sensitivities, selective sensitivity analysis can quickly clarify the economic impact of determining core analysis parameters. The output from reservoir simulation is used in the economic justification of the core analysis programme using value of information as discussed in Chapter 2.

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Other disciplines

Many other disciplines are involved in a core analysis programme development: •

Drilling engineering provides advice on drilling parameters and on coring bit and core barrel choices. New coring bits are continually being developed with the latest being low invasion, high density and anti-whirl bits. Selection of core bits is covered in Core Handling Manual, Chapter 2, and more discussion is available from section 4.4. Organisation of a pre-drilling meeting should be done in close cooperation with drilling engineering.

•

Production technology input is sought when core analysis programmes are to be designed to answer questions associated with well injectivity, sand control and rock strength issues pertaining to well-bore integrity, rock mechanical properties for fracture design, sieve analysis for gravel sizing and mineralogy for acid stimulation. An important area is cost estimation involving production technology. Different options frequently require different facilities which have a significant bearing on cost and value of information calculations.

•

Geophysicists should be consulted for input relating to measurements involving acoustic velocity measurements and use of core measurements in calibrating seismic data. The analysis of such samples for mineral constituents that can affect acoustic response often plays an important role.

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4.3

Core measurements

4.3.1

Basic core analysis

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Basic core analysis is carried out to assist in log calibration for formation evaluation such as porosity, density and mineralogy. Basic core analysis data and its uses are summarised in Table 4.2. Basic core analysis consists of the core preparation together with the measurement of the basic petrophysical parameters of atmospheric porosity, air permeability, grain density and fluid saturations for samples taken at regular intervals throughout the cored interval. A sampling rate of one per foot is common.

Parameter

Application

Residual fluid saturations at surface

• • •

Hydrocarbon presence Hydrocarbon type Fluid contacts

Atmospheric Porosity, Ø

• •

Define storage capacity Calibration of wire line density log

• • • •

Basic flow capacity Permeability profile Vertical flow Completion design

Air Permeability, kair: Horizontal (parallel to bedding plane Vertical (perpendicular to bedding plane) Permeability, ka (Klinkenberg corrected)

•

Liquid permeability from gas measurements

Grain Density, ρg

•

Wireline density log calculations (for logØ)

Lithology (from geological analysis, core photos, etc.)

• •

Rock type Rock characteristics

Table 4.2 - Basic core analysis parameters and their uses

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General points regarding basic core analysis are as follows: •

basic core analysis is often referred to as routine core analysis. However, it is sometimes not particularly routine and thus the authors prefer the term "basic" to denote the measurement of basic rock properties;

•

the definition of basic analysis is not rigid and some contractor laboratories include a few of the simpler measurements listed in the next section 4.3.2, Special Core Analysis, such as sieve analysis and cation exchange capacity, in their basic analysis;

•

geological and petrographic evaluations, covered briefly in Appendix 3, are performed both before and during petrophysical/reservoir engineering analysis;

•

basic core analysis is not performed by KSEPL but can be done at BTC;

•

after the initial preparation of core material, contractor laboratories tend to treat basic analysis as a single service with porosity, permeability and grain density measurements offered in one package. KSEPL recommended procedures should be used wherever possible;

•

particular care should be taken over the handling and storage of samples used in basic analysis; undamaged samples can be used in future special analyses which do not require fresh or native state samples.

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Special core analysis

Special core analysis is the term applied to core measurements that are not part of basic core analysis. For example, capillary pressure, relative permeability, compressibility and resistivity index are considered as part of special core analysis. Some measurements are made where the fluid distribution is at equilibrium while others are done under conditions of fluid flow. Special core analysis complements the data from basic core analysis; allowing for more thorough log calibration and reservoir modelling. Special core analysis is carried out to determine the following: •

wireline log evaluation parameters and log response calibrations;

•

refined volumetric calculations and reservoir modelling;

•

rock fluid flow properties for assessment of reservoir productivity, stimulation design and reservoir modelling;

•

rock water retention properties; fluid saturations, pore geometry and structure, and wettability;

•

the mechanical rock properties for the control of fines production and stimulation design;

•

the significance of reservoir compaction as a drive mechanism and to predict surface subsidence and for casing design;

•

rock/fluid interactions for the selection of drilling, completion, workover, stimulation, and injection fluids;

•

the magnitude and distribution of residual oil saturations for reservoir management and improved oil recovery.

Samples should be representative of the various lithologies, porosity and permeability ranges found from basic analysis.

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Typical special core analysis measurements are as follows: •

stressed porosity and permeability;

•

capillary pressure;

•

electrical properties such as resistivity index;

•

wettability and relative permeability;

•

mechanical rock properties such as compressibility;

•

waterflood sensitivity for injectivity and well performance;

•

acid solubility.

Many of these measurements are performed at reservoir conditions necessitating the reproduction of in situ pressures and temperatures.

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Coring considerations and well-site planning

It is worthwhile to understand the impact that the coring process can have on a core. Events that happen during coring can later play a critical role in the interpretation of the measurement results. Such considerations include the type of bit, type of core, drilling mud composition, length and type of core barrel, overbalance, rate at which core should be brought to surface. Proper considerations maximise the amount of core recovered. Core handling at the well-site will be considered in the next section and frequent reference will be made to the "Core Handling Manual", EP 93-2200. • coring bit type In general, low invasion face discharge coring bits are recommended especially for unconsolidated cores. Throat discharge bits are not recommended because they can cause invasion. Eastman Christensen's RC412 and Security DBS CD93 are examples of excellent coring bits with face discharge or modified face discharge with law or minimal invasion characteristics. Check section 2.1.3 of the Core Handling Manual. • type of core There are a number of coring methods such as conventional, sponge or even pressure coring. Most coring is done conventionally. Check section 2.1.4 of the Core Handling Manual. • drilling mud considerations An important consideration for a core analysis programme are the drilling mud components. Mud filtrate can invade the core and make the core behave unexpectedly with regard to desired measurements such as permeability or grain density. In the case of permeability, some drilling mud additives such as certain polymer types can be very difficult to remove from the core and make permeability measurements uncharacteristically low. In the case of grain density, hematite frequently found in mud is difficult to remove leading to significant overestimation of the grain density resulting in high porosity values.

