Advanced Technique Inhibition-Removal Paraffin deposition in oil wells.pdf.pdf

INVESTIGATION OF AN ADVANCED TECHNIQUE TO SELECT AN OPTIMAL INHIBITION AND REMOVAL METHOD OF PARAFFIN DEPOSITION IN OIL WELLS by BIKRAM M. BARUAH, B.Tech.

A THESIS IN PETROLEUM ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN PETROLEUM ENGINEERING

Approved

August, 200

^ ^ ^ : ;,

•

ACKNOWLEDGEMENTS

I am grateful to the Department of Petroleum Engineering at Texas Tech

University for accepting me into its graduate program and for providing the financial support which allowed me to pursue my educafion and to carry out the research work. I would like to express my gratitude to Dr. Lloyd R. Heinze for ser\ mg as my thesis committee as the chairman. His constant guidance and valuable advice \\ hile carrying out this thesis project as well as at different points of my entire slay at Texas Tech have been a great asset for me. I am also thankful to Dr. James C. Cox for agreeing to be on my thesis committee and patiently answering all my questions. His suggestions were very helpful as I worked through my thesis. The valuable advices and the encouraging words of Dr. Akanni S. Lavval, our graduate ad\isor, were of great help during my stay at Texas Tech University. His words will continue to encourage me in all my future endeavors. 1 consider the exchange of knowledge that I had w ith all the facult\ members, particularly Drs. James F. Lea and Scott M. Frailey, to be my life-time treasure. 1 would also like to thank the staff members of the Department of Petroleum Engineering who were very supportive and helpful to me. I thankfully acknowledge the useful communications that I had with Mr. 1 on\ Smith of Unichem, Mr. K. M. Barker of Baker Petrolite, Tony Cunningham of Pctroplex Acidizing, Dercck Cummings of Guardian Chemicals, J. Brent Johnson and Walt GloNcr of Halliburton Energy Services and Shaun Mesher of Trysol. Last but not the least, I proudly acknowledge the encouragement and support that my parents Mr. Chandra M. Baruah and Mrs. Anjali M. Baruah have given to their son for pursuing higher educafion at a place half way across the globe. Ponkhi, my only little sister, brightens my day. when her instant message '"oi! ki koriso'.^" (He\! What are you doing'.') coming across the globe, over the internet, often pops up h\ the corner ol m\ computer screen in the midst of writing this thesis!

II

TABLE OF CONTENTS

ACKNOWLEDGEMENTS

ii

ABSTRACT

vi

LIST OF TABLES

vii

LIST OF FIGURES

viii

NOMENCLATURE

ix

L

1

II.

INTRODUCTION 1.1

Background

1

1.2

Objective

2

1.3

Necessity

2

LITERATURE SURVEY 2.1

4

Technical Paper Survey

4

2.1.1

Historical Prospective

5

2.1.2

Composition of Deposits

6

2.1.2.1

Paraffin Waxes

6

2.1.2.2

Microcrystalline Waxes

7

2.1.2.3

Resins and Asphaltenes

7

2.1.3

Theory of Deposition/Crystallization

9

2.1.3.1

Precipitation

9

2.1.3.2

Deposition

9

2.1.4

Causes of Paraffin Problems

11

2.1.4.1

Naturally Occurring

11

2.1.4.2

Operational Causes

11

2.1.5

Remedial Measures

12

2.1.5.1

Mechanical Methods

12

2T.5.2

Thermal Methods

16

2.1.5.3

Chemical Methods

19

2.1.5.4

Other Methods

23 111

2.1.6

Selecfion of Treatment Methods

24

2.1.7

Common Terminology

27

2.2

Commercial Survey

29

2.2.1

The Internet

29

2.2.2

Product Catalogue

30

2.2.3

The Outcome

30

EXPERT SYSTEMS

33

m. 3.1

What is an Expert System

33

3.2

How it works

34

3.3

When Expert System Is Recommended?

36

3.4

Survey of Expert System Shells

37

3.4.1

C Language Integrated Production System

37

3.4.2

Java Expert System Shell

38

3.4.3

Expert Master

39

IV.

METHODOLOGY

4.1

40

System Requirements (Defining the 'Envisioned Sxstenf)

40

4.1.1

Requirement No. 1

41

4.1.2

Requirement No. 2

41

4.1.3

Requirement No. 3

42

4.2

The Project Plan

42

4.3

The Project Work

44

4.4

Anal\sis of the Literature Surveyed

44

4.4.1

Abundance of Theory

44

4.4.2

Lack of Comparison Among Different Treatment Options

45

4.4.3

Lacking and Inconsistent Information

46

4.4.4

No Analytical Method

46

4.5

Feasibility of Applying Expert Systems

47

4.5.1

Is Expert S\ stems justified'^

4"

4.5.2

Are The Resources Available?

48

i\

4.6

The Database

48

4.7

Stafisfical Approach

50

V.

4.7.1

The Problem

51

4.7.2

TheSolufion

52

CONCLUSIONS AND RECOMMENDATIONS

54

5.1

Achievements

54

5.2

Conclusions

54

5.3

Recommendafions

55

REFERENCES

56

ABSTRACT

From the very beginning of the business of hydrocarbon exploitation, the problem of paraffin deposition was encountered with varying degrees. With oil exploitafion expanding into exotic frontiers like deep-water and the Arctic Circle, wax deposition became a greater challenge for the operators. Various mechanical, thermal and chemical methods are used to remove and prevent wax deposition. Howe\ er, it is often difficult to select the most effecfive and economic remedial measure for a siven situation. Due to uniqueness of every crude, there is no single technique that is most effective for all t\ pes of crude oils. The main objecfive of this thesis project is to explore the feasibilities of using computer-based consulting systems, commonly known as expert systems, to select the best remedial measure of wax deposition in a given situation. Extensive literature survey was carried out to understand and collect infomiation on the phenomena of wax deposition and removal/prevention techniques. A separate survey was conducted to understand expert systems in general and also to find out the criteria and resources required for building one. Then a feasibility study of building an envisioned computer system was conducted. Steps were also taken to initiate the building of an expert svstein.

VI

LIST OF TABLES

2.1: List of Manufacturers and Their Products

31

4.1: Details of the Data-Tables in The Database

49

\ II

LIST OF FIGURES

2.1: SchemaUc representafion of a crude/precipitate system

8

2.2 : Common Tools for Paraffin Removal by Scrapping (a) Paraffin Scratcher and (b) Paraffin Cutter or Tubing Gage

14

2.3: Pipe-line or flow-line scraper

14

2.4: Casing Scraper

15

2.5: Details of a vacuum insulated tubing

20

2.6: The idea of wax crystal modificafion

22

2.7: Chromatogram showing effect of inhibitor to hold paraffin in solution

29

3.1: Typical Components of an Expert System 4.1: The Acfion Plan

35 43

4.2: The Inter-Relationships of the Data-Tables in the Database

50

Mil

NOMENCLATURE

Y

= specific gravity of the crude oil at 60 °F

Dbefore = Original paraffin depositional rate of the crude sample before treatment Dqfrer = Ncw paraffin deposifional rate of the crude sample after treatment A

=R35,b-R70,b

B

= R35,a - R70,a

fe,a

= Flow Resistance (pressure drop / flow rate) at 35 "F after treatment

Ryo.a = Flow Resistance (pressure drop / flow rate) at 70 °F after treatment 7^35^

