Anticollision 300 03 Jun 02

Drilling and Measurements Procedures

Standard Anticollision

Standard Anticollision Procedures Version 3.00

Proprietary Notice This information is confidential and is the trade secret property of Schlumberger. Do not use, disclose, or reproduce without prior written permission of Schlumberger. Schlumberger makes no warranties; express, implied, or statutory, with respect to the product described herein, including without limitation, any warranties of merchantability or fitness for a particular purpose.

Unpublished work © 2002 Schlumberger All rights reserved under copyright law All marks mentioned are trademarks or registered trademarks of their respective owners.

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Drilling and Measurements Procedures

Standard Anticollision

Document Information Document Type

Standard Anticollision Procedures

Software Version

Microsoft Word 2000 for Windows 2000 Anticollision - 3.00.doc

Source File

Author Author Information

Chris Chia Drilling Planning and Surveying Product Champion Sugarland Product Center 150 Gillingham Lane MD 150-2, Sugar Land, Texas 77478, USA Tel: (281) 285 7350 email [email protected]

Revision History

03-Jun-02 First Version Released to Field V-3.00

Review and approval

John McCullagh Manager – Surveying & Telemetry

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Drilling and Measurement Procedures

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Table of Contents STANDARD ANTICOLLISION PROCEDURES ..................................................................................................................... 1 1.1 1.2 1.3 1.4 1.5 1.6

Scope ........................................................................................................................................................................... 1 Application ........................................................................................................................................................... 1 Competency ........................................................................................................................................................... 1 Database Quality and Verification............................................................................................................................... 2 Anticollision Scan in Well Design File ......................................................................................................................... 2 Definitions ........................................................................................................................................................... 3 1.6.1 1.6.2 1.6.3 1.6.4 1.6.5 1.6.6 1.6.7 1.6.8 1.6.9 1.6.10 1.6.11

1.7

Standard Anticollision Procedures..................................................................................................................................3 Separation Factor.............................................................................................................................................................3 Oriented Separation Factor..............................................................................................................................................4 Center to Center Distance ...............................................................................................................................................4 Anticollision Scan Report - Local Minima .....................................................................................................................5 Allowable Deviation From Plan......................................................................................................................................5 Minimum Allowable Separation .....................................................................................................................................5 3D Least Distance ...........................................................................................................................................................6 Normal Plane ...................................................................................................................................................................6 Horizontal Plane ..............................................................................................................................................................7 Traveling Cylinder Plot...................................................................................................................................................8

Well Classification......................................................................................................................................................... 9 1.7.1 Single Well ......................................................................................................................................................................9 1.7.2 Nearby Well.....................................................................................................................................................................9

1.8

Survey Program Design ................................................................................................................................................ 9 1.8.1 General ............................................................................................................................................................................9 1.8.2 Survey Redundancy.........................................................................................................................................................9 1.8.3 Survey Program Parts....................................................................................................................................................10

1.9

Anticollision Scanning................................................................................................................................................. 10 1.9.1 1.9.2 1.9.3 1.9.4

1.10

Anticollision Rules....................................................................................................................................................... 13 1.10.1 1.10.2 1.10.3 1.10.4 1.10.5 1.10.6 1.10.7

1.11

Global Scan ...................................................................................................................................................................10 Proximity Scan ..............................................................................................................................................................11 Treatment for Sidetracks ...............................................................................................................................................11 Error Models and Dimensionality .................................................................................................................................12 General Rule..................................................................................................................................................................13 Alert Zone......................................................................................................................................................................13 Minor Risk Well............................................................................................................................................................14 Major Risk Well ............................................................................................................................................................14 Surface Hole Anticollision............................................................................................................................................14 Standard Anticollision Rules Summarized ...................................................................................................................15 Minimum Separation Rule Summarized .......................................................................................................................18

Anticollision Reporting ............................................................................................................................................... 19 1.11.1 Summary Scan Report...................................................................................................................................................19 1.11.2 Detailed Scan Report.....................................................................................................................................................19

1.12

Traveling Cylinder Plot .............................................................................................................................................. 20 1.12.1 1.12.2 1.12.3 1.12.4 1.12.5 1.12.6 1.12.7 1.12.8

1.13

General ..........................................................................................................................................................................20 Traveling Cylinder Coordinates....................................................................................................................................20 Relative Depth...............................................................................................................................................................21 Highside Referenced Traveling Cylinder Plots ............................................................................................................21 Tolerance Lines .............................................................................................................................................................21 Drawing Tolerance Lines ..............................................................................................................................................22 Use of Traveling Cylinder Plot While Drilling.............................................................................................................22 Traveling Cylinder Plot - Document Control ...............................................................................................................22

Anticollision Monitoring ............................................................................................................................................. 23 1.13.1 1.13.2 1.13.3 1.13.4 1.13.5

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Execution.......................................................................................................................................................................23 Wellsite Survey Validation ...........................................................................................................................................23 Close Approach to a Tolerance Line.............................................................................................................................24 Violation of Tolerance Lines ........................................................................................................................................24 Unexpected Collision Detection ...................................................................................................................................25

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Table of Contents (cont) 1.14

Anticollision Monitoring Program............................................................................................................................. 26 1.14.1 1.14.2 1.14.3 1.14.4 1.14.5 1.14.6

1.15

General ..........................................................................................................................................................................26 Application ....................................................................................................................................................................26 Roles and Responsibilities ............................................................................................................................................27 Surveying Procedure .....................................................................................................................................................28 Shut-in Criteria ..............................................................................................................................................................28 Poorly Surveyed Offset Wells.......................................................................................................................................29

Magnetic Interference ................................................................................................................................................. 28 1.15.1 General ................................................................................................................................................................................29 1.15.2 Changeover Between Gyro and MWD Surveys .................................................................................................................30

1.16

Appendix A – Standard Anticollision Procedures: Guidelines................................................................................ 30 1.16.1 Guideline 1 - Anticollision Scanning by Survey Program Parts ........................................................................................31 1.16.1.1 Survey Program........................................................................................................................................31 1.16.1.2 Survey Program Parts...............................................................................................................................33 1.16.1.3 Anticollision Scanning by Parts...............................................................................................................33 1.16.1.4 Interpretation of Anticollision Scan Reports ...........................................................................................34 1.16.1.5 Iteration ....................................................................................................................................................34 1.16.2 Guideline 2 - Drawing Tolerance Lines on a Traveling Cylinder Plot...............................................................................35 1.16.2.1 General .....................................................................................................................................................35 1.16.2.2 Basic Method ...........................................................................................................................................35 1.16.2.3 No-Go Circles ..........................................................................................................................................36 1.16.2.4 Tolerance Lines........................................................................................................................................37 1.16.2.5 Transferring Tolerance Lines...................................................................................................................38 1.16.2.6 Color Coding Tolerance Lines.................................................................................................................38 1.16.2.7 Traveling Cylinder Plots Versus Spider Plots .........................................................................................39

References

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Standard Anticollision Procedures 1.1

Scope

This Schlumberger Drilling and Measurements (D&M) Standard Anticollision Procedures are written in support of the D&M Well Surveying and Anticollision Policy. They cover all the surveying elements associated with the avoidance of well collisions during both the planning and execution phases of a directional well. Unlike the policy document that is open to all, the procedures are confidential as they describe methodologies that are internal and also refer to specific Schlumberger software. The target users are DEC (Drilling Engineering Center) personnel, D&M Line Management, Directional Drilling Coordinators, Directional Drillers and MWD Engineers. Drilling Engineers in the IPM (Integrated Project Management) Segment of Schlumberger, who are involved in well construction activities, may also find the document useful.

1.2

Application

The Standard Anticollision Procedures apply to all normal Schlumberger Drilling and Measurements directional well planning and execution activities.

In this context, “normal” is defined as those well

trajectories that can and do comply with the “drill ahead” criteria embodied in the anticollision rules. In the exceptional cases where such criteria cannot be complied with, an exemption process must be followed and a separate Risk Based Anticollision Procedure invoked. Strict adherence to the relevant procedures is mandatory, as the consequence of an unplanned collision can be far reaching in terms of risk to human life, damage to the environment, lost revenue to our clients and damage to our reputation.

1.3

Competency

The local sign-off authority can be a level 1 or level 2 DEC Manager, a Drilling Engineer or a Survey Specialist, who is recognized by the level 3 Area DEC Manager as being capable of acting as the local owner and custodian of these procedures and who has been granted well design sign-off privileges on this basis. These privileges can be revoked at any time, and transfer of a person who has sign-off authority to another location, does not automatically confer transfer of sign-off privileges. For the well design process, it is the responsibility of the sign-off authority in the local DEC to ensure that these procedures are followed and that all the personnel involved in planning are adequately trained and competent in their implementation. It is the responsibility of the Directional Driller to ensure that these procedures are followed during the execution process. The Directional Driller must have reviewed the well design file so as to be completely familiar with all of the requirements of the well design prior to execution. It is the responsibility of D&M Field Management to ensure the competency of the Directional Driller to follow the plan and adhere to the procedures.

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It is also the responsibility of D&M Field Management to ensure that, should a change in well trajectory be required once drilling has commenced, all anticollision planning is correctly redone and signed off by a competent authority. This last point is very important and requires that the contingency to re-plan a well at short notice due to unforeseen circumstances has been discussed in advance, and that the competent sign-off authority in such a situation, has been identified.