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In general, coring should be performed in such a way as to minimise invasion and with a bland mud, so that if invasion occurs, the rock fabric remains unaltered. A bland mud is one composed of components that do not have high interfacial activity. Components such as xanthum gum biopolymer and other polymeric products should not be present in the mud. Drilling mud additives that are known to be surfactants should be explicitly avoided. This is especially critical in the measurement of capillary pressure, relative permeability and wettability where mud filtrate can cause a change in wettability and deliver results that are not appropriate for input into reservoir simulation. • rate of penetration, weight on bit Excessively high rates of penetration (> 75 ft/hr) and weight on bit (> 30 klbs) should be avoided to minimise core disturbance, • rotary speed Moderate rotary speeds are recommended such as between 60 to 100 rpm. • overbalance control Again, invasion during coring should be minimised and therefore overbalance should be as small as possible. However, safety and environmental guidelines should be reviewed when deciding upon the appropriate overbalance. • core barrel considerations Core barrels for containerised coring can be made of aluminium or fibreglass and can be made up to varying lengths. Either aluminium or fibreglass barrels can be used as long as the material can withstand downhole conditions. The barrel length should be carefully considered especially in the case of unconsolidated samples. The typical barrel length is 30 feet (10 metres) but longer barrels such as 60 feet (20 metres) or even 90 feet (30 metres)

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are possible and can reduce the drilling time necessary for coring. However, core quality can be impacted by having too long a core barrel. For unconsolidated samples, 30 feet core barrel is, in general, recommended. Longer core barrels can be used for more consolidated rock material. • core jamming Review methods of detecting core jamming as it is better to trip out immediately following jam detection than to continue. This is very critical to good recovery in fragile or unconsolidated formations. • bringing core to surface After coring, the core barrel is brought to the surface which involves changing the temperature and pressure of the core material from formation conditions to surface conditions. The more gradual the process the better the core material can adjust to new conditions. This is particularly true for unconsolidated samples and poorly consolidated samples. • laying the core down The process of bringing the core to a horizontal position should be done carefully so that no bending of the core material occurs. Unnecessary bending causes core disturbance which then leads to measurements on disturbed or altered core material resulting in unreliable data.

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Core handling

It is insufficient to only maximise core recovery, but it is important to obtain high quality core. For core analysis to yield correct results, the core material should be damaged as little as possible by careful consideration of each step involved with core handling. There are two periods during coring when there is extensive core handling, namely: •

at the well-site and;

•

upon arrival in the laboratory.

4.5.1

At the well-site

Wellsite handling is covered in "Core Handling Manual" in sections 2.2 and Appendix 1. In general, handling of the core should be done in such a way as to minimise core damage and accomplish the following: •

identify cores in such a way that orientation and depth as is known is clearly and unambiguously marked to the point of redundancy;

•

cut cores into consistent lengths to allow for easier handling;

•

minimise handling steps and avoid hammering to remove the core material;

•

minimise exposure to the elements - the core should not be allowed to dry in any way;

•

minimise core disturbance. Use suitable equipment, for example, some saw blades are better able to cut core material;

•

transport core to the laboratory as soon as is practicable;

•

alert core analysis laboratory that core has been shipped and indicate expected time of arrival;

•

perform core analysis as soon as possible after coring. This improves data reliability and applicability.

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Upon arrival at the Laboratory

Preparation for the receipt of core is advisable to minimise poor laboratory handling which can result in undue delay in core analysis programme scheduling. It can also prevent core damage by preventing improper handling which can happen if inadequate preparation is made for core arrival. Planned activities so that core measurements can be initiated as soon as possible may include: •

inspect core sections; check sequence and mark driller's depth. Determine whether cores should be taken out of any barrels;

•

describe core to provide basis for sampling for special core analysis;

•

depth match with wireline logs;

•

core gamma scan or core spectral gamma scan;

•

slab (usually into 1/3 and 2/3 portions) but this should be carefully considered because some analyses need long plugs taken before slabbing. Consequently, the time of slabbing should be clearly specified;

•

photograph (normal and UV);

•

core imaging such as CT-scanning (can be done before or after slabbing and can be done before or after plugging);

•

other X-ray techniques such as (fluoroscopy, etc).

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Core screening

Additional steps for screening whole core can be specified beyond the scope traditionally defined in section 4.5.2. Additional screening techniques are described in Table 4.3 and in Appendix 2

Technique

Information

Natural Gamma Ray Scan (Core Gamma)

•

Gamma Ray Spectroscopy (Spectral Gamma)

•

Qualitative estimate of U, Th, K concentrations - correlate with spectral gamma ray log.

Gamma Ray Attenuation

• •

Pseudo bulk density profile. Correlation with wireline density log.

Probe Permeametry

•

Pseudo permeability profile (low accuracy).

X-Ray Computer Tomography (CT- Scan)

•

High resolution enhanced images of core material Density profile. Porosity indication. Observation of core recovery and core damage.

X - Ray Fluoroscopy

•

• • • •

Natural radioactivity – pseudo gamma ray profile Correlation with wireline GR log.

Examination of sleeved unconsolidated material.

Main applications of various scanning techniques are to aid in depth matching the core to wireline logs and to estimate the degree of material homogeneity for sample selection. Table 4.3 - Information derived from core screening

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Core sampling

Sample selection for core analysis is crucial to the success of the core analysis programme. Proper sample selection ensures that the data are measured on the most representative samples where the degree of heterogeneity is minimised or at least the degree of heterogeneity can be quantified. Sample selection must be done with regard to the needs of geology, petrophysics, reservoir engineering and other disciplines such as geophysics. Ideally, sampling should result in a statistical representation of the core material. However, that is to be balanced against cost. 4.7.1

Sampling for basic core analysis

Basic core analysis is usually done on a one per foot sampling (or 3 per metre). A quick perusal should be done to check that core competence allows such sampling. If too many plugs fall in regions of poor quality core, it is possible to plug every foot but beginning at a different position perhaps 3 to 6 inches away. In general, emphasis should be paid to the cutting of plugs as close to the one foot spacing as possible without any regard for variations in lithology. Otherwise a bias towards apparently better formation properties may be unwittingly introduced which can lead to improper log calibration. For some cores, where variations occur on about the one foot scale, it is recommended to consider a closer sampling such as 1 sample for every 6 inches (or 1 per 15 cm).

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Sampling for special core analysis

In special core analysis, most attention is paid to determining the relevant rock types upon which measurements are to be made. Samples for special core analysis are usually not taken at regular intervals but are taken so that samples are representative of the appropriate lithology. Perhaps it may mean only 1 or 2 plugs to be cut or as many as 20, but rarely as many as in a basic core analysis programme. The data for special core analysis is then interpreted as to apply to the appropriate formation in quantifying the reservoir model. There are two criteria that must be met when selecting samples for special core analysis namely: •

plug sample should be representative of rock type under consideration;

•

plug sample should be as homogeneous as possible given the prevailing geology.

It is recommended for special core analysis to begin with at least twice as many plugs as you think will be needed for special core analysis measurements. For example, to have 6 relative permeability measurements, at least 12 plugs should be cut to maximise the possibility of selecting the most representative plugs. For consolidated plugs, it is often more efficient to select from the plugs used for basic core analysis measurements. It is most important to take more plugs for any measurement involving fluid flow such as permeability and relative permeability. Static measurements such as capillary pressure are less affected. The larger number of plugs initially taken allows screening to be applied so that representative samples are obtained. For formations that are very heterogeneous, such as some carbonates, it may be necessary to select three (3) times as many plugs as will be needed for measurement. Without a screening process, special core analysis projects run the risk of using plugs that are not suitable for measurement. However, too much sampling may reduce the amount of available core material for other purposes. For very heterogeneous material, whole core measurements should be considered. Again, each lithology should be represented by a number of whole core samples.