= Flow Resistance (pressure drop / flow rate) at 35 °F before treatment

Ryoi^

= Flow Resistance (pressure drop / flow rate) at 70 °F before treatment

IX

CHAPTER I INTRODUCTION

1.1 BACKGROUND From the very beginning of the business of hydrocarbon exploitafion, the problem of paraffin deposition was encountered with varying degrees. Mention of this problem can be found in literatures as eariy as in 1865. Eariier, mechanical methods like scrapping and pigging and thermal methods like hot-oiling, etc., were used to remo\ e paraffin from producfion strings and flowlines. However, over the years operators started preferring chemical inhibifion methods rather then these crude and expensix e mechanical and thermal methods. More sophisficated methods like ultra-sonic removal, magnetic fluid condifioning, microbial inhibition etc. are also considered. With oil exploitation expanding into exotic frontiers like deep-water and the Arctic Circle, wax deposition became a greater challenge for the operators. The\ started looking for more economic and reliable technique to handle the challenge. Chemical inhibition is one of these methods that are widely used. However, the characteristics of crude oil differ from field to field. They may even differ when sampled from the same well, but from different zones. Therefore, every crude oil is distinct and no single method is universally effective for all types of crude oils and environments to remove and inhibit paraffin deposition. Hence, no one additive proves to be effective for all crudes. Selection of an additive is very crucial for a successful prevention of wax deposition. Laboratory pre-testing and screening of various chemicals solvents, inhibitors and/or dispersant for waxy reservoir fluids help eliminate unnecessary and more expensive field practices and trials. Even such laboratory pre-testing can also be eliminated if a vast database of experiences exists. Otherw ise, only an experienced person with a vast knowledge of previous treatment operafions can give a suitable decision. With the advent of computer systems and tools like artificial intelligence, it is now possible to capture human expertise into a computer system. In this project, an 1

investigafion will be made to explore the possibilities of using an expert system for help selecting the best method of paraffin inhibition and removal.

1.2 Objecfive In addifion to invesfigafing the possibilifies of using an expert system to help selecting the best method of paraffin inhibifion and removal, a number of other objecti\ es are also set. The list of all the objecfives of this project is as follows: •

To carry out extensive literature survey.

•

To understand the phenomena of paraffin deposition.

•

To investigate remedial inhibition and removal techniques in vogue.

•

To understand various aspects of expert systems.

•

To define and/or specify a computer system that has been envisioned to store and utilize expeilise to help selecting the best remedial measure of paraffin deposition in a given situation.

•

To study various commercially available software which can be used to implement the envisioned system.

•

To explore the feasibilifies of using expert systems to select the best remedial measure of paraffin deposifion in a given situafion.

1.3

Necessity

With the deposition of paraffin, the effective diameter of flow path of crude oil gets reduces. Eventually it may altogether block the flow path that includes flowlines, production strings, perforations and the connected pores of formation. As a result, the effectiveness of the natural drive-mechanism(s) and/or any artificial means of transporfing the crude from the formation to the surface and then to collecting/processing station are highly reduced. Therefore, for efficient and smooth exploitation oi' hydrocarbon, it becomes necessary to remove and inhibit precipitation and subsequent deposition of paraffin and other wax systems.

Significant monies are spent per year to address this problem. To minimize this unavoidable expense, it is also necessary to select the most efficient and economical technique. Due to uniqueness of every crude, there is no single technique \iable for all crude types. One can decide either by experimental trials or from experiences. With the paucity of human experts, it is necessary to capture their expertise into a computer system so that it can be used by non-experts. Often the human experts are biased or limited to a specific technique, so a system with a wider scope that can give neutral but technically appropiate response is essenfial.

S

CHAPTER n LITERATURE SURVEY

One of the most important aspects of this thesis project is to gather infoiTnation. Particularly to meet requirement discussed in secfion 4.1, an extensi\e literature survey is required. Moreover, before a funcfional expert system can be built, thorough understanding of the subject and a sufficiently huge knowledgebase are also required (details discussed in Chapter IV). Therefore, a vigorous sur\ e\ of available literature and commercially available products was carried out. The goal was to gather information and organize it in the form of the following: •

Theorefical aspects of the deposifion process;

•

A list of available remedial measures;

•

A list of manufacturers and companies involved in producing tools, chemicals pertaining to paraffin/asphaltene inhibition/removal;

•

A database of commercially available chemicals;

The survey was carried out in the following two phases, one after another: •

technical Paper Survey, and

•

commercial Literature Survey.

All these phases are discussed in the subsequent secfions. 2.1

Technical Paper Survey

It was a survey of technical literature. Much information was collected from the SPE image library. Other two sources of technical papers were South Western Petroleum Short Course and Canadian Journal of Petroleum Technology. These resources were searched with the following keywords: paraffin, wax and asphaltene. A number of papers showed up in the search results. This search result w as pnnted and the relevant papers were selected for further studies. These papers were printed and reviewed lor rele\ant information.

4

This survey attempts to invesfigate the problem from a historical prospecti\ e, idenfify the composifion of deposits, understand the theory of deposifion. identify the remedial measures available and review some common relevant terminolocries

2.1.1 Historical Prospectixe Menfion of paraffin related problem can be found in eari> literatures of 1865 by 19

Bone. The first scienfific invesfigafion of the problem in oil wells was conducted by Mills'^^ in 1923. Later in 1932, Reistle^^ published a comprehensive report on paraffin deposifion and other related problems in different areas of the oil industr\. His work has become a classic reference for the industry and is regarded as one of the most complete and thorough studies ever presented. Later in the 1960s, laboratory studies canied out with cold spot testers by Hurt,^^ Jorda^" revealed that roughness is one of the major factor contributing to paraffin deposition. Bucaram'^ studied the phenomena along with effect of some chemical inhibitors using a microscope with a camera attachment. After Bucaram, many other in\ estigators""* carried out an extensive study on various chemical inhibitors. However, no chemical w as found to be effective in all kinds of crude oils and often they are environmentally hazardous or economically prohibifive. The chemicals used so far were either solvents or dispersants. Knox, Waters and Amold^^ discovered a new class of chemicals called crystal modifiers. Due to low cost and non-hazardousness, these chemicals were thought to be a promising option for paraffin control. Moreover, once applied, these chemicals are effectixe through out the life cycle of crude oil production and transportation. Howexer, the selection of a suitable crystal modifier has remained a challenge ex en today. Thermal methods are comparatixely straight forward and effective in all kind o{ crude oils that deposits predominantly paraffin xvax. Its use in various w ays can be found from the very beginning. In his report, Reistle"^^' mentioned hot-oiling, exothermal reaction, steam injection and such other thermal methods. The concept of thermodynamics xxas first used by Erickson. Nicsen and Brown' m 1993 in an attempt to predict paraffin precipitation in crude oil. Thermodynamic

studies were also carried out by Firoozabadi-' and Mansoori et al.'" Two simulation studies were carried out in 1994 to predict paraffin deposifion. Simulation of paraffin deposifion and removal in wellbore was carried out by Keafing,^^ w hile that for deposition in reservoir was carried out by Ring et al."^' These predictive models are limited to only thermal methods of removal/ inhibition. The understanding of the phenomena is sfill primifixe to predict a suitable chemical inhibitor for given crude sample.