1.4

Database Quality and Verification

In satisfying the standard anticollision procedures, verification of the definitive survey database is a key element. It’s identification and location must be indicated in the well design file and it is incumbent on the DEC’s to take all reasonable precautions to ensure that it is complete and accurate. Each borehole, sidetrack, fish and cased or abandoned well must have a separate concatenated top to bottom definitive survey that uniquely describes the wellpath position from start to finish. In the ideal case, the DEC would be performing all Survey Management services for the client and so would have total control and be directly responsible for database quality. In many circumstances however, this will not be the case and the upkeep of the database will be the responsibility of the client or a third party contractor. Such a database may contain legacy data acquired over a period of several years by many different service providers and its quality may be suspect. When D&M takes over a directional drilling contract from a competitor, it is vitally important to audit the information received on existing wells and verify that it is complete and accurate. More details on this are contained in the database and data handling procedure. If this verification cannot be made for whatever reason, then Schlumberger needs to be indemnified in writing by the client concerned. For any database other than the definitive one to be used for anticollision scanning, a clear and auditable process must exist and must have been adhered to ensure that it is identical to the definitive database, even though it may only contain a subset of the data appropriate for the drilling area. During drilling, the definitive survey database must contain the most up to date as-drilled surveys to remain valid, until such time as it is updated with the final definitive survey.

1.5

Anticollision Scan in Well Design File

It is mandatory that the well design file, as specified in the Well Design Procedures, contains a section that lists adequate anticollision scanning information to demonstrate that the well proximity situation is known at all stages of drilling, and that the clearances between all nearby wells are sufficient to avoid any potential unplanned collision. The Global Scan (see section 1.9.1) must always be completed, and a summary scan report (from the proximity scan) must be included in the well design file which lists all wells identified by the Global Scan.

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1.6

Standard Anticollision

Definitions

Particular regard must be paid to the definitions below, as many of the terms listed, although commonly used throughout the industry, do not always mean the same thing. 1.6.1

Standard Anticollision Procedures

Standard Anticollision Procedures specifically refer to Schlumberger Drilling and Measurement’s treatment of the well proximity problem using separation factors, oriented separation factors, allowable deviation from plan and clearance distances as defined below. These procedures are compliant with Schlumberger Drilling Office software.

The use of Oriented Separation Factors requires the use of

approved Drilling Office software versions from V-3.0 onwards. 1.6.2

Separation Factor

The traditional definition of separation factor used by Schlumberger, is defined as the ratio of the centerto-center distance between wells and the sum of the radii (major semi-axis) of the ellipsoids of uncertainty, between the subject and offset wells being scanned, with allowance being made for the hole diameters. Subject Well

Center-to-Center Distance Allowance for Subject Well and Offset Well Hole Radii

Offset Well

Major Semi-Axis + Hole Radii Projected in a Sphere

Ellipsoids of Uncertainty

fig 1: Separation Factor = 1 (Projected Spheres are Touching)

Well collision risk has traditionally been managed by considering the clearance between spheres that contain the Ellipsoids of Uncertainty (EOU) as shown above. However, using this simplistic approach, it is possible to have two collision scenario’s with the same Separation Factor, but which have very different probabilities of collision because the orientation and shape of the EOU’s are not accounted for. This can result in overly conservative well planning, which can be unnecessarily restrictive. Oriented Separation Factor (OSF) is a new method of Safety Factor definition that takes into account the geometry of the EOU’s so that all scenarios with the same Safety Factor have the same probability of collision.

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The adoption of this new Oriented Separation Factor (OSF) by Schlumberger Drilling and Measurements for all future standard anticollision procedures is one of the major advances introduced in these procedures. Older versions of the anticollision scanning software, which can produce Separation Factor based results may of course continue to be used to satisfy these procedures, however results in some circumstances will be more conservative. 1.6.3

Oriented Separation Factor

The Oriented Separation Factor (OSF) is defined as the ratio of the center to center separation between wells and the ellipsoid of uncertainty separation, taking into account a fixed probability of collision as representing a separation factor of one, and treating the separation factor problem as described by engineering report 2001-016 Orientation Sensitive Risk Analysis1. This method compensates for the variation in probability of collision when the separation factor is equal to one. An allowance is made for the hole diameters for the subject well and each of the offset wells being scanned.

Subject W ell

Allowance for Subject W ell and Offset W ell Hole Radii Line of Probability Analysis

Offset W ell Center-to-Center Distance Ellipsoids of Uncertainty fig 2: Oriented Separation Factor = 1 (Ellipses are Touching for a Fixed Probability of Collision)

The mathematical derivation for Oriented Separation Factor is outside the scope of these procedures, however the capability to plan wells using OSF separation criteria, is contained in all Drilling Office software releases from v3.0 onwards. Obviously if a well is drillable using normal Separation Factor minima, it would also be drillable using Oriented Separation Factor minima, however the converse might not be true. 1.6.4

Center to Center Distance

The center-to-center distance is defined as the distance between the subject well (well being planned) and the offset well being scanned when the scanning method used is either 3D least distance or normal plane. Other scanning methods (such as horizontal plane) may be used to produce relative geometrical calculations but will not produce a measurement acceptable for use with anticollision calculations.

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Drilling and Measurement Procedures 1.6.5

Standard Anticollision

Anticollision Summary Report – Local Minima

The local minima indicated in the anticollision summary scan report from Drilling Office are defined as the points of inflection of the approach of the offset well. These are all of the closest points of approach for each of center-to-center distance, ellipse of uncertainty separation and separation factor, and are determined regardless of any scanning frequency chosen by the user. A box appearing around the result indicates the specific parameter that has triggered the reporting of the local minima. 1.6.6

Allowable Deviation From Plan

The allowable deviation from plan (ADP) is defined as the “drilling tunnel” which is created as a result of the avoidance of any close approach violation identified by the use of oriented separation factors. It is therefore represented as the radial distance from the plan at any point, to which the driller may be allowed to depart from the plan during the drilling process for the purposes of drilling efficiency, without any violation of the “drill ahead” anticollision rules. 1.6.7

Minimum Allowable Separation

The minimum allowable separation (MAS) is defined as the minimum center-to-center distance between the subject and offset well that is allowable without any violation of the ‘drill ahead’ anticollision rules. Therefore the allowable deviation from plan and the minimum allowable separation should sum to give the actual center-to-center distance observed under all normal drilling circumstances, when allowance has been made for the respective hole diameters. Offset Well

Minimum Allowable Separation (MAS) for OSF = 1.5 ((R1+R2)*1.5)

R1

Center-to-Center Distance (CtC) Subject Well

R2

Separation Factor Allowance ((R1+R2)/2)

Allowable Deviation From Plan (ADP) (CtC - MAS)

fig 3: MAS (For OSF = 1.5) = ((R1+R2)*1.5) or (CtC - ADP)

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1.6.8

Standard Anticollision

3D Least Distance

The 3D least distance method of proximity scanning calculates the nearest distance to each offset well by stepping down the subject well at specified intervals. At each step this analysis scans the offset well to determine a plane that is normal to the offset well survey, and which intersects the subject well at the interval point. Mathematically, this distance is the shortest (least) distance between the subject well and the offset well from each of the respective subject well scanning points. Subject Well

Offset Well

Scanning Points

3D Least Distance fig 4: 3D Least Distance Scanning Method

1.6.9

Normal Plane

The normal plane method of proximity calculation steps down each offset well at the specified intervals. This stepping down of each offset well is done to ensure that the proximity of the entire offset well is analyzed, and to ensure the scanning of any potential perpendicularly approaching wellbore. At each step down the offset well this method scans the subject well to determine where a plane normal to the subject well intersects the offset well at the respective scanning point.

Offset Well

Subject Well

(3D Least Distance

Scanning Points

Normal Plane Distance fig 5: Normal Plane Scanning Method

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It should be noted that both scanning methods, 3D least distance and normal plane, suffer from different but distinct boundary condition weaknesses, and therefore both methods must be used during the anticollision scanning process in order to fully investigate the potential for collision. As a result of the possibility of relative scale distortion, the normal plane method is the preferred method of anticollision scanning for the purposes of producing the Traveling Cylinder plot (see section 1.6.11). 1.6.10

Horizontal Plane

The horizontal plane method of proximity scanning steps horizontally down the subject well at specified intervals. This proximity scanning method is not to be used as an anticollision tool and is mentioned here for the sake of completeness only.

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Drilling and Measurement Procedures 1.6.11

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Traveling Cylinder Plot

A traveling cylinder plot is a polar plot centered on the subject survey. This plot displays the intersections of the offset surveys with the selected projection plane. The Schlumberger standard for the traveling cylinder plot will be Normal Plane scanning method and North Referenced orientation. No-go circles or tolerance lines may be used on the traveling cylinder plot to ensure compliance with these procedures. Their preparation and use will be discussed in more detail later in this document (see section 1.16.1). NORTH Offset Well

40

Subject Well

30

Travelling Cylinder Plot View

20 10 Offset Well

Subject Well

Plot Rings are projected normal to subject well at all times

Travelling Cylinder Plot View

Plot is Projected in Normal Plane to Preserve Relative Scale

fig 6: Travelling Cylinder Plot

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1.7 1.7.1

Standard Anticollision

Well Classification Single Well

A well is considered to be a single well when its surface location is at least 24,000 meters (80,000 feet) distant from the surface location of any other well. This distance is based on the fact that the global scan currently conducted by all commercial anticollision software is limited to scanning the proximity of the offset wellheads (ie. surface locations of offset wells) only. In the worst scenario possible with today’s ERD technology the upper limit on surface well separation at which a well collision could still potentially take place, is 24,000m, which represents two 12,000m horizontal wells drilled directly towards each other. Until such time as improvements in software and computer hardware technology make it possible to do global scans using down hole survey data and not just surface locations, this global scan threshold must be observed for every anticollision scan. Evidence of the absence of any other nearby wells must be included in the well design file, and before conferring the status of “single well” in any well planning exercise. 1.7.2

Nearby Well

Any well that is not a single well is a nearby well.