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Sampling considerations

•

Proper planning and cross discipline communication of requirements is important for proper sampling and ensuring that the appropriate rock types are included in the core analysis programme.

•

For basic core analysis, emphasis is placed on regular foot by foot (every 30 cm) sampling with little regard for variations in geology.

•

For special core analysis, emphasis is placed on rock type within the formation. It is recommended that twice as many samples as needed for a given measurement be taken for the purpose of maximising representativeness of the samples.

•

CT-scanning (cross-sectional and every inch) is critical to determine locations for plug drilling. Longitudinal CT-scans on each plug after drilling is needed to confirm acceptability of the plug. Without CT-scanning before drilling, rejection rate of plugs can be as high as 90%.

•

Probe permeability measurements can assist in assessing heterogeneity.

•

Depending on the reservoir, a statistically representative data set may require tens of plugs; the laboratory experiments have to be planned efficiently so that such a data set can be obtained.

•

Always work in conjunction with the analysis laboratory on the sampling strategy.

•

Make sure KSEPL recommended drilling/plugging methods are used to minimise damage.

•

For unconsolidated material, sampling and analysis should be done as soon as possible because unconsolidated material is not easy to store long term.

•

Prioritise sample taking; more important analyses usually merit samples from optimum positions.

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Core sample preparation

Having selected samples for screening, the next step is core preparation. Detailing core preparation procedures can minimise mistakes and damage to the core material. Details are given in Chapter 5 on core preparation. Steps in core preparation can be divided into several steps: • plug location identification As noted in section 4.7.1, the selection of plug locations for basic core analysis and special core analysis is based on different philosophies. In basic core analysis the emphasis is on the regular spacing of plugs namely 1 per foot. In special core analysis, the emphasis is on taking plugs that are representative of a particular lithology or rock type. The locations of the plugs should be chosen in cooperation with the geologist, reservoir engineer, petrophysicist and geophysicist. • plug drilling As noted in the next Chapter, the method of drilling plugs should be clearly outlined. • preparation Cleaning, drying, saturation and wetting restoration steps (for relative permeability) are critical to specify before embarking on the core analysis programme. • Whole core samples In the case of vuggy carbonates or very heterogeneous rock, more representative results will be obtained using whole core samples. Whole core analysis is detailed in section 7.3. Slabbing of the required core section is not carried out. Samples are usually taken from each foot of core, have a minimum length of 6 inches (15 cm), and are trimmed into right cylinders. Samples for geological evaluation must be taken before experiments are performed.

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Core sample screening for special core analysis

Sample screening provides an important step in quality control in the core analysis programme for special core analysis measurements. Sample screening is generally only applicable for samples used for special core analysis. Samples used in basic core analysis with quality problems are generally noted and treated as questionable data. As mentioned in the previous section, for any special core analysis measurement plan, at least twice as many plugs are cut as the number used for actual measurement. Thus, the screening process selects the better samples for measurement. Screening consists of a number of measurements aimed at determining basic properties and degree of sample heterogeneity. These measurements are analysed with respect to rock typing and facies identification to provide a clearer picture of the variety of rock types. It occasionally happens that additional plugs must be cut before acceptable samples are obtained. The activities and measurements which are recommended for special core analysis screening are: •

Core description

provides a permanent record of lithological, depositional, structural and diagnetic features of a whole or slabbed core. It provides the basis for routine core analysis sampling, facies analysis and special core analysis studies. •

Thin sections

provide a microscopic view of the rock sample and a general measure of the degree of heterogeneity. Thin section analysis on a number of samples for a given rock type generally will better characterise the formation of interest. It is also important to note the presence of minerals that can affect measurements or the possibility of any core alteration or core damage. Thin section preparation and analysis is a critical component of core screening. More detail about thin sections is given in Appendix 3.

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• Plug photography Photographs of the plugs are recommended. Plug appearance can be used for subsequent interpretation of core data. It is a simple but frequently overlooked step. • Longitudinal plug CT-scanning (2 views) provides a non-invasive and non-destructive view of the internal structure of the rock and provides a measure of core scale heterogeneity. This is done by presenting the density through the plug using two different slices through the sample position 90 degrees apart. CT-scanning is nondestructive and longitudinal scanning is an excellent method of screening for internal heterogeneities not visible from the surface. Figure 4.3 shows typical longitudinal CT-scans, of a plug sample. Variations in colour are due to clay richness and provide a measure of plug quality. Such a plug may be questionable for flow experiments. More detail about CT-scanning is given in Appendix 2. • Porosity, permeability and grain density Porosity, permeability and grain density are used to ensure values from special core analysis are in line with basic core analysis. Sometimes porosity-permeability cross-plots will show the separation of the rock types into recognisable lithologies which are important in identifying and selecting appropriate samples. • Mineralogy Sample mineralogy is useful in quantifying the abundance of unusual minerals. Variations in core analysis data can be related to rock composition. More detail on mineralogy determination is given in Appendix 3.

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• Capillary pressure (on an endpiece) Capillary pressures curves define pore throat distributions which can be used to differentiate rock type. A number of methods of screening capillary pressure curves are used such as the Leverett J function which is one of the most common methods of comparing and contrasting rock types. Each method of rock type classification has merits and any particular one may be more useful than another for a given regional geology. It is advisable to check the manual Capillary Pressure Saturation Height Functions by R. van den Oord and S.J. Adams, EP 93-0001, for a review of methods of using capillary pressure curves in rock typing. More information about screening is given in Appendix 2 and Appendix 3.

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Core preservation

Core preservation is aimed at maintaining core for future use. In any core analysis project, it is very difficult to anticipate every need core analysis should address. This occurs because present needs do not always coincide with future ones and because the demands placed on core analysis may have to be met with core that has been preserved. There are a number of ways of core preservation and these are described in detail in the Core Handling Manual by Okkerman and van Geuns, EP 93-2200. The main types are as follows: • Core freezing Shell has long been a proponent of freezing although freezing of core for preservation has been controversial. Possible effects of freezing include rock damage if water saturation is too high, fracturing if freezing time is not sufficiently slow, salinity variations because of freezing, and alterations to rock fabric because of freeze thaw cycles. It is advisable to cake the core with ice, and to store the frozen core at temperatures below 22°C while minimizing sublimation by ensuring that the frozen core is packaged in appropriate core package materials such as special core wrap, aluminium foil and boxing. Core takes about 3-4 hours to freeze when in contact with dry ice. Water saturation should not be too high less than 60% or at least 10% gas saturation which generally is the case because of the hydrocarbon-bearing formation of interest or when the core is brought to surface. Frozen core is easy to handle and plugs can be easily drilled with liquid nitrogen. CT-scanning can also be done on frozen core. •

Resin stabilisation In many places around the world, dry ice is not available and the opportunity to freeze the core and maintain the core at low temperatures is not possible. Resin stabilisation is an alternative to freezing. Resin stabilisation is done at the well-site, where epoxy resin is introduced between the core and the liner. Once the epoxy sets up, the core is stabilised and can be transported. Unfortunately, it is not always possible to completely fill the annulus with epoxy. Other problems are possible contamination of the core by resin, core invasion by resin and possible safety hazards with use of resin.