2.1.2 Composition of Deposits A wide range of different materials is deposited in oil xvells. Broadly, thex can be classified as organic and inorganic. A detailed classification was done by New benV' classified these deposits as follows; •

•

Organic o o Inorganic o o

Paraffinic Asphalfic; Scale, iron sulfide, iron oxide, etc. Clay, drilling mud, etc.

In this thesis, organic deposits, especially paraffin will be studied. According to Bucaram,'^ organic deposits range from almost pure white paraffin wax to pure asphaltic in nature. However, most deposits are between these two extremes and contain paraffin waxes, microcrystalline waxes, asphaltic material, resin, oil, water, sand and silt.'^"^''

2.1.2.1

Paraffin Waxes According to Bucaram,'^ pure paraffinic deposit constitute 40 to 60'/( of an

average crude deposit. They are long chained hydrocarbons xvith about 26 to 50 carbon atoms. Solid paraffin waxes consist of large, well-formed, needle-shaped crystals that agglomerate and form large masses.

2.1.2.2

Microcrystalline Waxes These consfitute about 10% or less of an average paraffin deposit. These are long-

chain hydrocarbon compounds with branched chain and cyclic-ring molecules located at random along the carbon chain. The crystal structures of these compounds are verx small and irregular and tend to remain dispersed in fluid and show little tendency to a^sjlomerate.

2.1.2.3

Resins and Asphaltenes Asphaltic materials are sficky, dark, semi-solid amorphous substances.'^ The> are

heterocyclic geomacromolecules consisfing of carbon, hydrogen and minor components like sulfur, nitrogen and oxygen. They also contain nickel, iron and x anadium in trace. Resm molecules are little smaller and have higher proportion of paraffinic chains attached to aromatic rings. However, the exact chemical structure and physicochemical properties of these materials are not well understood. Their operational definition are based on the solubility in different diluents.

Asphaltenes are defined as the fraction of

crude oil insoluble in excess normal alkanes like n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fractions of crude oil insoluble in excess liquid propane at room temperature and are adsorbed on silica, alumina or other surface-active materials.

7

la

Monomeric

C ^ -Asphaltene Monomeric Resin

7

(^^^

.Asphaliene-fiee oil species

Micelle

Precipitated Phase

Figure 2.1: Schematic representation of a crude/precipitate sxstem Source: Firoozabadi"' The amorphous, polar asphaltene molecules do not ha\ c melting points and decompose on heafing above 300-400 °C. Asphaltene molecules are insoluble in crude oil. A commonly accepted viexv is that, in crude oil, asphaltenes form micelles that are stabilized by absorbed resins kept in solufion by aromafics-'" (Figure 2.1). .At ^'normal" reservoir conditions, asphaltenes, resins and other chemicals remain in thermodynamic equilibrium with the oil phase.^^ When this equilibrium is disturbed, either by change in pressure, temperature or many other parameters like pH and concentration of resin, etc., can lead to flocculation of asphaltene. Flocculation of asphaltene in crude oil is known to be irreversible. Due to their size and their adsoiption affinity to the solid surfaces, flocculated asphaltenes ofien cause such inexersible

deposition. The amount present m crude oil deposits has been reported'' to range from 10 to about 60 %.

2.1.3 Theory of Deposition/Crystallization Paraffin to accumulate as deposit, it must go though two processes"^^: (a) Precipitafion and (b) Deposifion.

2.1.3.1

Precipitafion The process of precipitafion is defined as the solidification of w ax crystals from

the liquid phase. This process takes place" ^^'- due to either (1) drop of sxstem temperature below the cloud point, or (2) the solubility of the system get reduced so that the liquid phase no longer keep the solids in solufion. The causes that lead to these condifions are discussed in secfion 2.1.

2.1.3.2

Deposition When the solid particles precipitate out of the liquid phase, it must deposit on a

solid surface for a sizable amount to hamper in the production process. This process lakes place in two stages: (1) movement of the precipitated solid particles towards the static pipe wall and then (2) adherence of these particles on the wall. Burger, Perkins and Striegler'^ extensively invesfigated the mechanism of movement of the solid particles towards the pipe wall in an attempt to determine the expected nature and thickness of deposition as a funcfion of time and distance in the Trans Alaska Pipeline System, fhe theories they and other investigators'•^"•^•^"'" postulated are discussed in the suhsequest paragraphs. The mechanism of the second stage, that is, hoxv the solid particles adhere to the pipe surface is mainly attributed to surface roughness by Hurt" and Jorda."' A process called 'nucleation' is observed at the rough edges of the pipe wall. The process of crxsial growth also contribute to this accumulation.

Molecular Diffusion: For all flow condifions, oil will assume a laminar flow either throughout the pipe or at least in a thin laminar sublayer adjacent to the pipe w all. When oil is being cooled, there will be a temperature gradient across the laminar sublayer. If this temperature is below the WAT, a concentration gradient of dissolxed wax will result and the dissolved material will be transported towards the w all bx molecular diffusion. Shear Dispersion: A solid particle in a moving fluid tends to attain the x elocity of the streamline at its center. However, as the particle approaches a stafic solid boundarx (the wall), its both linear and angular velocifies are reduced. When the particle concentration is high and a significant number of multi-particle interactions occur. This results in a drag force from the pipe wall towards the center of the pipe and exentuallx leads to a lateral transport and a dispersing of particles toxvards the pipe w all. Brownian Diffusion: Precipitated wax crystals suspended in oil are continuously bombarded by thermally acfive oil molecules leading to random Brownian movements. In presence of a concentration gradient, Brownian motion will lead to a net transport towards the wall similar to molecular diffusion. Gravity Effect: Precipitated wax crystals are denser than the suiTOunding liquid phase. Unless prevented, these particles would settle down in a gravity field and deposited on the bottom of pipes or tanks. Electrokinetic Effect: When a crude oil is flowing in a conduit (porous media, well-tubular or flowline), a electrical potential develops along the conduit due to the mofion of charged particles present in the fluid."' This electrical potential could then cause a change in charges of the colloidal particles, which in turn gel electrically attracted towards the walls of the conduit. Thus, the colloidal particles start depositing on the sides.

,0

2.1.4 Causes of Paraffin Problems There might be several reasons why paraffin related problems occurs. Some of these reasons are occurs naturally in the process of producing and storing the crude oil. However, in many cases, these are artificially created by various operational procedures.•^•^•^ The following paragraphs describe some of them.

2.1.4.1

Naturally Occuring Gas Expansion Cooling. When gas seep through a small passage, into a lower

pressure region, it expands and cools. This is known as the Joule-Thompson Effect. When the well produces, the gas cools at various places like the choke, perforations, etc. This greatly contribute to wax precipitation at the constrictions. High Production Levels of Gas and Oil. exen if the proportion of high moleculer weight wax is very low in the over-all crude oil composition, when produced over a large period of fime, it will contribute to a large quantity of wax. Geothermal Gradient. Due to the decrease in temperature as the crude oil flows up to the surface, wax tends to precipitate and deposit in the tubing. Presence of Cold Water Zones.'' If the xxell trajectory passes through a cold water zone, the temperature of the crude oil will reduce so that wax is deposited. Evaporation of volatile light hydrocarbon.'^ These volatile hydrocarbon acts as naturally occurring solvents. With their evaporation due to pressure drop, the solubility of the liquid system become so that less that it no longer hold the solid paraffin in solution. Surface Roughness/Foreign Matter.''^'"'^'"^''"^" These forms nucleus that help initiate deposition. 2.1.4.2 Operafional Causes When a choke is opened for well tests like a draxx-down test, a pressure drop occurs in the well-bore. It leads to cooling due to Joule-Thompson Effect and subsequcsi paraffin deposition at the formation face and at perfomations.