1.8 1.8.1

Survey Program Design General

The fundamental purpose of the survey program is to ensure that sufficient quality surveying is carried out in order to achieve the target at the minimum cost while avoiding unplanned collisions. In doing so, it is also highly desirable to provide sufficient redundancy of data to ensure that each dataset included in the final well trajectory has been independently verified.

Many operators make this a mandatory

requirement, as it is one of the best methods for the early detection of gross errors due to input of wrong declination or grid correction etc. This procedure will deal with the aspects of survey program design specifically related to anticollision (see Appendix A), with target requirements and survey quality having been defined and satisfied in other procedures. 1.8.2

Survey Redundancy

As mentioned above, for all well designs being executed, it is strongly recommended that no one individual survey instrument shall be used to define the definitive well trajectory in any single hole section without its performance having been independently confirmed by another survey instrument. In the case of a magnetic survey tool this may be done by comparison with overlapping data from any other survey tool of equal or greater accuracy or alternatively, by the application of an approved multi-station analysis

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Standard Anticollision

In the case of a gyroscopic survey tool this may be done by comparison with either

confirmatory magnetic survey data, or sufficient overlapping data from another gyroscopic survey tool. Every effort must be made to persuade those clients who remain unconvinced, that all our experience, especially in a crowded subsurface environment, strongly points us in the direction of having survey redundancy in our survey program design. 1.8.3

Survey Program Parts

A survey program will consist of one or more parts, identified by the various planned drilling stages of the well (see Appendix A). Each of these parts must be the subject of a separate anticollision scan. The anticollision scan results for each individual survey program part must be satisfied independently of each of the other parts. Details of the survey program to be used, and any contingency planned surveys are to be included in the well design file. In addition, details of each of the survey program parts used for the scan are also to be clearly indicated. For single wells it may be possible that the survey program consists of a single survey program part consisting of one or more planned survey instruments. For multi-well installations, or when dealing with the proximity of other nearby wells, a survey program consisting of multiple program parts may be required. Generally, a new survey program part can be identified when the position accuracy of any previously surveyed section of the well improves as a result of having now been resurveyed by a more accurate survey tool occurring at a later stage in the survey program.

1.9 1.9.1

Anticollision Scanning Global Scan

The global scan is the initial scan made in the anticollision planning process in order to scan through the entire database project for all nearby wells that fall within the user specified scan radius. It is strictly made on the surface location of the wells under consideration. Subsurface survey data is not considered during this first step. As indicated previously, the scan radius must be set to 24,000 meters (80,000 feet) in order to identify all nearby wells, or to establish the single well status of the subject well. This global scan is required to identify the wells that are required to be included into the next step of the anticollision planning process, the proximity scan.

Careful data management, as specified by the survey

management procedures, is required in order to ensure the effectiveness and completeness of the global scan. The nature of the global scan makes it imperative that fields are stored in a logical order in the database projects. Nearby wells that have been stored in different database projects cannot be scanned against each other (see Survey Management Procedures).

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Proximity Scan

On completion of the global scan, a proximity scan must be performed on all the wells that have been identified as being “nearby wells”. The proximity scan uses the subsurface survey data associated with each nearby well in order to calculate the distance from each well to the subject well at every point along its length. In addition to the surveys of existing wells, the definitive database may also contain dummy trajectories with dummy uncertainties for future planned wells in order to protect empty slots. There are various filtering options available in the software and the Well Design File must state whether the proximity scan includes or excludes the definitive plans. This statement will be acceptable where the sign-off authority has evidence that the procedures and principles of good survey management have been observed, and that use of the filtering tool is valid. In all other cases, all surveys and plans must be scanned. Particular care must be taken that all recently completed nearby wells are included in the scan as required. There are two possible outputs from the proximity scan, the summary scan report for all wells outside of the alert zone (OSF > 5.0) and the detailed scan report for all nearby wells which fall within the alert zone (OSF < 5.0). This will be explained in more detail under section 1.11. 1.9.3

Treatment of Sidetracks

When the subject well is a sidetrack from an existing parent well it is possible for the sidetrack to exist in the database in one of two formats. In the first case it may simply be tied onto the parent well at some subsurface depth, whereby it shares a common set of position data above the tie-on point with the parent well or, in the second case it may exist in the database as an independent complete well which extends to surface with the shared surveys being copied over to the sidetrack wellpath. Despite the fact that the second case, where each sidetrack is treated as a complete well is the preferred method, it may not be possible for the software to recognize that part of the parent wellpath is shared, particularly if the transferred surveys were transferred as interpolated points at evenly spaced intervals.

When an

anticollision scan against the offset parent well, with a sidetrack (often a sidetrack plan) as the subject well is required, then it will be necessary to make a temporary copy of the definitive plan or survey and apply the ZERO tool error model on one or both of the temporary wells (depending on whether the sidetrack exists as case one or as case two above) from surface down to the sidetrack tie-on point so that the anticollision scan can effectively examine the proximity between the sidetrack and parent. Final definitive surveys must not have their survey tool codes changed in any way, and must always be copied if this facility is required. Where this procedure is followed, it must be clearly documented in the well design file, and it is recommended that the sign-off authority has in place a method to positively confirm that the original survey tool codes have been reset on the parent and sidetrack wells respectively. Future versions of anticollision software may be able to conduct this procedure automatically.

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Drilling and Measurement Procedures 1.9.4

Standard Anticollision

Error Models and Dimensionality (1D, 2D or 3D)

The default standard tool error model type for use in all Schlumberger Drilling and Measurements anticollision calculations will be the Schlumberger version of the Industry Steering Committee for Wellbore Survey Accuracy2 model (SLB-ISCWSA). However, the older Topographic model may also continue to be used. Survey tool error models within the industry are in a continuous state of evolution to take into account advances in well surveying technology, improved sensor accuracy and attempts at standardization. Because some clients have a specific preference based on their own experience, there is provision within the Schlumberger planning software to make use of error model types other than those listed above however these are not approved for use in anticollision calculations without going through the exemption process. Where a client or operation requires the use of a non-Schlumberger software package or error model type for use in any anticollision calculations, then that client is expected to provide the alternative software application and appropriate training if required. In any case, anticollision calculations must be duplicated using approved Schlumberger software except where a client or case specific Area level exemption has been granted by the Area DEC Manager. It is imperative that the minimum requirements of this procedure are satisfied regardless of which system used for the actual calculations. The default dimensionality for all anticollision calculations will always be three-dimensional, and at an uncertainty level of 95% (2.79 sigma). A sign-off authority may grant exemption from the uncertainty requirement subject to the fulfillment of the appropriate exemption procedure.

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1.10 Anticollision Rules 1.10.1

General

The function of the alert zone is to allow the user to clearly identify from the summary scan, which wells are required to be the subject of a further detailed scan report. In order to do this the separation factor alert type is used in the Drilling Office Close Approach program at the 95% (2.79 sigma) confidence level and the alert zone threshold set at an oriented separation factor (OSF) of 5.0. At the same time, the minor risk oriented separation factor threshold should be set to 1.5, and the major risk oriented separation factor threshold to 1.0. These are the program defaults in any case. Any well identified that violates the major risk separation factor (OSF < 1.0) will not be considered for exemption without an exhaustive risk assessment being conducted and only then in very exceptional circumstances.

In this case the

exemption to proceed at the increased risk must be signed off by an appropriate Schlumberger Area DEC Manager and the Area Business Manager. In accordance with policy, it is a fundamental requirement that all nearby wells exceed the minor risk separation factor of 1.5 and in addition, also satisfy the surface hole anticollision requirements listed in paragraph 1.10.5. If they do not, the wellbore trajectory must be re-planned to rectify the problem, or an exemption obtained and the Risk Based Anticollision Procedure invoked as necessary. During planning, the standard anticollision rules will be applied to any planned well being scanned against any nearby well, and during execution these rules will be applied to the projected position ahead of the bit by at least one survey interval as stipulated in the survey program (see appendix A). Any Schlumberger directional well design that does not fully satisfy this procedure at the planning stage must be revised or if determined at the execution stage, drilling must immediately stop until a review is conducted. 1.10.2

Alert Zone

The Alert Zone, which is triggered when any offset well falls between OSF = 5.0 and OSF = 1.5, is designed for use as an alert tool which allows the user to quickly identify which wells are in closest proximity to the planned (subject) well, and therefore most likely to be the cause of a proximity issue during the execution of the plan. The purpose of the Alert Zone is therefore only to provide an indication of the wells to be included in the “detailed scan” report. For all wells falling outside of the Alert Zone, evidence of having scanned these wells in the form of the “summary scan” report is sufficient for the purposes of this procedure.