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Fluid measurements

4.11.1 Brine measurements There are several types of brine measurements of interest in core analysis: • Obtaining water samples In cases where there is water production, sampling is relatively straightforward. Select a well that has not been worked-over and no additives have been recently used in the well. Water samples should be taken in a plastic container and not a metal one. Water sampling can be difficult if there is no water production. Estimating water composition from core water may be possible, see section 13.2.3. • Brine composition Composition has ramifications because it can playa role in the preservation of the state of minerals in the formation. Abundances of cation and anion types are important. In addition, pH is a very important brine property because it may alter the wettability of the formation. • Brine resistivity Brine resistivity plays a role in all electrical properties measurements. The brine resistivity used in the measurements should duplicate as closely as possible the formation brine resistivity. However, only few measurements are done at reservoir temperature. It is usually thought to be more important to match the brine composition than its resistivity. • Brine-rock compatibility Some consideration should be given to compatibility of brine for flow experiments. Incompatibility results in poor results and unusable data. See section 13.3 for details.

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4.11.2 Oil measurements Oil properties that can play a role in core analysis or are critical information evaluation: •

PVT properties such as bubble point;

•

Gas Oil Ratio - is sometimes necessary for adding hydrocarbon gas to dead crude to make it live. Although usually only methane is added, it is often that methane, ethane and propane are added to obtain a better simulation of live crude properties. Live crude oil is sometimes used in preserved or restored state experiments. Recent work has suggested that it is more important to be at the correct reservoir temperature than the correct gas-oil ratio and that dead crude results are similar to live crude results;

•

API gravity;

•

whole oil analysis for oil quality characterisation;

•

high temperature gas chromatography for hydrocarbon typing;

•

density - necessary for computation of oil volumes and for use in such analyses as oil/water capillary pressure by centrifuge;

•

viscosity at various temperatures- necessary for determining relative permeabilities to oil;

•

acid and base number - useful parameters for characterising wetting tendencies of oil;

•

cloud point and wax deposition rate are important for oil refinery processing considerations;

•

presence of trace elements such as sulphur, nickel, vanadium which are also important in processing considerations.

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Fluid handling considerations

Although this manual is primarily devoted to core analysis, some comment on the handling of fluids is in order because modern special core analysis techniques require proper fluid samples, especially oil samples, to deliver properly measured special core analysis data such as relative permeability. •

Sample oil volumes. Oil volumes can be obtained from formation testing logging tools such as RFT, which may be the only source of oil in new discoveries. Note that before fluid transfer, the tool should be warmed to 55°C as well as pressured to 100 psi greater than formation pressure. Agitating the tool during transfer is important. After as much fluid as possible has been transferred, the tool should be rinsed with chloroform/methanol and the rinses saved. Rinsing should be repeated two to three more times. In remote locations, these procedures may be difficult.

•

Dead oil. Sampling from a well should be done at the wellhead. A well that has not been worked-over recently should be selected and where no additives have been used.

•

Live crude oil. For live crude oils, appropriate volumes of separator gas are required.

•

Required volumes for analysis. It is important that required volumes be obtained. Oil volumes should be sampled as follows:

Analysis

Volume required

Relative permeability at least 300 ml (for both aging and actual measurement; some relative permeability techniques require significantly more oil such as up to 5 litres.) PVT

around 1 liter

geochemical analysis

500 ml.

Approximately 500 ml of brine are required for brine analysis.

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Sequencing and scheduling

The sequencing and scheduling of the core analysis programme allows each team member to know when input is required and when to expect data. The process of sequencing and scheduling is the key to the proper management of a core analysis project. Sequencing is critical to ensure the maximum amount of data can be measured on each plug sample and takes into account the time each step from core preparation to final data reporting should take so that proper time allowance can be made. Some contingency is generally allowed for so that unforeseen circumstances can be incorporated. Scheduling ultimately reduces to the issue of when data can be expected. By recognising data needs and schedules within the larger project, core analysis data should be scheduled to maximise its impact.

4.14

Costs

Costs for the entire coring and core analysis programme can be determined from appropriate price lists. Although KSEPL does not provide prices, this manual does provide an estimate of cost for services described herein. The prices are based on an average of US based and European based prices as of 1994 and may vary by region.

4.15

Economic impact and justification

Project approval should be based on the economic impact of the project and not just on cost alone using Value of Information concepts as discussed in Chapter 2. Core analysis data provides data for decisions used in economic development of hydrocarbon resources. Recognising the balance between cost and impact is the responsibility of the integrated PE team.

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Project reporting

A core analysis programme is not complete until properly documented. Core analysis data are reported in tabular, graphical, image and or digital form. These reports become permanent records of the observations made at the time of coring, core handling, screening, preparation and testing. Any commentary which may assist in the interpretation of the different data forms at the present or in the future should be recorded, e.g.: •

well-site activities should be summarised in a well-site report;

•

a copy of the core analysis programme should be included;

•

all raw data used to generate the final data should be included in the core analysis report;

•

unusual testing circumstances and data anomalies should be clearly noted.

It is important to specify the number of core analysis reports required by the Opco so that appropriate stakeholders (including appropriate central files) receive a copy. Tabular report Tabular report should include all data, positively identified and tabulated in some convenient, but usually spreadsheet, form. Identification comes in two forms: • Wellsite report: well identification; type of well (vertical, deviated, side track, etc); type of core; core recovery; drilling parameters; mud composition; core depths; well-site handling; well-site preservation; core transportation; report timing of activities.

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Core analysis laboratory data: a copy of the complete core analysis programme (see Chapter 3 and Chapter 4); core condition upon arrival at the laboratory; note any variations to core analysis plan; core screening activities; core storage; core disposition; methods and conditions applied (drying, cleaning solvents); results (both raw and calculated data etc.).

The exact presentation of data may be determined between the user and the analyst. Suggested significant figures to be reported are as follows: Porosity values to Grain density values to Saturation values to Pore volumes to Grain volumes to Bulk volumes to Permeability values to Pressure values to Stress values to Resistivity values to Relative permeability values to Compressibility

3 (e.g. 0.203 or 20.3 %) 3 (e.g. 2.65 g/ml) 3 (e.g. 49.9 % pore volume) 4 (e.g. 12.68 ml) 4 (e.g. 20.34 ml) 4 (e.g. 25.78 ml) 3 (e.g. 1.33 mD) 3 (e.g. 75.7 bar) 3 (e.g. 789 bar) 3 (e.g. 25.1 ohm m) 3 (e.g. 0.00123) 2 (e.g. 1.1x10-5/bar)

Graphical report Graphical presentations are often included to provide the user with a pictorial overview of various data. Through the continued advances of computer graphic software an unlimited selection of pictorial formats are readily available including crossplots, histograms, or core data profiles (logs). Specific formats are left to the discretion of individual Opcos. However, some essential recommendations are made below: • Basic core analysis Two graphical figures have been widely accepted and are recommended for inclusion in every basic core analysis report: - Permeability vs Porosity plot - Core Data vs Depth plot.