Cold Complefion/Frac Fluids. When the well is completed with complefion fluids or hydraulic fracturing is used to sfimulate the well, hardly any consideration is gix en to maintain the temperature of the fluid. These cold fliuds goes into the formation main cools down the formafion the fluids and even the rocks, leading to wax deposition inside the formation itself Water/C02 Floods. The low temperature of the huge volume of water during Water/C02 flooding causes the over-all temperature of the reserx oir to decrease. This may often lead to wax-deposifion within the reservoir. Hot-Oiling. If proper precausion is not taken, it may lead to more w ax-problem rather then cleaning them. It may melt the wax deposited in tubing, but later redeposit them in the formation. Moreover, the oil used for hot-oiling is often from the stock-tank.

2.1.5

Remedial Measures

The remedial and prevention techniques for paraffin deposition that are in use and/or previously used are discussed here. These techniques can be broadly classified into three categories: Mechanical, Thermal and Chemical methods. Mechanical methods are removal techniques. However, thermal and chemical methods can be either remox al or preventive techniques. The preventive methods act by txvo mechanism: (1) by preventing precipitation of solids from the liquid phase, and (2) by preventing deposition and/or accumulation of precipitated solids on the pipe walls. All the thermal methods fall under the first category, whereas some of the chemicals methods belong to the first category, but others fall under the second.

2.1.5.1

Mechanical Methods Mechanical methods are the simplest method of paraffin removal. These methods

are bi-oadly classified into two basic categories: (1) removal of the deposits bx applying mechanical force and (2) preventtion of accumulation of deposits. The first category encompasses scrapping, pigging and a comparatixely sophisticated meihtul of ultrasonic

12

removal. Applying smooth coafings inside the pipe comes under the second categoix. All these are described in the subsequent secfions.

2.1.5.1.1 Scrapping Scrapping is one of the most common mechanical methods of paraffin remox a!. Descripfion of various mechanical scrapper like "paraffin scraper,"' "paraffin knife."" etc. can be found in Reistle's report.^^' The scrapper can be classified as: (1) wireline Scrapper, (2) Rod Scrapper and (3) Floxxline Scrapper. Some of these tools are presented in Figures 2.2, 2.3 and 2.4. Over the years, more sophisticated mechanical w ax scrapping tools were devised. However, this method later appeared less cost effectixe, particulariy when deposition rate is very high. Moreover, the scrapping process leads to increased wear on the tubulars.

n

I !

i

p-r

i

' ^

n^ hi iii

iia,

CIn

(a)

O.Q

(b)

Figure 2.2 : Common Tools for Paraffin Removal by Scrapping (a) Paraffin Scratcher (b) Paraffin Cutter or Tubing Gage Source: Wireline Opearions and Procedures^

Figure 2.3: Pipe-line or flow-line scraper 46 Source: Reistle

Figure 2.4 : Casing Scraper Source : Bilco Tools, Inc.^

2.1.5.1.2 Anfi-Sfick/Plastic Coats When experiments carried out by by Hurt,^^ Jorda"^" and Jessen et al.^^ revealed that rouhness is a major governing factor for deposition, attempts were made to minimize roughness by introducing plasfic coats. However, it has been found that plastic coats solve the problem only temporarily as it starts eroding over time.47

2.1.5.1.3 Pigging Ward^"^ and Snedeker^" mentioned pigging as a means of paraffin removal in the mid-1950s. Ward describes an economical method of using a short cylindrical plug made of a soluble material to pig out paraffin from flow-lines. These plugs are such that they dissolve when exposed to crude oil for a long fime, but maintain their shape long enough to push deposited paraffin down a line when fluid pressure is applied behind it.

15

2.1.5.1.4 Ultra-Sonic Removal Laboratory experiments^^ carried out with core samples revealed that ultrasonic remoxal is another viable technique for paraffin removal, especially if combined with solvent soaks. The dominant physical mechanisms seem to be increased re-suspension and solubility of paraffin due to mechanical agitafion. However, its actual field tnal is xet to be carried out.

2.1.5.2

Thermal Methods The main objective of any thermal method is to maintain fluid temperature well

above the Wax Appearance Temperature (WAT). These methods can be broadly classified as: (1) Heafing and (2) Insulafing. Thermal methods are effectixe only near its point of applicafion. Further away from the point of application, the problem reoccurs."*^ Moreover, in asphalfic deposits, thermal methods are ineffective. In the heating process, extra heat energy is supplied to the sxstem to melt the deposited paraffin xxax and to maintain sufficient temperature in the system to halt further precipitafion. The most common methods used in supplying the extra heat eneigx to the system are: (1) heating by a hot fluid (Oil, Water/Steam) (2) deploy downhole heater, (3) by means of exothermal reactions. The fluid temperature is also maintained by insulating the pipe and preserving the heat energy. One oi' the heat loss mechanisms in oilx\ells is free conxeciion in the annular space. This source of heat loss can be minimized by either (1) wrapping the pipe by some insulating material, (2) vacuuming the annulus, or (3) Insulafing Packer Fluid.Some of these thermal methods are discussed in detail in the following sections.

2.1.5.2.1 Hot Oiling Hot oiling is one of the most commonly used and old method of paraffin remox al. A detailed study of hot-oiling was canied out under a project called "Applied Production

16

Technology" at Sandia Nafional Laboratories during mid-1990s. The inxestigators found that the chemical and thermal processes that occur during hot-oiling are xerx complex."^^ "Key issues include: (1) During a typical hot oiling job, most of the fluid injected into the well goes into the formafion, and hence, particulates and chemicals in the fluid have the potential to damage the formafion.' (2) Hot oiling can xaporize oil in the tubing faster than the pump lifts oil. This interrupts paraffin remox al from the xvell, and thus the wax is refined into harder deposits, goes deeper into the w ell. and can stick rods such as sucker rod in beam pump installafions". (p.6) The following good "hot oiling" pracfices designed to maximize xvax removal and minimize formafion damage are inferred from these insights: •

Paraffin treatment should be w ell specific.

•

The frequency of treatment should be minimized.

•

Good quality fluid should be used.

•

The fluid should be injected down the annulus.

•

The tubing should be full and the well producing oil, not just gas.

•

The BTUs / hour injected should be maximized.

•

The volume injected should be limited.

•

Thermodynamics alone should not be the deciding factor in choosing betw een hot oil and hot water.

2.1.5.2.2 Hot Watering Hot water was introduced to provide an alternative to hot oiling. Apart from eliminating the some of the drawbacks of hot oiling, it can provide more heat energy to the target area due to the greater heat capacity of water. However, the process has its ow n drawbacks. Plain hot water treatment cannot provide the solvency that hot oiling can. Surfactant are frequently added to disperse the wax molecules in the w atcr phase. Substantially large amount of demulsifier is required to separate out these water-w et w:a\ from the water before its disposal.