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Drilling and Measurement Procedures 1.10.3

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Minor Risk Well

A minor risk well is an offset well which falls within the proximity filter of OSF = 1.5, but does not violate the major risk OSF threshold of 1.0 (i.e. the OSF falls between 1.5 and 1.0). This OSF threshold of 1.5 represents the “drill ahead” separation threshold as per policy. Wells which fall below the OSF threshold of 1.5 will be required to be included in a detailed scan report, in addition to being subject to the anticollision rules below, and may require exemption for violating the minor risk threshold if the well trajectory cannot be replanned. 1.10.4

Major Risk Well

A major risk well is an offset well which falls within the proximity filter of OSF = 1.0. This threshold represents the stop drilling condition, and ordinarily Schlumberger will not proceed with drilling until major risk wells have been dealt with either by replanning to increase clearance beyond the minor risk threshold, or replanning to attain minor risk status and subjecting the minor risk well to the risk-based exemption process. 1.10.5

Surface Hole Anticollision

Surface hole collision risk is the most common proximity problem, particularly where the slot separation provides minimal clearance from other wells drilled from the same template. It is well known that standard separation factor anticollision rules are technically weak in this scenario because of the nature of the separation factor ratio calculation, and the potential for rapid change in the propagation of survey errors at or near surface. Surveying frequency can also have a large effect on the surface hole anticollision problem, particularly when steerable drilling assemblies are used for kicking off. It is therefore crucial that the survey frequency requirements for surface hole as specified in the survey program are adhered to. The Well Reference Point is defined as the last known point of departure for any well. For land wells this would be the wellhead location at ground level, and for offshore wells, this is typically the wellhead location at seabed. For wells sharing the same physical drilling template or pad, the slot separations are known exactly from engineering drawings, and therefore because the lateral relative survey errors at this point are zero, the Allowable Deviation from Plan (ADP) is effectively the side wall to side wall distance between well conductors. For wells which do not share the same template the relative lateral position uncertainty between wells is driven by the position fixing accuracy of the survey system used to define the wellhead location. Modern day systems such as Global Positioning Systems (GPS) which are very accurate, may be able to reduce this surface uncertainty to within a few meters depending on geographical location, but more traditional systems are likely to be much less accurate. Therefore any surface hole rule for wells not sharing the same template must also make allowance for variations in surface position accuracy.

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Subject to the survey frequency requirements being satisfied, the additional mandatory surface hole anticollision criteria is that a minimum separation of no less than 80% of the ADP at the WRP for wells sharing the same physical drilling template or pad be maintained, and a 10m minimum separation in all other cases. In situations where multiple wells are to be drilled from the same slot or caisson, then the well is effectively being commenced from a collision situation, and the surface hole will require a detailed close proximity anticollision monitoring plan (see section 1.14) to be executed at least until the clearance reaches 10m and the anticollision rules are satisfied and projected surveys clearly indicate divergence from other sibling wells. Therefore, for all wells to satisfy the standard anticollision procedure for the drillahead condition, the following summary of these requirements must be met: OSF ≥ 1.5 in addition to Minimum Separation not less than: (80% of ADP at WRP OR 10m) 1.10.6

Standard Anticollision Rules Summarized: •

OSF > 5 in addition to Minimum Separation > 80% ADP (or 10m) Outside of Alert Zone, Summary Scan Report only required, Drill Ahead. S ubject W ell

O -E O U 1

O riented E O U Separation > 4*(O -EO U 1 + O -EO U 2 ) C enter-to-C enter D istance

O -E O U 2

O ffset W ell

fig 7: O S F >5 in addition to M inim um S eparation > 80% A D P (or 10m )

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•

Standard Anticollision

OSF >1.5 in addition to Minimum Separation > 80% ADP (or 10m) Inside Alert Zone, Detailed Scan Report required, Drill Ahead

Subject Well

O-EOU1

O- EOU Separation > 0.5*(O-EOU1+ O-EOU2) Center-to-Center Distance

O-EOU2 Offset Well

fig 8: ·OSF >1.5 in addition to Minimum Separation > 80% ADP (or 10m)

•

1.5 > OSF >1 in addition to Minimum Separation > 80% ADP (or 10m) Minor Risk Well, shut in interfering well and resurvey subject well with a more accurate survey tool to increase OSF above 1.5, or invoke Risk Based Anticollision process and plan to drill ahead with Line Manager approval and Client written exemption.

Subject Well

O-EOU1

O-EOU2

O- EOU Separation < 0.5*(O-EOU1+ O-EOU2)

Offset Well

fig 9: 1.5 > OSF >1 + Minimum Separation > 80% ADP (or 10m)

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•

Standard Anticollision

1 > OSF or Minimum Separation < 80% ADP (or 10m) Major Risk Well – STOP DRILLING, Replan trajectory and/or survey program (during planning phase) to increase OSF above 1.5, Plug back and redrill (during execution phase) to increase OSF above 1.5, or very exceptionally, invoke Risk Based Anticollision process and plan to drill ahead with Area approval and Client written exemption.

Subject Well

Stop Drilling Condition for Projection Ahead of Bit

Offset Well

EOU's Have Overlapped

fig 10: 1 > OSF or Minimum Separation < 80% ADP (or 10m)

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Drilling and Measurement Procedures 1.10.7

Standard Anticollision

Minimum Separation Rule Summarized M inim um S eparation = 80% *(C tC - (R 2 + R 2)) O ffset W ell

Subject W ell W ell R eference P oint (W RP )

SeaBed

R2

R1

AD P

At the W RP the relative lateral survey errors are zero, and so the A DP is equal to the sidewall to side-wall distance

Center-to-C enter (C tC )

fig 11: W ells Sharing the S am e Tem plate or Pad : M inim um S eparation = 80% of A DP at W RP

O ffse t W e ll

S u b je ct W e ll

W e ll R e fe re n ce P o in t (W R P )

SeaBed R2

M in im u m S e p a ra tio n = 1 0 m

R1

C e n te r-to -C e n te r fig1 2 : W e lls N o t S h a rin g th e S a m e T e m p la te o r P a d : M in im u m S e p a ra tio n = 1 0 m

Anticollision Monitoring Plan (see Section 1.14) Req'd Until Separation =10m and OSF>1.5 Multi-well Slot or Caisson Well Reference Point (WRP) SeaBed

W2

W1

Separation Distance

fig13: Wells Sharing the Same Slot

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1.11 Anticollision Reporting 1.11.1

Summary Scan Report

The anticollision summary scan report is obtained by manually running the proximity scan close approach software for each survey program part. It is required to be listed in the well design file. This summary report details each of the center-to-center, ellipse of uncertainty and separation factor minima, as well as the separation factor alert zone and the anticollision rule violation status of each well scanned. The purpose of the summary report is to demonstrate that all wells exceed the alert zone separation criteria (OSF = 5), and therefore the anticollision scan procedure is complete for these wells. In the case where the proximity of any nearby well triggers any of the alert zones, or crosses the minor or major risk thresholds, a detailed anticollision scan report as indicated below, must be made on these wells. 1.11.2

Detailed Scan Report

The detailed scan report is obtained by manually running the close approach software for each object well to be scanned for each survey program part, and is required to be listed in the well design file for every well that fails to exceed the alert zone separation criteria. The detailed report contains sufficient information to closely examine the proximity condition of nearby wells that have failed the alert zone filter. The detailed scan report is also used to examine cases where the major alert or minor alert status has been violated, in order to offer the well design team a geometrical statement from which to begin any redesign work that might be required to eventually satisfy these procedures. A secondary purpose of this report is to indicate where any restrictions in the allowable deviation from plan (ADP) exist, even when the proximity requirements have been satisfied, in order to review and optimize for any drilling efficiency issues that may result.

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1.12 Traveling Cylinder Plot 1.12.1

General

The traveling cylinder (TC) plot is a polar plot where the center of the plot depicts the relative position of the subject well plan used to create the plot.

Provided the wellpath is drilled exactly on plan, the

observed position will always be at the plot center. This is not realistic however, and so the traveling cylinder coordinates are usually plotted on the TC plot in real time to monitor the observed position relative to the plan. In this way, the traveling cylinder is a graphical description of the anticollision scan, and all of the relevant information on the plot can be obtained from the detailed scan report. At least one wall-chart sized traveling cylinder plot is a requirement to be sent to the wellsite, and for inclusion in the well design file, for every well design in which any nearby well, nearby well design or slot bin exists. In many cases, particularly in high well density areas, it may also be desirable to use more than one traveling cylinder plot; each one appropriately scaled and detailed for each hole section.

The

Schlumberger standard for the traveling cylinder plot will be Normal Plane scanning method and North Referenced orientation. In some cases it may also be desirable to produce a wall-chart sized spider plot. The spider plot is not a substitute for the traveling cylinder plot, and cannot be accepted in its place in the well design file. The sign-off authority will have the discretionary power to require additional traveling cylinder plots, and will advise on the most appropriate scale. 1.12.2

Traveling Cylinder Coordinates

Two coordinates can define any point on the polar traveling cylinder plot. The radial distance is the distance from the center of the plot (when plotting the position of any offset well, this represents the center to center distance for a given measured depth on the subject well), and the traveling cylinder azimuth, is the angular coordinate measured relative to the north reference. The angular coordinate represents the sum of the angle clockwise from the well design highside and the well design azimuth at a given depth. These coordinates can be obtained from the detailed scan report of the anticollision scan reporting software and from the anticollision reporting panel in the DDToolbox software. Although both methods can be used in real time at the wellsite, the use of the close approach software is the primary planning tool, and the use of the DDToolbox software at the wellsite is the primary execution tool.