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• Special core analysis Graphical representation of special core analysis reports is important. Common graphical representation is necessary for the following data: - Porosity (linear) vs stress (linear) - Permeability (logarithmic or linear) vs stress (linear) Capillary pressure curves - Pressure (logarithmic) vs saturation as % pore volume (linear) - Pressure (logarithmic) vs saturation as % bulk volume (logarithmic) Resistivity curves - Resistivity Index (logarithmic) vs water saturation as % PV (log) - Formation resistivity factor (logarithmic) vs porosity as % PV (log) Relative permeability curve - Relative permeability (logarithmic or linear) vs saturation as % of PV(linear) Compaction curve - Axial displacement (linear) vs pore fluid pressure (linear). Image report An important component of current core analysis procedures is the reporting of core and plug screening procedures which specifically include image data such as photographs, thin section images and CT-scans. The ability to cross-check tabular and graphical reports with images is critical to assessing quality of core analysis data and fundamental to the process of project review (see section 4.17). Digital report Most core analysis data are now automatically acquired. Initial (raw) data and calculated (final) data processing and storage, as a result, are being done increasingly on computer systems. A standardized method of data reporting minimises the cost and effort to transfer data between various computer platforms. The exact format of logical data organisation and type of physical storage media (i.e. disk, tape, optical disk, etc.) applied should be agreed between Opco and core contractor. At KSEPL, all data is represented in printable ASCII characters in EXCEL spreadsheet format. The layout is the same as that for the hard copy of both the tabulated and graphical reports. Digital reports should include image files where possible.

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Project review

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A follow-up meeting of the PE team to review core analysis results is very important in order to better apply core analysis data, to maximise impact of core analysis data and to ensure that the needs of the PE team are met. This is shown in diagrammatic form in Figure 4.4. Having completed a series of measurements, data review leads to a much clearer picture of which actual data merit the highest consideration. It is most worthwhile to have the core analysts present at the data review. This could be difficult with problems of distance and time, but perhaps modern technology such as videoconferencing can be of assistance. Even basic core analysis projects, although usually uncomplicated, merit review. The project review process should include: •

review all measurements made on the same plug especially core photography, CT-scanning, thin section, SEM and XRD results which will have a bearing on values measured. The quality of the plug data can be checked, for example, if the sample coincidentally has a higher clay content than normal coupled with an unusual resistivity, etc.;

•

review measurements in the geological context of the formation. Thus the implications of core analysis data can be grasped by examining all measurements on the same facies;

•

determine the quality of the measurements. This is done with the core analysts who performed the work being present;

•

check consistency of related measurements: - pore volumes, and thus porosities, should be checked if a single sample is subject to multiple measurements, - initial water saturations from capillary pressure should be consistent with resistivity-index measurements and relative permeability measurements;

•

obtain input and consensus from the multidisciplinary PE team.

As a consequence of the follow-up review, the best data will have been determined and a consistent picture of the nature of the data will be evident. This will lead to better application of the core analysis data in quantifying reservoir models in the multi-disciplinary team by proving the proper contextual interpretation of the core analysis results.

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Literature

Niko, H., Maas, J.G. Special core analysis as seen from reservoir engineering. Contribution to the PW04 Advanced Reservoir Engineering Workshop, Noordwijkerhout, October 1993 Cuiec, L. The Effect of Drilling Fluids on Rock Surface Properties SPE 15707 Sharma, M.A. and Wunderlich, R.W. The Alterations of Rock Properties due to Interactions with Drilling Fluid Components Worthington, A.E., Gidman, J. and Newman, G.H. Reservoir petrophysics of poorly consolidated rocks 1. Well-site procedures and laboratory methods Transactions of the 28th Annual Logging Symposium London Lamb, C.F. and Ruth, D.W. Laboratory Programme Design for Unconsolidated Heavy Oil Reservoirs: A Case Study SCA 9104 paper presented at the 5th Annual Technical Conference of the SCA. Bateman, R.M. Building a reservoir description team - case study. Paper EE presented at the fifteenth formation evaluation symposium, 1993, May 5-7. Okkerman J.A. and van Geuns L.C. Core Handling Manual EP 93-2200

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Core preparation

The process of core preparation can be divided into three steps •

core plug drilling;

•

core cleaning;

•

core drying.

After selecting plug locations and cutting plugs for analysis, the proper methods of core cleaning set the stage for later measurements. Improper core preparation can impact subsequent data quality. Avoidable errors often occur in these first steps of core analysis so it is advised to be as specific as possible about core preparation steps. This chapter begins with the appropriate considerations for proper drilling of core plugs followed by methods of core cleaning and core drying. After core preparation, the clean dry samples are ready for measurement or for restoration of wettability prior to relative permeability measurement.

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Plug drilling

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Drilling consolidated samples

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Principle Whole core sections are slabbed into two sections typically with thicknesses 1/3 & 2/3 of the original diameter after the optimum slabbing plane is determined through CT-scanning. Plugs are taken from the 2/3 section and can be drilled with either fresh (or tap) water, brine, kerosene, air or liquid nitrogen. Points • Fresh (or tap) water is used for clean sands and carbonates. • Brine is used for cores from high salinity environments. • Kerosene (or petrofree) is used for shales and halite bearing samples. • Air is used for fluid saturation determination studies. • Liquid nitrogen is used for shales and when consolidation is questionable. • For most measurements cylindrical plugs of diameter 2.54 cm and length 2.5 to 5.0 cm should be drilled from the slabbed core using fresh water as a drilling fluid (see also Chapter 10 if wettability must be preserved). • Horizontal plugs should be drilled parallel to the apparent bedding plane while vertical plugs are drilled perpendicular to the apparent bedding plane. Care needs to be taken over the direction of the bedding since fluid flow properties may vary with sample orientation. If horizontal and vertical properties are to be compared the plugs need to be drilled as close to each other as possible. • Samples should preferably be taken from the centre of the core to minimize contamination from drilling mud invasion. • Plugs should be machined into right cylinders; the offcuts (also known as endpieces or trimmings) of this process can be used in analyses which do not require plug samples, such as thin sections, mineralogy, and cation exchange capacity determination (see chapter 9). • Small diameter cores must be plugged before slabbing to ensure plugs of adequate length, see Appendix 4 for more details. Price/timing • < US$25 per plug drilled • Plugging usually takes about 10-15 minutes per plug.