17

2.1.5.2.2 Exothermic Reacfion Exothermic Process involves internal heat generation through an exothermic reaction. One such process is mixing of nitrate/nitnte compounds in a reduction/oxidation reaction that generates an exotherm. This process relies on water as the solxent to delixer the reactants to the problem areas, and may require addifional chemicals to disperse the melted paraffin. Such aqueous based processes may result in difficult emulsion and can cause shut-down of producfion by upsetfing dehydrafion processes. Furthermore, such gas generating reacfions can become uncontrollable explosion and can cause extensixe damage to the dovxnhole well hardxx'are.'"^ To overcome the drawbacks described above. Brown and Dobbs'^ came up w ith a novel method of using mild exothermic reactions of organic acid and organic bases. These exothermic reaction produces enough temperature ( > 212 '^F) to melt paraffin adequately and the resultant salts act as excellent dispersants. Most common acids that can be used in such reactions are alkyl-substituted analogs of benzene sulfonic acid and naphthalene sulfonic acid while xarious organic amines, alkali hydoxides, etc. can be used as bases.

2.1.5.2.3 Annular Packer Fluid Ashford et al.^ described the use of annular packer fluids as insulating material to prevent heat loss. They canied out various laboratory and field tests and concluded that gelled fluids proxides effective annular thermal insulation, thereby inhibiting paraffin precipitafion. Some of the fluids that can be used as insulafing fluid are Arctic Diesel, Gelled Diesel, Solid Laden Gel, etc.

2.1.5.2.4 Vacuum Jacketed Pipe Bunton'^ described the use of vacuum jacketed pipe, a system of two pipes one inside another (Figure 2.5). A earner pipe is coxered with radiation barners. fitted with spacers at regular intervals, and then inserted inside the jacket pipe. The two ends of the pipes arc welded together to form a thin annular spacing that is exacuated through a port 18

in the jacket pipe using a mechanical xacuum pump. .After desired lexel of xacuum is achieved, the port is also sealed. The key benefit of vacuum insulated tubing is supeiior thermal insulafion that is achieved with a minimal gap between the carrier and the jacket pipe. Such vacuum insulated tubings were demonstrated to be technically successful method of paraffin inhibifion by Purdy and Cheyne.^^ Howexer. they found economic success only in some limited wells.

2.1.5.3

Chemical Methods The chemicals used to prevent deposifion of paraffin wax are commonly referred

to as "paraffin inhibitors." Depending upon the mechanism by which thex work to control paraffin, these chemicals can be broadly classified into there categories: (1) Solvents. (2) Dispersants, and (3) Crystal Modifiers. The following sections discuss them in detail.

iukilij "liU

ri

' SviiScr

CcnlQ'j2Cf

1.^

"— Ccapling lrsc;lalo' Couplinc

"hfus! Cone

^

O.rcr Casing

hk).s lutie ,-

- ^ - • < .

Figure 2.5: Details of a vacuum insulated tubing Source: GrantPrideco's Website"

20

2.1.5.3.1 Solvents Solvents are are added to the crude oil to restore its ability to dissolve w ax crystals due to loss of dissolved gasses or reduction in temperature.'^ Some commonlx used solvents are: Produced condensate, Casing head gasoline, Pentane, Butane, X\ lene. Toluene, Chlorinated Hydrocarbons, Carbon Disulfide and Terpenes, etc. Hoxx ex er. according to Dobbs, some efficient solvents like carbon tetrachloride are refinerx catalysts poisoning. Some other extremely good paraffin solxents like Carbon Disulfide are hazardous as they hax e very low flash points. Solvents are adequate if the problem is only minor; however, for moderate to severe occurrences of paraffin the quanfifies of solvent required are usually prohibitix e. Moreover, some best solvents are prohibited because of health hazards and refinery catalyst poisoning.^^

2.1.5.3.2 Dispersants Dispersants are also known as Wetting Agents and Emulsifiers. In theory, dispersant should work like plastic coats. They form a film on the pipe wall to change the contact angle and thus retard the deposition process.^^ They also neutralize the attractive forces that bind the paraffin particles together.''^ The chemical structure of these molecules is such that one end is attracfive to the paraffin molecule, xvhile the other end is soluble in either oil or vxater, depending upon the phase in w hich the paraffin is to be dispersed. Thus, they keep the precipitated paraffin particles to disperse in the produced fluids without deposifing on the walls. Some common dispersants are: naturally occun-ing asphaltenes in crude oils, sulfonates, alkyl phenol derivatives, ketones, teipenes, polyamides and naphthalene, etc.

2.1.5.3.3 Crxstal Modifier Laboratory investigations'^"^'^^ indicated that certain chemicals change the manner in which paraffin crystallizes from solution. These chemicals were found to have the ability to reduce the tendency of paraffin crvstals to clump as they separate from solution. 21

They co-precipitates and co-crystallizes with the wax by taking the place of a wax molecule in the crystal latfice. While doing so, it prevents the crx stal to grow further b\ placing a steric hindrance that interferes with the proper alignment of new incoming paraffin molecules' (Figure 2.6). Thus, thex help reducing paraffin deposition tendency as well as lower the pour point. These chemicals are also knoxx n as Ciystal Modifier, Pour Point Depressant or Paraffin Inhibitors.

CRUDE OtU WAX CRYSTAL UaoVECUOUOPOINTi

WAX CRYSTAL COHESION (JUST BELOW THE CLOUD POINT)

WAX CRYSTAL ADHESION IBEIDW CLOUD POINT); AND OCCLUDED 85 6 W

0 D D WtTH WAX CRYSTAL MODIFIER • •i»-«'t

Figure 2.6: The idea of x\ ax crystal modification Source: Bilderback and McDougall, 1999

90

After the successful field trails and promising results, Bilderback and McDougall'° declared in 1969 that, "The wax crystal modifiers are the only chemicals found thus far to solx'e irreversibly the paraffin control problem from the point of chemical contact down hole (or at the wellhead), through the tubing, through thefloxvlinesand through the suri'ace equipment, storage tanks and pipelines"(p.3). Typical crystal modifiers are'^: 2-hydroxy-naphthalene, Polyethylene. Copolymer esters, Ethylene/Vinyl acetate copolymers, Olefin/Ester Copolymers, Ester/\'inyl Acetate Copolymers, Polyacrylates, Polymethacrylates and Alkyl phenol resins, etc. One of the problems of crystal modifiers is that these freeze during xvinter months in geographical areas having cold climate, making it difficult to transport and delixer or pump at the desired location. Becker^ has come up w ith a nexv blend of product called 'Winterized Paraffin Crystal Modifiers' that remain pumpable even in the cold xveather.

2.1.5.4

Other Methods Apart from the conventional mechanical, thermal and chemical methods, there are

some other innovative non-conventional methods being tried. The following articles describe some of them.

2.1.5.4.1 Kinetic Cell In 1978, a new product called 'Linear Kinetic Cell" was introduced.^' Paraffin molecules are in dipole stage, having a positive and a negative end, similar to a microscopic magnet. When crude oil containing paraffin crystal is passed through a strong electromagnetic field, it polarizes paraffin molecules, thtis helping them to remain in suspension. This theory is used by this new technology.