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Drilling and Measurement Procedures 1.12.3

Standard Anticollision

Relative Depth

Every object well point plotted on the traveling cylinder must have an associated subject well relative measured depth for it to provide useful information. This is the surveyed depth along the subject well design wellpath or in other words, the progress made against the plan. The relative depth for comparison will generally be the measured depth at that point as a result of using the normal plane scanning method. This is not the case when other scanning methods (3D least distance or horizontal plane) are used for this purpose, and the true scale distortions that may arise as a result of using other scanning methods make them unsuitable for use with the traveling cylinder plot. 1.12.4

Highside Referenced Traveling Cylinder Plots

The use of Highside referenced traveling cylinder plots is not approved for general anticollision. The reason for this is that the predominant usage requirement for traveling cylinder plots is in the low angle, high well density area, usually at or near surface. In these circumstances, where the subject well is at or near low angle, the Highside reference can ‘flip’ around the subject well dramatically as a result of small changes in orientation. This can have the visual effect of ‘spinning’ the offset wells around the traveling cylinder artificially, and does not provide any practical usefulness in this scenario. Although it could be argued that for wells (e.g. sidetracks) which kick-off quickly, or begin a higher angle, the Highside reference can be a useful traveling cylinder plot, practical experience (of near miss events) suggests that the risk for misinterpretation of the plot and mistakes is too high to support it’s use. The preferred method therefore, is to use north referenced traveling cylinder plots, upon which the Highside azimuth may be indicated if desired. 1.12.5

Tolerance Lines

The main advantages of traveling cylinder plots over any other type of graphical display are their ability to clearly and accurately display the drilling tolerances or “drilling tunnel”. For any point on a nearby well that is displayed on the traveling cylinder plot, a line may be drawn around that point which represents the minimum distance from that point to which the well being drilled can approach without violating the anticollision rule in force. This line is generally known as a no-go zone, and the distance from the center of the plot to the edge of the no-go zone represents the allowable deviation from plan. The no-go zone therefore, is a combination of the separation factor, position uncertainty and hole radii of both the subject and the offset well in question at that depth. The minimum approach distance defined by the no-go zone will vary based on the relative orientations of the subject and offset wells, and combinations of no-go zones for various wells at common depths relative to our planned well may be joined together to encompass a depth range specific area called a tolerance line.

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Drilling and Measurement Procedures 1.12.6

Standard Anticollision

Drawing Tolerance Lines

Drawing these tolerance lines around every offset well at every point would result in plots that display several nearby wells quickly becoming unreadable. It is therefore desirable to draw the tolerance lines such that they summarize the information given by a number of no-go zones for different nearby wells at a common depth. This can be done with the resulting tolerance lines color-coded and styled for a specific depth, but must be done in such a way as to prevent the inadvertent violation of the minimum separation required by the anticollision rules at any depth. An example of how this might be done is given in the appended guidelines to this procedure, and the sign-off authority is expected to be able to advise accordingly. 1.12.7

Use of Traveling Cylinder Plot While Drilling

The major premise of the traveling cylinder plot, and all of the anticollision calculations supporting it are that they are valid for a given survey program, which must be executed accordingly to validate the plan. Major changes (such as the removal of a survey tool run, or a required survey interval being significantly truncated), will not be made to the survey program without a review of all anticollision calculations, and regeneration of the traveling cylinder plot. Even minor changes such as start and end depths of survey intervals for different survey types may have a significant impact on anticollision and should also be validated accordingly. Any survey frequency requirement stipulated by the survey program must be observed at all times, and the first response to a failed survey, or failed survey program part must be to resurvey the section in order to maintain the integrity of the plan and the anticollision calculations. 1.12.8

Traveling Cylinder Plot – Document Control

Each traveling cylinder plot must be verified and signed-off by the appropriate sign-off authority and the line manager prior to it’s release for client sign-off. A process must be in place to control the version number, location and method of recall for all copies of all drilling plots as described by the Well Design Procedures. All traveling cylinder plots required and stipulated in the well design file, must be received at the rigsite prior to drilling each section. A complete set of the current authorized plots for use at the wellsite must be held at the DEC, and/or by the D&M Drilling Engineer in the client office, for use and reference at any time.

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1.13 Anticollision Monitoring 1.13.1

Execution

The active and careful monitoring of the position of the well being drilled compared to the well design plan and its proximity to nearby wells, is critical to the avoidance of any unplanned collision. Drilling performance and efficiency with regard to allowable deviation from plan must always be secondary to collision avoidance. The scope for the wellsite team to alter the well design based on operational circumstances must either be clearly defined and contingency planned, or not conducted without documentation and the authorization of the sign-off authority. Any instructions sent to the wellsite team as part of the well design information, or as any subsequent clarification, must be clear on which instructions are mandatory and which are discretionary or advisory in nature. 1.13.2

Wellsite Survey Validation

Throughout the full quality control life cycle of a survey, the survey tools used must be calibration checked before and after the survey run. This often takes several days to complete, particularly if the tools have to return to the base for this to be done. Therefore, there are several levels of survey validation that are required during this life cycle, one level of which must be at the wellsite at the time of the survey. Any survey validation conducted at the wellsite must be sufficient to enable a decision to be made about survey tool change-out or resurvey to be made. When validating surveys during any well proximity phase of the operation certain mandatory restrictions apply. Tolerance lines are not to be crossed, and each survey contractor’s own tool specific procedures and quality control procedures for the tools in use, must be strictly adhered to.

The Survey Specialist (or the sign-off authority) may

recommend a suitable contingency for any failed survey program part, and will stipulate the replanning, or revision of the anticollision calculations and the well design file, if required, as part of that recommendation.

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Drilling and Measurement Procedures 1.13.3

Standard Anticollision

Close Approach to a Tolerance Line

A close approach to a tolerance line occurs when the oriented separation factor at a position at least one survey interval projected ahead of the bit reduces sufficiently to threaten the minor risk tolerance line threshold (OSF = 1.5). In addition, it is good practice that a projection ahead of the bit by at least two drilling stands is also monitored. Where any projection ahead of the bit will result in a close approach to a tolerance line the interfering well should be closed in, an attempt must be made to steer away, or the well must be resurveyed at that point with a more accurate survey tool to increase the oriented separation factor sufficiently to allow drilling to continue. If neither of these actions is successful and the point is reached where the projection ahead of the bit crosses the tolerance line, then it is mandatory that drilling must cease until the situation and plan forward have been fully reviewed by the office based drilling engineering team. Rig site personnel must not unilaterally cross any tolerance line within the depth interval to which it applies under any circumstances. 1.13.4

Violation of Tolerance Lines

The team back at the office that has responsibility for the well will review the tolerance line close approach to determine whether the tolerance line can be relaxed without violating any no-go zones (e.g. if the line was drawn to smoothly join together two no-go zones), or in case the tolerance line is protecting another planned well (or wells), and these could be replanned at a later date. Where this proves to be possible, the team must update the anticollision plots and/or redo the anticollision calculations and communicate them to the rigsite. In some cases this may be possible by email or fax for minor changes and where the rigsite has the capability to produce revised plots. If, after review at the office, the projection ahead of the bit by one survey interval approaches a major risk (OSF = 1), the subject well must be plugged back, cemented and redrilled before violating the major risk threshold. Alternatively, drilling must cease pending the well design being replanned having invoked the Risk Based Anticollision Procedure which will require a risk assessment and written exemption by both the Client and Schlumberger management. It is important that the Directional Driller remains in control of the situation at all times and is not afraid to exercise his/her authority to stop the drilling process immediately if the “drill ahead” criteria are violated.

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Drilling and Measurement Procedures 1.13.5

Standard Anticollision

Unexpected Collision Detection

Experience has shown that despite best efforts at avoidance, an unexpected well collision can occur, particularly where the legacy survey data is incomplete or inaccurate. Obviously, there is a higher risk of an unexpected collision when traversing areas of high well density. Extra vigilance is required to look for indications that this may have occurred including rough, erratic or high torque drilling, especially where drilling is expected to be smooth or a sudden unexpected change in penetration rate, especially where the field conditions are well known. Any unexpected cuttings returns, unexpected magnetic interference of the MWD surveys and vibrations detected at the wellhead of a nearby well may all give prior warning before the drilling mud is lost or produced hydrocarbons from the nearby well appear at surface. Note that it has been known for the bit to pass or glance off a nearby well, while a stabilizer or even drillpipe hard-banding some distance back has penetrated the casing. Extra diligence is required at least until the top stabilizer has passed the close approach point of a nearby well and not just the bit. The opinion expressed by some in the industry stating that when the bit is approaching an existing well at a shallow (glancing) angle, it will always “bounce off” has been proven by hard experience to be a myth and has resulted is some very expensive remedial work.

1.14 Anticollision Monitoring Program 1.14.1

General

Every well design file must reference the client or project specific anticollision monitoring program for which the well design has been provided. Where such a program does not exist, or it’s existence is in doubt, then the well design file shall include a detailed anticollision monitoring program. This program shall include details of the conditions and circumstances under which the program shall be executed and anticollision monitoring carried out, with the minimum requirement being described below. It shall also include roles and responsibilities for each of the key wellsite and onshore personnel involved in anticollision monitoring, and the manning and resource requirements to ensure successful execution of the program. 1.14.2

Application

Active anticollision monitoring and close proximity measures should be implemented whenever there is an increased risk of collision with nearby wells. This would normally be required when nearby wells are subject to any anticollision shut-in requirements, when there is the possibility or potential for the OSF to reduce to below 1.5 and trigger the minor risk proximity or whenever any risk based anticollision process is being executed under an exemption. Each project should set it’s own additional conditions and criteria to take into account local or governmental regulations, client policies and the known effects of lithology etc on the behaviour of drilling assemblies in use.