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Drilling unconsolidated samples

Principle Frozen unconsolidated whole core sections are slabbed into two sections typically with thicknesses 1/3 and 2/3 of the original diameter after the optimum slabbing plane is determined through a CT-scanning. Plugs are taken from the 2/3 diameter section. KSEPL recommends the 2/3 diameter core section be frozen in dry ice, if not already frozen, and that a plug be drilled using liquid nitrogen as a coolant (see Figure 5.1). Plug samples should remain frozen prior to measurement.

Points • • • •

Analysis should be performed as quickly as possible to prevent deterioration of the samples. This is particularly true for samples containing high salinity brine. Plugs should be stored at temperatures below -220C. Some contractors take plugs by 'punching' the sample out of the unfrozen core material. This is NOT recommended. Plug samples should be drilled with liquid nitrogen. Points from section 5.1.1 are applicable here.

Price/timing •

< US$25 per plug drilled

•

Plugging usually takes about 10-15 minutes per plug.

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Core cleaning

5.2.1

Cleaning consolidated samples

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Principle Cleaning is accomplished by means of a hot solvent extraction (Soxhlett) technique, in which an azeotropic mixture of methanol and chloroform is heated and diffused into the sample. This should be done below the boiling point of water, to avoid removing any water before the oil. The technique also ensures that salt is not precipitated. Points • •

• •

•

• • •

Certain measurements require samples that have NOT been cleaned; fluid saturations by the Dean-Stark method (section 6.3.1) is one example. Gas-drive solvent extraction (e.g. CO2 saturated toluene extraction where a pressure drop causes the gas to expand and flush the sample) used at some contractor laboratories is considered acceptable, but must not be used for soft rock types such as chalk. Types of solvents include acetone, chloroform/methanol azeotrope, cyclohexane, ethylene chloride, naptha, tetrahydrofuran, toluene, trichloroethylene, xylene. Usual criterion for plug cleanliness is a clean extract (check for fluorescence in solvent). However using a second solvent can remove additional hydrocarbon and is recommended when the first solvent takes more than three or four days to clean the sample. The usual sequence is to begin cleaning with toluene and chloroform/ methanol. If it is found that samples are not sufficiently clean more aggressive solvents are then used such as xylene and tetrahydrofuran. Combinations of solvents such as alternating chloroform/methanol and toluene can be used for samples that are difficult to clean. Tetrahydrofuran is recommended for cleaning to water-wet state prior to restoration of wettability. High boiling point solvents can cause clay dehydration. Naturally occuring halites may be removed by toluene.

Price/timing • < US$25 per plug. • Cleaning by this technique usually takes one week. If this is not sufficient to remove all the oil, an alternative solvent can be applied.

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Cleaning unconsolidated samples

Principle At KSEPL, the frozen sample is mounted in a core holder and a confining stress of 30-50 bar is applied. The sample is allowed to thaw. After thawing, the sample is cleaned by cold solvent flushing with chlorothene and toluene alternately. Points • KSEPL apparatus for cleaning unconsolidated samples is shown in Figure 5.2. • Cleaning in a stress cell prior to a stressed measurement is normally done and eliminates one core handling step. • It may be difficult to ensure that the sample's pore structure is representative of the reservoir. Computer Tomography (CT) scans can be used to recognise core disturbance and core recovery in case of fibreglass or plastic liner contained cores. • Studies have found that if core material is frozen slowly by refrigeration or with dry ice, damage to the microstructure is not induced. • Many core contractors apply a screen capped teflon method (SCTM) to unconsolidated plugs in which a teflon sleeve confines the cylindrical surface while the ends are capped with a wire mesh. Thereafter, for cleaning, drying and measurement the sample is treated as if it were consolidated. The teflon is non-conductive so electrical properties can be investigated. Other contractor labs use heat shrink tubing or lead sleeves. • Further preparation steps are usually: the sample is dried by purging with nitrogen, saturated with a 10 g/l NaCI solution and frozen again. The frozen sample is then placed in the experiment sample holder, subjected to a confining stress and allowed to thaw. The NaCI solution is removed by flushing with methanol. Price/timing • < US$75 per sample • Cleaning can take as long as several weeks depending on oil type.

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Core drying

5.3.1

Oven drying

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Principle The sample should be dried in a vacuum oven. KSEPL advises that samples should be dried at 95°C. Samples may show induced fractures at higher drying temperature. Points • Use an explosion-proof oven. Conventional ovens as shown in Figure 5.3 are acceptable. • Each core sample should be dried until constant weight is obtained. • Drying times may vary substantially. . • Care is critical in handling samples with hydrated materials. In some cases a lower temperature should be used.

Pricing/timing • About US$5 per plug. • Drying is done in batches of up to 100 plugs overnight (at least 16 hours).

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Critical Point Drying (CPD)

Principle The CPD technique prevents the development and advancement of a gas/fluid or fluid/fluid interface within the rock by raising the fluid within the pores above its critical point (see Figure 5.4). The prevention of interface formation is achieved by replacing the oil and brine in the pore space successively by methanol and liquid CO2 through diffusion. CO2 is preferred because its critical point of 32°C and 72 bar is more convenient than other solvents. The gas is vented without an interface being formed. Methanol is used as an intermediate liquid to ensure full miscibility. Points • Diffusion time depends on sample size and permeability and can range from one day to one month. • Temperature and pressure are chosen such that liquid carbon dioxide becomes a gas without a phase change. • Critical point drying preserves the structure of the clays in the pores. It should be used if there are delicate clay minerals, such as fibrous illite, which are sensitive to the conventional oven drying method. If these minerals are damaged in the drying process, the air permeability of the samples is profoundly affected, by up to a factor of 10 or more. Figures 5.5 and 5.6 show the damage which can be caused to delicate clay minerals by conventional oven drying. • The CPD technique can only be used on consolidated material. • CPD is done on fresh cores. • After critical point drying, the sample can be resaturated without the appearance of interfaces by using the critical wetting method; essentially CPD in reverse. • Permeability estimates can be obtained by using a cleaning solvent such as methanol which avoids using brine. Price/timing • About US$50-100 per plug. • CPD takes about 2 weeks to 2 months, depending on air permeability.

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When the pressure and temperature of the pore fluid are raised above its critical point, the fluid is brought into a supercritical state in which no phase transition exists between gas and liquid. The supercritical fluid can then be removed from the sample without damaging fragile clay minerals.

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Humidity controlled drying

Principle Core contractors employ humidity drying where the sample is heated to 60°C at 40% relative humidity. Because of the low temperature, the drying may take several days. It is claimed this method preserves the water adsorbed on the clays (clay-bound water). Points • This technique is NOT recommended by KSEPL as the resulting 'effective' porosity has no unique relationship with either log-derived effective or total porosity. • Salt is not removed and causes erroneous weights. Salt must be removed by flushing with methanol/water before measurements of total porosity are made. • Small variations in relative humidity can impact porosity measurement of some plugs. Price/timing • < US$25 per sample • Humidity controlled drying usually takes several days.