2.1.5.4.2 Microbial Treatments Certain bacteria have the ability to decompose heavier hydrocarbon into smaller hydrocarbons. ^' If these bacteria are cultured at the wax depositing places, the problem can be minimized. When applied downhole a producing well, for proper grow th of bacteria, the following criteria must be fulfilled. •

The well must produce certain amount of water.

•

Downhole temperature must be within an optimal range (~ 90 to 210 °F).

•

pH of the fluid must be within an opfimal range (~ 3pH).

•

The concentrafion of chlorine, H2S, etc. must also be optimal.

2.1.5.4.3 Magnefic Fluid Condifioning Though the exact mechanism is sfill not well understood, it has been found that when the crude is passed through a strong magnefic field, the rate of paraffin deposition decreases. However, laboratory experiments confirmed that magnetic fields do alter the paraffin crystallizafion process and that this phenomenon is time dependent.^^ This technique is generally known as Magnefic Fluid Conditioning (MFC) and Biao and Lijian have reported that a large number of MFC units are being successfully used in China. They also found that MFC is not ideal for crudes with high pour point and high wax contents and is generally economic for wells with more than 50 % water-cut.

2.1.6 Selection of Treatment Methods The most common remedial measures of paraffin deposition has been discussed in the previous secfions (2.1). The selection of these methods in a given situation is ofien confusing and controversial. In many cases, when a particular method is deployed, other alternatives are ignored. In the following paragraphs, a critical rex ievx of method selecfion procedures available in the published literature is presented. In modem days, mechanical methods like scraping are generally avoided as it involves extensive labour and mechanical wear and tear of xvell hardxvare." How ex er. in case of operational emergency, they may also be employed. Application of plastic or 24

glass coafing on the pipewall gained only a one-fime popularity during the earix 1960s.^^'^^ With the advent of cheaper and effecfive chemical methods, particulad} crystal modifier, popularity of these methods seem to diminished. Thermal methods are used when the problem area is limited because thex are effecfive only near the point of applicafion. For examples, if exothermic reaction, hotoiling, etc. are carried out in the wellbore, they will not help if the w ax> oil also needs to be transported through a long surface flowline in the cold winter (w hen temperature drops below the cloud point). In case of sub-sea floxvline, flowlines in the arctic region etc. where maintainfing the heat energy is of importance, vacuum insulated pipe sx stems are most effecfive due to their superior thermal insulation."" However, economx will dictate whether such a system is preferable over a chemical treatment. Among the chemical methods, solvent treatments is used primarily to remox e already exisfing deposits while crystal modifiers can not be used for such purposes. Thex are used for only inhibifing or preventing deposition. Dispersants are also less effectix e for removing existing deposits: however, they are extensivelx used along xvith other methods like water-based thermal washing, crystal modifier, etc. The selecfion of a suitable crystal modifier is often a difficult process. Dobbs describes the generally adopted approach as follows: 'A carefully collected and transported sample is tested for wax content, asphaltene content and pour point. Next a determination is made of the type of x\ ax that is causing the deposition problem. This is determined by placing a sample of the crude in a cold finger apparatus and collecting a sample of the depositing wax. The scraping is analyzed by gas chromatography. The retesfing of the crude with various paraffin inhibitors will show this same concentrafion peak, but hopefully the proper inhibitor will reduce it (Figure 2.7). The best inhibitor will be the one that reduces the peak to its lowest volume", (p.4) Another emerging technology is the magnetic fluid conditioners (MFC). .MFC is not ideal for crudes with high pour point and high wax contents and is generally economic for wells with more than 50^'^ water-cut.^ Howexer, the possibilitx of using this economic method is almost always ignored where chemical treatment is administered.

After an extensive study in various oil fields of China, Biao and Lijian^ haxe made some useful conclusions regarding the criteria of choosing different types of inhibifion techniques. These conclusions are: a.

For the oil wells with water cut below 50%, wax content less than 30%

and Carbon Number distribution of the wax in the range of C13-C40. a good efficiencx may be obtained by using chemical removing and inhibiting techniques and some of these wells could be effecfively treated with magnetic paraffin-inhibiting technique. b.

For the wells with water cut more than 50%, the magnetic paraffin-

inhibifing technique is generally more economic to apply. c.

For the wells xvith xvax content more than 309c, pour point higher than 40

"C, in'espective of the water cut, the best choice is to adopt chemical paraffin-remox al or thermal washing method. d.

For the oil wells with very high Carbon Number of wax, both magnetic

and pure chemical paraffin removing techniques are ineffectix e. The best economic choice is to apply chemical paraffin inhibitor or glass/plastic coating or lining of tubing. This type of additional unbiased conclusions are needed to make an usable knowledgebase (discussed in section 4.5), but there are hardly any in the published literature.

26

2.1.7 Common Terminology At this point, it is important to discuss some of the attributes of a crude oil. These attributes or properties are used to describe a given crude oil quantitatixely. The statistical method descrided in secfion 2.1 may also require to store and compare these attributes. One of the most common properties often measured and documented is API Gravity. It is a measure of the density of the crude oil and is related to specific gravity as follows: API =

r

^-^1^-131.5.

M

API Gravity decreases with increasing molecular weigh. Presence of asphaltenes also lowers API Gravity. This parameter is most often used to describe the qualitx of the crude oil in trade negofiations or biddings. Bubble Point is another important parameter of crude oil. It is that state of pressure and temperature at which gas separates from solution. This parameter is helpful in predicting reservoir pressure and gas-cut. Another important parameter is Cloud Point or Wax Appearance Temperature (WAT). It is defined as the temperature at which crystallized wax start appearing in a liquid system. This parameter is particularly important to know for crude oils having xvax deposition problem. Pour Point is another parameter required for a high wax-containing crude oil. It is the lowest temperature allowing the crude oil to flow. This parameter is influenced by presence of branched hydrocarbons in the crude oil. Moreover, one of the results of applying the chemicals that modifies the crystal growth of paraffin (crystal modifiers) is lowering of pour point.'^'*^ Therefore, these chemicals are also known i\s pour point depressants. To characterize the flow behaviour of crude oils, a property called viscosity is used. It is the measure of resistance to flow exerted by a fluid.''^ In the oil industry, the term viscosity usually refers to apparent viscosity, which is measured in centipoises.

17

Like other physical properties, viscosity is affected not only by pressure and temperature, but also by the type and size of the compounds present in a given fluid. The variafion of liquid viscosity is not known with any exactness, howex er, it is experimentally shown that viscosity of hydrocarbon increases with the molecular complexity.'''^ The effecfiveness of a paraffin inhibition technique is a measure of the degree to which it is successful in inhibifing or prevenfing the deposifion. In the literature, a number of different ways of defining effecfiveness of an inhibitor can be found. In terms of deposifion rate. Brown, Niesen, and Erickson'^ defined effectix eness as % Inhibifion = e = ^''""' " ^""'' x 100 % .

2.2

^before

The unit of deposifion rate can be mass/time (gram/sec) or volume/time (cm3/sec). The deposition rate can be found in a Cold-Finger Apparatus. The percentage inhibifion described above is a static parameter. It does not take "J A

into account the effect of flow resistance. To overcome this drawback, Fulford" defined another effectiveness that takes into account the reduction in flow resistance. A- B % Inhibifion (Flow) =

xlOO% .