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Drilling and Measurement Procedures 1.14.3 •

Standard Anticollision

Roles and Responsibilities

Directional Reference Corrections Each of the Directional Driller, Survey Engineers and Drilling Engineers is individually responsible for taking all precautions for checking and double-checking the survey references and reference correction (i.e. Grid Convergence and Declination) as appropriate, which are in use for any aspect of this procedure. A useful guide for this check is the signed-off drilling plot provided to the wellsite, which should clearly display this information on the plot header.

•

Directional Driller Maintain and update the local definitive survey database (which may be a copy of or a subset of the main definitive survey database as defined by Section 1.4) and conduct anticollision calculations, including Oriented Separation Factors (OSF), at all times as required using Schlumberger approved software as new surveys become available and during the execution of the survey program. Confirm onsite survey quality control requirements, corrections, reference data and their use by survey engineers, and confirm that independent survey and position calculations performed by surveyors correlate with the local definitive database.

•

Gyro / EMS Survey Engineer Maintain a complete record of all raw and computed survey data, and independently confirm all reference corrections and positional calculation data for correlation against the local definitive database. At the end of each multishot survey, or at least daily, report to the Directional Driller a hardcopy of all surveys and surveyed position calculations to date.

•

MWD Survey Engineer Maintain a complete record of all raw and computed survey data, and independently confirm all reference corrections and positional calculation data for correlation against the local definitive database. At the end of each section and at least daily, report to the Directional Driller a hardcopy of all surveys and surveyed position calculations to date.

•

(Onshore/Office) Drilling / Operations Engineer Verify all anticollision calculations and surveyed position calculations in use by the Directional Driller on a daily basis, or more frequently as required, using the definitive survey database, or another independent and verified copy of the database as appropriate.

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Drilling and Measurement Procedures 1.14.4

Standard Anticollision

Surveying Procedure

During periods of anticollision monitoring where there is a risk of infringing the minor risk rule, or where a risk based exemption is in force, surveys should be taken every 10m (30ft) or more frequently with either Surface Readout Gyro (SRG), Northseeking Gyro (NSG), or MWD (providing external magnetic interference is not an issue). At each survey station the Directional Driller and Operations Engineer should confirm the survey has been recorded and plotted correctly, and produces the same surveyed position results on independent systems.

In order to protect against gross errors, particularly

unrecognized calibration problems arising, where wireline conveyed gyro survey tools are being used for singleshots, the survey tools provided at the rigsite should be cycled at least every four runs in hole or two stands drilled (whichever occurs first). A process should be maintained which confirms surveyed position across tool changeovers, and the positive seating of the gyro in the UBHO sub. Projections ahead of the bit should be calculated based on both straight line and trend analysis, based on tool settings and the last three surveys, with any drilling progress decisions generally taking into account the worst possible case between the two projections. The OSF should be calculated at the projected position ahead of the bit, and during periods when the minor risk rule has been infringed, or a risk-based exemption is in force, the clearance calculations should be independently confirmed at the time of drilling, by the (Onshore or Office-Based) Drilling or Operations Engineer. Drilling should be delayed until this confirmation has been completed. Once clear of the minor risk area, or risk based exemption zone, and diverging from any wells shut-in for anticollision, normal drilling and surveying operations may resume. Drilling must cease when any inconsistency in any recorded survey is observed, or the anticollision status or clearance calculation at any stage cannot be confirmed, until the problem is resolved. 1.14.5

Shut-in Criteria

The shut-in criteria must be agreed with the client at the planning stage, and fully documented in the well design file, particularly where the client has provided written exemption from any of the standard shut-in criteria below. When the minor risk OSF has been, or is likely to be infringed during drilling operations, the following shut-in criteria should be observed. Provided that the potential point of collision is above any shut-in and depressurized subsurface safety valve or plug, drilling may continue, however the following recommendations should be followed where possible: •

Use a rock bit in preference to a PDC or diamond type.

•

Where a drilling motor is used, use low speed in preference to high speed.

•

Where there are mud returns, monitor for the presence of cement,

•

Where there are mud returns, install a ditch magnet upstream of the shale shakers and monitor for the presence of metal shavings.

•

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•

Closely monitor drilling torque.

•

Annular pressures on potentially intersecting wells should be monitored continuously for fluctuations and any such fluctuations reported immediately.

•

A listening device may be used on the wellhead of the nearby well that is at risk from collision.

•

When drilling in close approach situations, ensure that the top stabilizer and not just the bit has safely passed the potentially intersecting well, before drilling ahead at full speed.

When the minor risk OSF has been, or is likely to be infringed during drilling operations and the potential point of collision is below the depth of any shut-in and depressurized subsurface safety valve or plug, then the well should either be plugged back and redrilled, or a deeper plug must be set so as to be below the potential point of collision, and the offset well depressurized. If a deeper plug is set, then drilling may proceed as above. If, at any time the projected ahead position of the well gives an OSF of equal to or less than 1.0 then unless the well is immediately resurveyed with a more accurate survey tool to increase the OSF beyond the minor risk threshold, then the subject well shall be plugged back and redrilled. 1.14.6

Poorly Surveyed Offset Wells

Where the position of any offset well is in doubt as a result of having been poorly surveyed, or it’s position is not well known for whatever reason, it shall be assigned the worst possible position error model (SLBISCWSA: UNKNOWN) until it’s position can be properly verified. Where this procedure results in a potential proximity issue involving a well having an UNKNOWN tool code assigned to it then the well design must either be altered to avoid this problem, or the poorly surveyed well must be the subject of a risk analysis, and the Risk Based Anticollision Procedures should be invoked. No poorly surveyed well having an UNKNOWN survey tool code shall be allowed to infringe the minor risk OSF without having been the subject of a risk based written exemption.

1.15 Magnetic Interference 1.15.1

General

The use of magnetic surveys that have been influenced by magnetic interference is one of the major causes of modern day well collisions.

There are two main sources of this interference; external

interference usually coming from nearby casing(s) nearby fish or formations that may exhibit magnetic properties, and the second source is drillstring originated magnetic interference.

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External interference cannot be corrected for, and the only safe alternative is to run gyro surveys during well proximity situations until clear of it. Drillstring magnetic interference can be corrected for using the latest Schlumberger multi-station correction algorithm (D-Mag).

The underlying assumption for all

anticollision operations involving survey quality control, and the comparison of MWD versus Gyro (or any other) surveys, is that each survey type has independently met it’s own wellsite quality control requirement. In other words, the use of Gyros for anticollision proximity surveying will not be halted unless the comparative MWD survey meets it’s own independent field acceptance criteria and sufficient confirmatory overlapping surveys have demonstrated correlation. In every case, every offset well, partial well drilled, fish, sidetrack and abandoned well must be separately contained in the database in it’s own borehole, and must have a definitive survey in place with survey uncertainty tool codes selected appropriately to indicate current knowledge of the surveyed status of each well. An exemption is required if a client elects to drill ahead using only MWD measurements in situations where there is still evidence of external magnetic interference, even if the gyro and MWD agree at the last gyro survey. 1.15.2

Changeover Between Gyro and MWD Surveys

In situations where magnetic interference is expected on the MWD tool from multiple nearby wells, a simple calculation can be used during planning as a guideline for predicting where gyro surveys might be required. This is done by taking the root sum square (RSS) of the center-to-center distances of all of the offset wells surrounding the subject well at any given depth, and plotting this “overall effective clearance distance” against measured depth on an x-y plot. This planning technique needs to be calibrated with real surveys in each area to be useful. During execution, prior to any decision being made to halt taking gyro surveys, it must be confirmed that the MWD tool is clear of external interference. The only practical way to ensure that this is the case is by direct comparison of MWD versus Gyro surveys. In determining what the degree of correlation should be, some allowance must be made for the orientation, geographic location and BHA configuration (or amount of drillstring interference present), as well as the respective tool accuracies. This calculation can be done using a software utility, or worksheet, which utilizes the technique described in the 1997 Anadrill MWD Surveying Procedures3.

An upper bound for this

calculation, given the dependent variables are not extreme cases, is unlikely ever to exceed 2° in azimuth, and 0.5° in inclination.

The calculated correlation should be confirmed over at least two

successive surveys where the subject well is positively diverging from any offset well before halting the use of gyro surveys. A more detailed treatment of magnetic interference summarized here can be found in the Survey Procedures.

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1.16 Appendix A – Standard Anticollision Procedures: Guidelines The guidelines below are provided as an illustration on the use of the procedures and to provide context. Unlike the procedures, they are not mandatory. Rather, they serve to help provide a better understanding of the intent of the procedures. It is the responsibility of each location to ensure that the procedures are strictly adhered to and that the guidelines are understood. 1.16.1

Guideline 1 - Anticollision Scanning By Survey Program Parts

1.16.1.1 Survey Program The survey program is the planned series of survey instruments to be used, and surveying requirements to be met, during the execution of the well design in order to satisfy the Well Surveying and Anticollision Policy. In general, the survey program forms the link between how we plan to avoid any unintended well collisions, and how we ensure that we achieve our well positioning objectives and penetration of the drilling target. Designing a survey program is an iterative process where the objective is to meet all of the well positioning objectives with the best technical solution in terms of operational efficiency, cost and accuracy. This is discussed further in the Surveying Procedures, but for the purposes of illustrating the use of the survey program for anticollision scanning by parts, we can begin with a typical example of a survey program: In the table below, JORPS are defined as vendor specific Joint Operating and Reporting Procedures as agreed between the client and the service provider.