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Review of some contractor preparation procedures

The following tables are based on recent studies carried out by KSEPL/SIPM on a number of contractor analysis laboratories. The review summarises preparation procedures routinely available. The remark "acceptable" indicates that KSEPL agrees with the procedure offered; it is either similar to that used at KSEPL or expected to yield results of comparable quality. A blank space indicates lack of information. In general, contractor laboratories can offer preparation procedures of acceptable quality as long as recommended procedures are requested and followed. Note: Preparation costs are approximate.

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Literature

Dicker, A.I.M. and Bil, K.J. The influence of core preparation on the effective permeability of tight samples. KSEPL report RKMR 87.041 and RKRS.87.02. Dicker, A.I.M. and Bil, K.J. The influence of drying on the gas permeability of tight samples containing illites. KSEPL report, RKRS.86.11. Blehaut, J.F. The effect of laboratory drying techniques on clay morphology and permeability in Rotliegendes tight gas sandstones. TIGRE RKSR June, 1983 Schipper B.A. A critical review of two common core analysis measurements for reservoir evaluation. EAPG/RMC/SCA Workshop, Vienna, June 6, 1994. Soeder D.J. and Doherty M.G. The effects of laboratory drying techniques on the permeability of tight sandstone core. SPE/DOE 11629. Schipper, B.A, Aperen, A.E. van, Looyestijn, W.J. Quality assessment of core analysis procedures of Core Laboratories Aberdeen. EP 90-1886. Schipper, B.A, Aperen, A.E. van, Looyestijn, W.J. Quality assessment of core analysis procedures of Poroperm-Geochem Limited, Chester. EP 90-1901 Schipper, B.A., Hofman, J.P., Quality assessment of core analysis procedures of Corex Services Ltd, Aberdeen. RKTR.93.052, May 1993 (EP 93-1296). Schipper, B.A, Oord, R.J. van den, Adams, S.A Quality core analysis - essential to our business! Production Newsletter July/August 1992. Schipper, B.A. Quality Assessment of the Core Analysis Services of Simon Petroleum Testing, Aberdeen. RKTR.94.089, May 1994 (EP 94-0974)

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Basic core analysis

The most frequent measurements in core analysis are performed for the determination of basic physical properties. These are porosity, permeability, grain density, water and oil saturations. Simple relationships govern these parameters such as: •

the sum of pore and grain volumes is equal to bulk volume;

•

grain volume is given by the ratio of dry weight to grain density;

•

saturations are water and oil volumes normalised to pore volume.

Sampling for basic core analysis, as discussed in section 4.7.1 is generally foot by foot.

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6.1

Porosity and grain density

6.1.1

Bulk volume by buoyancy in mercury

Principle A clean, dry, consolidated sample is immersed in mercury. The sample weight in mercury is measured and Archimedes principle used to determine bulk volume, see Figure 6.1. Points • KSEPL recommended technique because of simplicity and speed. • Significant error in bulk volume may be introduced if large surface pores are present, e.g. in a vuggy sample. • Errors are minimised by using right cylinders and wrapping them in cling film before the measurement. • For carbonates which are particularly cracked or vuggy, more representative bulk volume measurements may be obtained by whole core or full diameter analysis (see section 7.3). Precision • 0.01 ml. Price/timing • < US$75 per sample. • A single measurement takes a few minutes. Peripheral measurements • Grain and/or pore volume are needed to determine porosity. • Dry weight is needed to determine grain density.

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Bulk volume by mercury displacement

Principle A clean, dry, consolidated sample is immersed in mercury and the volume of displaced mercury is determined by a piston displacement. Points • This is an acceptable technique. • Ensure that there is not a significant head of mercury on the sample during measurement. For this reason, bulk volume by mercury buoyancy is preferred. • Significant error in bulk volume may be introduced if large surface pores are present, e.g. in a vuggy sample. Errors are minimised by using well formed right cylinders and wrapping them in cling film before the measurement. • For carbonates which are particularly cracked or vuggy more representative bulk volume measurements may be obtained by whole core or full diameter analysis (see section 7.3). Precision • 0.01 ml. Price/timing • < US$75. • A single measurement takes a few minutes. Peripheral measurements • Grain and/or pore volume are needed to determine porosity. • Dry weight is needed to determine grain density.

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Bulk volume by caliper

Principle A caliper is used to measure sample dimensions to calculate bulk volume. Points • This is an acceptable technique for regularly shaped samples. • This method is slightly quicker than mercury displacement but is less accurate. • It can be used in cases where large surface pores/vugs are present. Precision • 0.15 ml (which is equivalent to a porosity precision of 1 porosity unit for a 20% porosity sample). Price/timing • < US$50 per sample. • A single measurement takes a few minutes. Peripheral measurements • Grain and/or pore volume are needed to obtain porosity. • Dry weight is needed to determine grain density.

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Pore volume by liquid saturation

Principle The weight of a clean, dry, consolidated sample is measured before the sample is 100% saturated with a fluid, either brine or an organic solvent. The saturated weight is measured and the pore volume is determined from the dry and saturated weights and density of the saturating fluid. Points • This is an acceptable technique although care must be taken to avoid handling errors such as can arise in measuring saturated weight if extraneous fluid drops adhere to the sample surface or if the sample is not fully saturated. • Pore volume is measured directly. • Consolidated samples only. Precision • 0.01 ml (which is equivalent to a porosity precision of 0.2 porosity units for a 20% porosity sample). Price/timing • < US$50 per sample. • A batch of 20 samples usually takes about a day. Peripheral measurements • Bulk volume and/or grain volume are needed to determine porosity and dry weight to determine grain density.

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Grain density by pycnometer

Principle Five to ten grams of clean, dried (unconsolidated or crushed) sample are placed in a pycnometer (which is a small glass flask of known weight accurately calibrated for volume; see Figure 6.2). After weighing, the pycnometer is filled with toluene or kerosene, and the solvent is degassed. The weight of pycnometer, sample, and solvent is then determined at a known temperature. Grain density is calculated from weights, pycnometer volume and solvent density. Points • KSEPL recommended technique for determination of grain density for unconsolidated and consolidated material. The techniques is recommended for its accuracy. • Technique is destructive. • Trimmings (or endpieces) are usually used. • Temperature control is critical. • Automated pycnometers as shown in Figure 6.3 are often used where temperature control is less critical. Precision • 0.002 g/ml. • This technique is used to calibrate other grain density methods. Price/timing • < US$50 per sample. • A batch of up to 20 takes about a day. Peripheral measurements • Dry weight and bulk volume are needed to obtain pore volume and porosity.