-3

A Dobbs'^ tned to define effecfiveness in terms of the composifion of the deposits in terms of the alteration of concentrafion peaks of their heavier carbon molecules in gas chromatography analysis chart. (Figure 2.7). Dong et al.^^ described effecfiveness of chemical control of paraffin in terms of the change of thermal washing (hot-oiling/watering) intervals. If the treatment is successful, deposition rate will decrease and the time interval between at which thermal washing is required will be elongated.

28

"Colcf Finger' Scrapings

250 p

200

h Z D

C16 C21 C26 C31 C35

-C2i)

56 4 ' :7 1 - ,

• C25 • C30 -C35 • C40

D IS

n ft

150

DTREATED

< fit:

z ill o z o o

6-CIS

~-rj

CO

Z 0

TREATED

Deposit Rsduction

n

-

•UNrREATEO

n «

100 UNTREATED

6 •• C15 C20 CI 6 Ci'5

h

50

11

0

^

0^ 0^ 0^' 0^^ 0^' 0^' 0^^ ^

1 C26 ; C31

C30

C35 C36 •- C40

it t.\ 6 5^ 36 41 2C 8> 1 0'

& & & & & & f}^ & ' of literature and software tools xxonld be can-ied out. After this preliminary survey, an evaluation of whether such softwaic is possible to build not with the limited recourses will be canied out. At the end o\' this exaluation, if it 42

is determined that more informafion is required, appropriate steps would be taken to gather the missing information. B

Investigate software tools Literature survey Industry survey

Study feasibility of building the software

Specify missing info/data, devise methodology to gattier info/data.

No

Ves Specify hurdles of available aigorlthm/methodoiogy, attempt to devise new ones

Specify hurdles of available software.

No

Figure 4.1: The Acfion Plan However, if it is determined that sufficient informafion is available, then attempts will be made to find out if there is proper algorithm/methodology to process those informafion into meaningful knowledge. Again, at this point, availability of software tools will also be determined. If the search/survey fails to yield proper software tools, a more vigorous search may need to be undertaken. However, it has been decided that no attempts will be made to construct software tools as it might befimeconsuming and would deviate the purpose of the project.

43

4.3 The Project Work The project work was initially carried according the plan discussed in the previous secfion (4.2). As a first step, an inifial literature survey of technical papers xx as carried out (path-A in Figure 4.1) and presented in secfion 2.1 of Chapter E. Apart from theoretical aspects of the subject, case studies, field/lab data were also collected. An analysis of the information gathered is presented in section 4.4. In a separate effort, attempts xx ere made to understand the details of an expert system and the resources required to build one (path-B in Figure 4.1). The details of this effort is presented in Chapter I\'. With the help of above information, a feasibility study was carried out to investigate the hurdles of creafing the envisioned system and the ways to mifigate them. Details of this studx can be found in secfion 4.5. This study revealed that with the available resources are not adequate to contract the knowledgebase module of the expert s> stem. Steps xx ere taken to make up for this shortcoming by emphasizing on acquiring more structured knoxxiedge and to create a database (secfion 4.6) in the later part of the project. As an effort to construct the knowledgebase, a commercial survey of available products xvas also cairied out. The result of this survey is presented in section 2.2. Hoxvever, within the time-frame of this thesis project, the stage at which steps can be taken to construct the expert system could not be achieved. The hurdles encountered are explained in section 4.5.

4.4 Analysis Of The Literature Surveyed This analysis of the information gathered in the technical literature survey \xas aimed at extracting sufficient information to be included in the proposed knowledgebase of the envisioned system. The results of this analysis are discussed in the subsequent sections. 4.4.1 .Abundance Of Theory There is an abundance of theory. A large number of published literature can be found about the deposition phenomena, xanous preventive measures etc. fhe findings are 44

presented in secfion 2.1 of Chapter III. This information will be helpful in safisfymg requirement #1 (secfion 4.1.1) of the envisioned system.

4.4.2 Lack of Comparison Among Different Treatment Opfions In field applicafion case studies, methods are described without detailing. "Whv this method has been chosen over other available methods." Hence, it is difficult to construct any reasonable decision tree by means of xx hich the appropriate remedial measure can be identified. For example, Shroyer and Haynes^' described the use of Linear Kinetic Cell (LKC) and cited 8 case histories where they found success in terms of increased production, decreased paraffin deposition rate, etc. Howexer, they haxe not mentioned anything about why they have preferred LKC to the other available methods. Probably other methods, including application of chemicals might be equally effectixe and economic. Another such example is the use of bacteria treatment described by Santamaria and George.^° The applicafion of this method saved thousands of dollars per month, reduced the hot-oiling frequency and it is an environment friendly remedy. However, in this paper, it has not been analyzed, if other chemical or thermal methods x\ ould hax e been more efficient technically as well as economically. Dobbs'^ cited two field application case studies of squeezing crystal modifiers in the near well-bore formafion. Such applications have increased and stabilized the producfion rates and elongated the thermal washing interval. Thus, it helped the operators earn more revenue. However, his study remained quite about the reason xx hy this particular treatment method has been chosen. Other options like doxvn-hole heater, microbial treatment, magnetic fluid conditioner or linear kinetic cell could have been used and they might have resulted in a more effective treatment option.

45

4.4.3 Lacking and Inconsistent Information As far as chemical methods are concerned, field application case studies do not detail the chemicals used and properfies of crude oil treated. Many of the chemicals used are proprietary products. Even if these data are available, they are not consistent. The followings are some of these instances. In the field applicafions cited by Dobbs,'^ exact composition or even a generic chemical formula of the crystal modifier used could not be found. The properties of the crude oil treated are not presented. Another field study, discussed by Garbis et al.,"^ provided sufficient information on the geology of the formafion, well history and properties of the crude oil treated. However, one of the two chemicals tested is a proprietary product. As the composition of this proprietary product is not published, it is impossible to make a correlation of exact type of chemical and their effect on crudes of various types, particularly if tested by different researchers and published separately. Barker et al."* described successful treatment of two Oklahoma xxells xxith paraffin problems. The paper contains a detailed well history, geological background of the xxells and Gas Chromatography results of the crude and the deposits. It also describes hoxx they analyzed the problem and designed and administered the treatment. However, it lacks informafion on the exact type/formulation of the solvents and crystal modifiers used.

4.4.4 No Analyfical Method The typical method of choosing a chemical inhibitor is by hit and trial in the laboratory as described in detail by Dobbs.''^ A systemafic way of carrying out such laboratory experiments to identify a "tailor made" preventive or remedial actions is also discussed by Garcia.^^ However, there is no analyfical method that can determine the effectiveness of an inhibitor in a given crude oil, provided the composition and properties of the both (inhibitor and the crude oil) are knoxx n.

46

4.5

Feasibility of Applying Expert Systems

With the understanding of the problem of paraffin deposifion and its remedial methods, and with an insight of what is an expert system, hoxv it xvorks and xx hat are the recourses required to build one, a close scrutiny of feasibility of developing an expert system to recommend remedial measures for paraffin deposition problems has been carried out. The first question that needs to be answered is that whether expert systems are recommended in solving the problem of paraffin deposifion or not. Secondly, it is required to invesfigate if the required resources available.