Hole

Casing

Depth From

Depth To

Survey Tool

Vendor

Survey Frequency

QC Requirements

Tool Code

26"

-

Seabed

865ft

Gyro Singleshot

Scientific

Maximum 30ft

SDI JORPS

NSG-SSHOT

@342ft

Comments No MWD in 26" BHA - Gyro in UBHO sub as close to Bit as possible Cycle between at least 2 tools every 6 runs taking correlation shots Drillpipe gyro multishot section on last gyro run in hole to confirm singleshots

17 1/2"

-

865ft

2975ft

MWD

SLB

Stand (~96ft)

SLB JORPS

MWD+SAG

MWD to be SAG corrected -

13 3/8"

12 1/4"

-

Seabed

2950ft

@342ft

Continuous

Scientific

25ft

SLB

Stand (~96ft)

Gyro Multishot

SDI JORPS

MWD benchmarks and checkshots required as per JORPS on any BHA change CNSG-CASING Continuous gyro multishot must reach a minimum depth of 2800ft, otherwise

S/Specialist

a cleanout trip must be done and the gyro re-run to this depth.

Acceptance Req'd

12 1/4"

-

-

9 5/8"

2975ft

2975ft

Seabed

7455ft

7455ft

7400ft

@342ft 8 1/2"

8 1/2"

-

-

7455ft

7455ft

MWD

EMS

Continuous

Scientific

Scientific

11200ft

MWD

EMS

SLB

Scientific

Gyro inrun/outrun and QC report faxed to S/S immediately on completion MWD+SAG

If section completed in a single BHA run, drop EMS multishot on completion.

6 overlapping

MWD to be

If multiple BHA's used, EMS not required, but ensure a minumum of six

repeats

SAG corrected

overlapping MWD vs MWD surveys somewhere during trip back in hole.

Stand (~96ft)

SDI JORPS

25ft

Gyro Multishot 11200ft

SLB JORPS

Gyro singleshots on standby for first 500ft of section in case of external magnetic interference. MWD Surveys to be SAG corrected

Stand (~96ft)

EMS+SAG

Contingency: Drop EMS multishot if 12 1/4" section is drilled with one BHA.

EMS to be

This is for confirmation of the MWD only, and is required over at least 1000ft

SAG corrected

of open hole, so need not be dropped at TD depending upon hole condition.

SDI JORPS

CNSG-CASING Continuous gyro multishot must reach a minimum depth of 7200ft, otherwise

S/Specialist

or

Acceptance Req'd

CNSG-DPIPE

SLB JORPS

MWD+SAG

the gyro must be pumped down in drillpipe on trip in hole to this depth. Gyro inrun/outrun and QC report faxed to S/S immediately on completion If section completed in a single BHA run, drop EMS multishot on completion.

6 overlapping

MWD to be

If multiple BHA's used, EMS not required, but ensure a minumum of six

repeats

SAG corrected

overlapping MWD vs MWD surveys somewhere during trip back in hole.

Stand (~96ft)

SDI JORPS

EMS+SAG

Contingency: Drop EMS multishot if 8 1/2" section is drilled with one BHA.

EMS to be

This is for confirmation of the MWD only, and is required over at least 1000ft

SAG corrected

of open hole, so need not be dropped at TD depending upon hole condition.

In order to identify the anticollision scanning steps, we first need to clearly identify the appropriate drilling stages. This is most easily done graphically, using a ‘Christmas Tree’ diagram. The Christmas Tree diagram is simply a plot of ellipsoid of uncertainty (EOU) major axis versus measured depth, and is used to graphically display the effect of each of the various sections of the survey program (using our example illustrated above).

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The plot has to be drawn manually (using Excel) but once the user has done this once or twice he/she will very quickly identify the parts.

0

Surface Uncertainty 26" Gyro S/Shots

865 1500

17 1/2" MWD

Part 1

2000 2975

13 3/8" CNSG

Measured Depth (ft)

4000 4500 5000

Part 2

12 1/4" MWD 5500 6000 6500 7455

9 5/8" CNSG

9000 10500

Part 3

8 1/2" MWD

11200 80

60

40

20

0

20

40

60

80

EOU Major Axis (ft) fig 14 : Christmas Tree Diagram for Example Survey Program

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Using our example above, we will simulate a surface uncertainty of 5ft, and then begin by plotting the resultant increase in EOU (given by the EOU report from the planning software) for each survey program item in turn. So, we have gyro singleshots in 26” hole down to 865ft, and then MWD to 2975ft. We set these tool codes in our well design package and generate the 3D 95% EOU report and extract the major half axis at 865ft and 2975ft. By continuing to complete the plot by changing the survey tool codes in the well design software to simulate the progress of the execution of the survey program, a Christmas Tree plot similar to the example above is generated. It can clearly be seen from this that the MWD surveys are driving the largest EOU generation and we need to complete our anticollision scan using three survey program parts as given below: 1.16.1.2 Survey Program Parts

Part

Hole

Casing Depth From Depth To Survey Tool

26"

-

Seabed

865ft

Vendor Survey Frequency QC Requirements

Gyro Singleshot Scientific

Maximum 30ft

SDI JORPS

Tool Code

Comments

NSG-SSHOT No MWD in 26" BHA - Gyro in UBHO sub as close to Bit as possible

@342ft

Cycle betw een at least 2 tools every 6 runs taking correlation shots

1

Drillpipe gyro multishot section on last gyro run in hole to confirm singleshots 17 1/2"

-

865ft

2975ft

MWD

SLB

Stand (~96ft)

SLB JORPS

MWD+SAG

MWD to be -

13 3/8"

Seabed

2950ft

@342ft

Continuous

Scientific

25ft

Gyro Multishot

SDI JORPS

MWD benchmarks and checkshots required as per JORPS on any BHA change CNSG-CASING Continuous gyro multishot must reach a minimum depth of 2800ft, otherw ise

S/Specialist

2

a cleanout trip must be done and the gyro re-run to this depth.

Acceptance Req'd 12 1/4"

12 1/4"

-

-

2975ft

2975ft

7455ft

7455ft

MWD

EMS

SLB

Scientific

Stand (~96ft) 6 overlapping

MWD to be

repeats

SAG corrected

Stand (~96ft)

SDI JORPS

C -

9 5/8"

Seabed

7400ft

@342ft

Continuous

Scientific

SLB JORPS

25ft

Gyro Multishot

8 1/2"

-

-

7455ft

7455ft

11200ft

11200ft

MWD

EMS

SLB

Scientific

If multiple BHA's used, EMS not required, but ensure a minumum of six overlapping MWD vs MWD surveys somew here during trip back in hole. EMS+SAG

Contingency: Drop EMS multishot if 12 1/4" section is drilled w ith one BHA. This is for confirmation of the MWD only, and is required over at least 1000ft of open hole, so need not be dropped at TD depending upon hole condition.

SDI JORPS

Stand (~96ft)

SLB JORPS

6 overlapping

MWD to be

repeats

SAG corrected

Stand (~96ft)

SDI JORPS

C

If section completed in a single BHA run, drop EMS multishot on completion.

EMS to be

Acceptance Req'd 8 1/2"

Gyro inrun/outrun and QC report faxed to S/S immediately on completion MWD+SAG

SAG corrected S/Specialist

3

Gyro singleshots on standby for first 500ft of section in case of external magnetic interference. MWD Surveys to be SAG corrected

SAG corrected

CNSG-CASING Continuous gyro multishot must reach a minimum depth of 7200ft, otherw ise or

the gyro must be pumped dow n in drillpipe on trip in hole to this depth.

CNSG-DPIPE Gyro inrun/outrun and QC report faxed to S/S immediately on completion MWD+SAG

If section completed in a single BHA run, drop EMS multishot on completion. If multiple BHA's used, EMS not required, but ensure a minumum of six overlapping MWD vs MWD surveys somew here during trip back in hole.

EMS+SAG

Contingency: Drop EMS multishot if 8 1/2" section is drilled w ith one BHA.

EMS to be

This is for confirmation of the MWD only, and is required over at least 1000ft

SAG corrected

of open hole, so need not be dropped at TD depending upon hole condition.

1.16.1.3 Anticollision Scanning by Parts Having identified the survey program parts, we have immediately established that we will require at least three separate summary scan reports, with the detailed reports being dependent upon the summary scan results. The first anticollision (a/c) scan will require the surface uncertainty to be set at 5ft, and the Gyro Singleshot tool code (NSG-SSHOT) to be set from seabed to 865ft. The next step is to set the tool code to MWD+SAG (in this example), from 865ft downwards. Next, conduct the global scan and then run the proximity calculation as described earlier in these procedures. Output the summary report and note any wells that infringe the Alert Zone OSF < 5.0. At this stage it is more efficient to stay in the Close Approach application and deselect the wells that are clear of the Alert Zone threshold. Then re-run the proximity scan, and output the detailed report. This completes the anticollision scan for part 1 of the survey program. The same process is repeated for part two, by returning to the well design package and setting the survey tool codes to Continuous NSG (CNSG-CASING), down to 2950ft, and then MWD+SAG onwards.