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Grain volume by buoyancy

Principle A sample is weighed when 100% saturated with a solvent such as chlorothene or toluene. The saturated sample is immersed in a bath of the same solvent and re-weighed while suspended below the surface of the solvent. Grain volume is calculated from the dry and saturated weights and solvent density according to Archimedes Principle. Points • This is the recommended technique (see Figure 6.4). • Consolidated samples only. • Temperature control is critical. Precision • 0.005 ml (which is equivalent to a porosity precision of 0.1 porosity units for a 20% porosity sample). • Error can occur if sample is not completely saturated. Price/timing • < US$50 per sample. • A batch of 20 usually takes about a day. Peripheral measurements • Bulk volume and dry weight are needed to obtain porosity and grain density.

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Grain volume by Boyle's law porosimetry

Principle A Boyle's law porosimeter determines volumes by the principle of gas expansion. A typical design is shown in Figure 6.5. It consists of two chambers, a reference chamber and a sample chamber of known volume, which can be isolated. Helium gas, at a pre-set initial pressure, is allowed to expand from the reference cell into the evacuated sample cell, which contains the clean, dry sample of unknown grain volume. The final equilibrium pressure is measured and the grain volume can be calculated using Boyle's Law.

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Points • Boyle's Law porosimetry is acceptable but KSEPL recommends that, on a regular basis, either the pycnometer or buoyancy method be performed on a subset of samples so that Boyle's Law technique can be checked or calibrated. • Sufficient time must be taken to reach pressure equilibrium otherwise porosity and grain density can be underestimated particularly for low permeability samples. • The technique is non-destructive for consolidated samples. • Both consolidated and unconsolidated samples can be run. Unconsolidated samples are placed in a cup of known volume Precision • 0.025 (which is equivalent to a porosity precision of 0.5 porosity units for a 20% porosity sample). Price/timing • < US$50 per sample. • A typical measurement takes less than 30 minutes per sample. Peripheral measurements • Bulk volume and dry weight are needed to obtain porosity and grain density.

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6.2

Steady- state gas permeability

6.2.1

Air permeability

Principle A core plug of known length and diameter is loaded into a “Hassler” type core holder (see Figure 6.6 ). The sample is subjected to a low confining stress (of about 15- 20 bar) to prevent gas flow around the plug. Gas (air or nitrogen) is allowed to flow through the sample by applying a pressure differential across the sample. Flow rate and pressure differential are measured and used to determine sample permeability using Darcy’s law, pressure differential, sample dimensions and gas velocity.

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Points • This is an acceptable technique. Liquid permeabilities are preferred but are much more expensive. • Unconsolidated samples can be measured but problems can occur with by-passing if the sample is not sleeved or if insufficient confining pressure applied. Permeabilities of unconsolidated samples are better measured at reservoir stress. • Specify orientation of the core plug; horizontal and vertical permeability can be quite different. If permeability anisotropy is required, vertical and horizontal permeabilities should be measured on samples as near to each other as possible within the same rock type. KSEPL RR/37 has cubical sample capability for permeability anisotropy (see Figure 6.7) Precision • Precision depends upon permeability: 0.01 - 1.0 1 - 50 50 - 2,000 2 - 10

mD mD mD D

accuracy: +20 % accuracy: +10 % accuracy: + 5 % decreasing accuracy with increasing permeability.

Price/timing • < US$100 per sample. • A single measurement takes about 5 minutes.

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Peripheral measurements • Gas viscosity at temperature must be determined. •

Klinkenberg correction. Using gas at low pressure leads to an apparent permeability which is too high because the mean free path of the gas is no longer negligible compared to a typical pore size. Corrected permeability values are obtained by measuring the permeability at a series of different mean pressures and using them in the following equation: kair = k∞ ( 1 + (b/P) ) where

kair k∞ P b

apparent gas permeability (mD) absolute 'Klinkenberg' permeability (mD) mean absolute pressure (bar) Klinkenberg gas slippage correction factor (bar).

Apparent permeability values are, in general, linear in reciprocal mean pressure. Extrapolation to infinite mean pressure determines the theoretical liquid, or Klinkenberg, permeability, k, and the slope is the Klinkenberg gas slippage factor, b. Values for b range from 0.1 for high permeabilities to 10 for permeabilities in the micro-Darcy range. •

Turbulence correction. Fluid acceleration and deceleration in the pore throats and bodies lead to inertial effects, becoming more prominent at higher differential pressures and flow rates. Darcy's equation will no longer describe the flow, as it is valid only for laminar flow. The Forchheimer equation can be used as described below: (∆P/L) = (Qµ/Ak) + βρ (Q/A)2 where

∆P/L Q/A µ/k β ρ

pressure gradient flow velocity reciprocal mobility (viscosity/permeability) coefficient of inertial resistance fluid density.

Setting β = 0 returns Darcy's Law.

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Probe permeability

Principle In probe permeametry, gas flows from the end of a small-diameter tube (or 'probe') that is sealed against the surface of a slabbed or unslabbed core (see Figure 6.9). Gas at a known pressure is delivered from the probe to the sample. The pressure in the probe is measured together with the corresponding volumetric gas flow rate. Gas permeability is determined from calibrations based on pressure and flow rate. Points • This is an acceptable technique as long as sufficient calibration is performed. • The permeability is localised to the region near the seal. • The method is non-destructive. • As the probe only investigates a small volume of rock, the measurement is well suited for investigation of spatial permeability variation in cores. Also directional permeability variation around the circumference of a whole core can be measured. • A permeability range of about 1 to 10,000 mD can be measured. • Data reliability depends heavily on the condition of the core. • Permeability data may reflect effective permeability at partial liquid saturations. Precision • 20% of measured permeability with proper calibration. Price/timing • < US$50 per sample. • Each measurement takes a few minutes. Peripheral measurements • Permeability should be checked against plug values.

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6.3

Fluid saturations

6.3.1

Fluid saturations by Dean-Stark extraction

Principle Dean-Stark extraction relies upon distillation of the water fraction and the solvent extraction of the oil fraction from a sample using the apparatus in Figure 6.10. A virgin sample is weighed and placed in the extractor. Vapour of the boiling solvent distils water from the sample. Solvent and water vapours condense in a reflux-type condenser and are collected in a calibrated trap. Additional extraction is sometimes needed to complete removal of oil and precipitated salts from the sample. After water and oil have been removed, the sample is dried. The oil weight is obtained as the difference between total loss in sample weight and water weight. Oil weight is converted to oil volume using oil density which is measured separately. Points • Dean-Stark is the recommended technique for determining fluid saturations. • Dean-Stark should be done as soon as possible after coring. • Consolidated samples usually remain undamaged and can be used for further testing. • Unconsolidated samples can be used but integrity is difficult to maintain. • Typical solvents are toluene, xylene, chloroform/methanol. • Reported saturations are at atmospheric conditions. • An extra methanol distillation can be used to remove precipitated salt which occurs with samples that contain high salinity brines. • Plugs should be drilled with air and not kerosene. Precision • Water saturation reproducibility is 3 saturation units Price/timing •