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Run the proximity calculation again, output the summary report, deselect the wells clear of the alert zone threshold, rerun the proximity calculation and output the detailed scan report. Repeat the process for part 3 of the survey program, by setting the tool codes to CNSG-DPIPE (assuming the gyro had to be pumped down in this case) from surface, and MWD+SAG to 11,200ft. Repeat the report generation process as described above. 1.16.1.4 Interpretation of Anticollision Scan Reports The above process is carried out so that it is now possible to identify any wells that present a danger of infringing our minor risk or major risk threshold. What happens if on the anticollision scan for part 1 there is a minor risk well at 5,400ft? Well, typically this is ignored because part 1 only covers from surface to 2,975ft. We would ensure that all of our anticollision rules are satisfied for part 1 over this interval, and then move on to ensure that each of the other parts do not show proximity problems over their respective intervals. 1.16.1.5 Iteration Anticollision planning is an iterative process that requires training and experience to come to an optimized solution. In many cases the user will have to modify the well design slightly, change the survey program or perhaps request a change in the geological target size in order to reach a practical compromise. When any of these changes are made the entire process of identifying the parts, conducting the anticollision scans and interpreting the results is required to be repeated as described above. Over the life of a well design, sometimes up to and during actual execution, this process may be repeated many times and over many iterations using variations in planning, and ultimately combinations of planned and actual surveyed well position as the well construction progresses. 1.16.1.6 Survey Tool Accuracy Hierarchy The following table provides a general guideline to the relative survey tool accuracies of the various generic survey instruments available today. Instrument

Rank

Type

Reference

Conveyance

Remarks

RIGS

1

Inertial

Initialization

Wireline

5.3" OD Min.

Continuous Gyro

2

Gyro

True North

Both

1.75" OD Min

Northseeking Gyro

3

Gyro

True North

Both

Not above 70deg angle

NSG Singleshot

4

Gyro

True North

Wireline

Not above 70deg angle

MWD / EMS

5 / 3*

Magnetic

Magnetic North

Drillpipe

EMS 1.375" OD Min

SRG

6 / 4**

Gyro

Sightline

Wireline

Runs not > 40min

Continuous DNI

7***

Magnetic

Magnetic North

Drillpipe

Accuracy not defined

Photo Singleshot

8

Magnetic

Magnetic North

Drillpipe

Verticality checkshot

Incliation

Gravity

Drillpipe

Verticality check only

Inclination Only

9

* MW D / EMS are more accurate than gyrocompassing Northseeking Gyro above 70deg inclination. ** SRG is equallt as good as Northseeking gyro in low angle at surface where SRG is less affected by noise. *** Continuous DNI improves trajectory definition for Geosteering, survey accuracy still being investigated

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Drilling and Measurement Procedures 1.16.2

Standard Anticollision

Guideline 2 - Drawing Tolerance Lines on a Traveling Cylinder Plot

1.16.2.1 General Having completed an acceptable anticollision scan and produced each of the required reports as described in the previous guideline, one or more traveling cylinder plots can be prepared. Tolerance lines can then be drawn on them to aid the driller’s interpretation of his “drilling tunnel” and to define the hard boundaries that represent the threshold of any minor risk zones. In the example given above, it would be prudent to prepare three separate traveling cylinder (TC) plots, one for each of the 26” and 17½” sections. A TC plot for the 12 ¼” and/or 8½” sections will only be required in the event that any offset wells are within the alert zone radius at these depths. The preparation of the TC plots, their scale ranges and depth labels requires some experience in order to provide something that is of practical use to the Directional Driller. If at all possible, the Directional Driller should be involved to personally review the plots in sufficient time to allow for any changes to be made and for regeneration of the plots if necessary. 1.16.2.2 Basic Method Having identified the alert zone wells over the interval of each part, a suitable scale range for the tophole TC plot can be chosen so that all alert zone wells are displayed at least down to a depth of 1,000ft (our 26” hole section finishes at 865ft giving us some early warning of what is ahead for the next section). If the well being drilled is a sidetrack, or starts at a greater depth than the example given, the initial scale may have to be larger to accommodate the increased minimum allowable separations (MAS) that will exist at depth, due to larger position uncertainties having been propagated to that point. Wells can be removed from the tophole TC plot for clarity only if they do not infringe the alert zone, or are clearly overshadowed by other wells, which will clearly prevent a proximity issue with the overshadowed more distant well. This is not recommended however, and a better strategy is to leave all wells in place but remove unnecessary no-go circles. Having identified the wells required to be placed on the plot, and chosen to display no-go circles based on the minor risk rule, we can now generate the TC plot (Normal Plane and North Referenced), by setting our survey tool error model codes to those for part 1 of the survey program and outputting the TC plot to the graphics display. A typical scale for the tophole TC plot is likely to be 1in = 5ft (2cm=1m), and for subsequent plots perhaps 1in = 10ft, or 1in = 20ft.

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1.16.2.3 No-Go Circles The no-go circles that are generated are given a frequency based as a function of the scan interval chosen. Therefore if the scan interval is 100ft, and the no-go circle frequency is set at 1, then no-go circles will appear at 100ft depth intervals. It is worth noting here that the depth intervals, and any depths depicted on the TC plot are referenced to the measured depth of the planned (or subject) well only. Therefore, no-go circles marked as 600ft, refer to a measured depth of 600ft on the subject well (the one we are drilling). The no-go circle radius should be found to match the Oriented Minimum Allowable Separation (OMAS), which can be obtained from the report output, and the direction and distance of the point on the offset well which to which the no-go circle belongs is generated from the TC Azimuth and Center-to-Center distance, also obtainable from the text report. At this stage it is sometimes useful to draw over the no-go circles in color, using the same color for the same depth no-go circles across wells. Thus all of the no-go circles for 400ft can be colored blue (for example), 600ft circles red, 800ft circles green etc…

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1.16.2.4 Tolerance Lines The drafting of the tolerance lines on a TC plot takes some experience and knowledge of the drilling area and BHA capabilities. Because the tolerance lines can be somewhat subjective, the basic premise is that providing they are adhered to and observed, then the possibility of infringing the anticollision proximity rule in force must be minimal. This should take into account drilling efficiency, practical common sense and obviously the presence of no-go circles at intermediate depths not shown on the TC plot as a result of the user defined depth filter. The use of tolerance lines is advisable but not mandatory. Their use should be encouraged as it will aid improved understanding and practical use of the traveling cylinder plot.

North

800 800 60 Do

no

tc

t 0f

600

600 ro

ss

be

fo

re

40

400

400 0f

t

800ft

Do not enter before 800ft

400 800

600

fig 15 : Traveling Cylinder Plot W ith Tolerance Lines

In the diagram above the 800ft tolerance line provides the main guidance for the shape of the tolerance lines, with an additional restriction for 400ft and 600ft (respectively) drawn in to the Northwest to complete the tolerance line plot. It is worth mentioning that just because we did not plot them, additional no-go circles exist for each of 500ft and 700ft, and common sense consideration must be given to these in order to produce a sensible result that is not overly complicated by too many lines. Generally too many lines make the plot hard to read, whilst too few tend to restrict the “drilling tunnel” or available drilling room. In general however, drilling into areas that must be vacated further down the well because of proximity problems is a very weak strategy.

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Occasionally, because of very high well density, this has to be done but should be considered carefully. In the past, when standard separation factors were used, the practice of using tolerance lines was occasionally restrictive far down the well because of the proximity of an offset well having very large uncertainties, and therefore requiring a large tolerance far beyond the real probability of collision that existed further up the well when the offset wells were much closer together.

The use of Oriented

Separation Factors (OSF) has effectively dealt with this problem so that tolerance line rules and practices can be consistently applied throughout the well. 1.16.2.5 Transferring Tolerance Lines Where multiple traveling cylinder plots are prepared for use, there must be an overlap of preferably two tolerance lines between plots (i.e. two deepest tolerance lines on shallow plot become the two shallowest lines on the next deeper plot). It is possible that further down the well, perhaps even beyond the range scale of the tophole or surface hole traveling cylinder plot, a proximity issue arises with another offset well that is approaching at depth. In this case it may be appropriate to transfer a more restrictive tolerance line which is required further down the well to the surface hole TC plot in order to “plan ahead” for this potential problem. This will also help to maintain continuity between TC plots. 1.16.2.6 Color Coding Tolerance Lines In many areas the practice of color-coding and symbol coding the tolerance lines has been adopted. Particularly with the use of fluorescent highlight colors. The reason for the use of symbol coded tolerance lines was that some reprographic systems used to reproduce plots could only do so in black and white or by using a two-tone dyeline system. The advent of color plotters has removed the specific need for this but it is still a useful practice to avoid any ambiguity and to positively highlight any unusual anticollision problem. 400ft 600ft 800ft fig 13 : Example Tolerance Lines Color and Symbol Coded

Further information on the use and construction of a traveling cylinder can be found in the SPE paper 19989, by Thorogood and Sawaryn4.

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1.16.2.7 Traveling Cylinder Plots Versus Spider Plots Many locations are still using spider plots as an aid to anticollision planning and execution. This practice should not be discouraged where the Directional Driller is unfamiliar with traveling cylinder plots. However, the Schlumberger standard is for traveling cylinder plots to be prepared and issued with every well design file, and over time and with ongoing training it is expected that the traveling cylinder plot will become both the anticollision tool of choice, as well as the mandatory standard.

References: 1. 2001-016 Orientation Sensitive Risk Analysis (Phillips) – Internal Schlumberger Engineering Report. 2. SPE 67616 Accuracy Prediction for Directional Measurement While Drilling (Williamson). 3. S-262734-AA 1997 Anadrill MWD Surveying Procedures Manual (Phillips) – MWD survey quality control manual accepted as Schlumberger MWD JORPS (Joint Operating and Reporting Procedures), by some major clients. 4. SPE 19989 The Traveling Cylinder Diagram: A Practical Tool for Collision Avoidance (Thorogood, Sawaryn).

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