4.5.1 Is Expert Systems justified? Secfion 3.3 discussed the criteria that a problem area should satisfy before application of experts systems can be recommended. The following paragraphs attempts to verify if the paraffin problem qualifies for applicafion of expert systems. As discussed in section 3.3, the chemical formulation of an inhibitor for paraffin deposifion needs to be tailor made for every specific problemafic well or field. This is done by hit - and - miss in the laboratory or by consulfing people with knoxvledge of previously carried out such trials, thus the knowledge of this chemical formulation is difficult to acquire or is based on the rules that can only be learned through experience. Therefore, experts systems can be useful in such situafions. In a given situation, a large number of varying options can be effeeiivc. It is often difficult to choose from the options that seem to be equally effective and economical. From the analysis of published case studies (section 4.4.2), it seems that the choice is often done arbitrarily. In such cases, experts systems can be helpful. The user of this system is supposedly the operatoi-s in the field xvith x cry limited knowledge about paraffin deposition and its remedial measures. For these people, a computer system which they can consult might be helpful.

47

4.5.2 Are the Resources Available? As discussed in secfion 3.2, the first step in building an expert sxstem is to acquire knowledge and reorganized and arrange them to construct the knoxx ledgebase. Knowledge acquisifion can be done from the following sources: (1) Human Experts, (2) Published Papers and (3) Database. However, there is a dearth of human experts and human experts contacted seem to have professional atfitude. Moreover, most of the experts are affiliated to a particular company, hence their opinions are likelx' to be biased.The published literature is inadequate to construct a full-fledge knoxx ledgebase due to the inherent drawbacks discussed in section 4.4. A database is constructed and attempts are being made to populate it (details in section 4.6).

4.6

The Database

A relafional database model has been created in MS Access 2000 to handle the problem by the stafisfical method described in secfion 2.2 or otherwise. This database consists of three main interlinked data-tables namely chem, samp and eff. The effectiveness data-table, ^j^^stores an effecfiveness factor (E) and an associated certaintx factor (C) corresponding to every chemical, details of which are stored in the data-table cheni, for every type of crude sample, details of which are stored in the data-table samp. A number of other supporting tables are also incorporated in the database. .A summary of these data tables can be found in Table while the inter-relationship betxveen the tables is shown in Figure 4.2 The commercial survey canied out and described in section 2.1 attempts to populate these tables with as many data as possible. It is hoped that some possible patterns xvill be recognized after sufficiently large data points xvere gathered, and translated into rules for the knowledge.

48

Table 4.1: Details of the Data-Tables in the Database Table Name Description Chem

Samp

Stores data about the chemical inhibitors. It contains x arious physical, chemical, hazard & environmental and commercial informafion of the inhibitors. Stores data about crude oil samples. It contains information about the geological setting of the oil field, reserx oir data, and various physical & chemical data of these samples. Stores the effectiveness of various chemicals on different crude oil samples. It also stores associates a certainty factor with each effecfiveness. This table contains details like contact address, xx ebsite. mailins address etc. of the manufacturers of x arious chemicals. This table can store detail compositional data (may be analyzed by high resolufion gas chromatography) of the crude oils. This table stores the detailed geological and reservoir related informafion of the oil field from xvhere the crude sample has been corrected.

49

flppID ReP ChernID SarnpID E C Result

ChenJD

SairpID SarnpNarne ResvID Ref APIGravity Viscosity CloudPoint PourPoint Color AsphPC PrPnPC AromPC H2S C02 pH

^SHHH ResvName Geographic Regie GeologicalAge DepoEnviron WatDrive EOR PI Pc BHT

TradNarne ManufID RefID toPrint ph_state color odor boiling_pt pour_pt PlashjDt sp_gr viscosity pH solj'vat sol_hc gendesp gcc hazards mechanism application handling compatibility features unit_cost unit inhibitor solvent dispersant _pfn _asp

Max/ID ManufName website Address askedCatalog toPnnt

One-to-Many Relationships <

Figure 4.2: The Inter-Relafionships of the Data-Tables in the Database

4.7 Stafisfical Approach The typical approach of selecfing the best inhibitor in a given situafion is by laboratory tests. ^^ If a large number of such laboratory data can be gathered and a database of effecfiveness of various inhibitors in different crude oils is built, it can subsfitute for a knowledgebase. In absence of analyfical approach for selecting the best inhibitor, a statistical method is postulated. This stafisfical approach relies on availability of experimental data 50

of effecfiveness of various inhibitors on different types of crude samples. The approach is described below

4.7.1 The Problem Let the set of Inhibitors invesfigated be / = [f,f_J^ .•••/„} Let the set of crude oil samples invesfigated be S = [S^ .S,,S^.,'--S^} where the/^ sample can be defined as Sj = [A^J . A,j, Aj^,• • • A,. } Akj = attributes (or properties) of the sample Let the effectiveness matrix be, e as

11

-n\

^ 1 .

im

. e nin

^nl

where eij is the effecfiveness of inhibitor // in crude sample Sj, The elements of this matrix can be found out by laboratory or field experiments. A number of ways of estimating these values are presented in section. As these values are experimental, their certainty depend upon the repetition of the same result in a large number of similar experiments. Hence, a matrix of certainly of these effectiveness xalues is also estimated. ^11

^i:

CT[

C T)

C, C,

c ^n\

^ nl

C,

One way of esfimafing these certainly values is by the folloxxing equation, Cjj = JNeij/iMij

where Neij = number of experiments that gave effectiveness = c,j ± z 51

(f = allowable error Nij = Total number of experiments conducted. Now, let S^ = (A,^ , A,^, Aj^ • • • A .^} be another crude oil sample that is not tested with any of the inhibitors f E I. The problem is to seek the probability distribufion of effectix enesses of all the inhibitors in crude sample S,^, provided the effecfiveness matrix e and certainty matrix c are known.

4.7.2 TheSolufion Let Fxj be the difference between crude sample Sx and Sj and is defined as _

1/

N

^ . =

4.1 k=\

where here w^^is the weight given to the ^''' attribute such that

j^^,=i-

4.2

Again, let the probability that ^v will behave exactly same as Sj can be P^j. When the attributes of 5v and ^S^exactly match, F^j is 0. In this case, the probability that Sx will behave exactly as Sj is assumed maximum. Let this maximum value be Pnm.\However, when the attributes oi Sx and 5"^ differ the most, the value of Fxj is maximum, say F,nux and the con'csponding probability is assumed to be minimum, say P„i/„.The following equation reflects these assumpfions P -P — """" '^"^ p

p =p v/

max

17 max

43 ^^ '

The reliability of this equation depends upon the validity of these assumptions and correct estimations of P,na.x, Pmm, E,„ax and Fxj. Now, using equation (3), Pxj can be calculated for all Sj e S. With respeci to the given inhibitor f there exists an effectiveness value ieij) for everx Sj If it is assumed that, the effectiveness of inhibitor /in sample Sx xxill be Cjj with a probability of P, . then a

table of Cij vs. Pxj can be prepared. This table will represent the probabilitx distribufion of various effectiveness values of Ii in sample Sx. The expected value the effecfiveness of inhibitor /, in crude sample 5vCan be estimated as

1^

-y

u ij

/

These esfimates of e,., for all available /, will give the required probability distribufion.

53

CHAPTER V CONCLUSIONS AND RECOMMENDATIONS

5.1

Achievements

An extensive technical literature survey was carried out. Information on composition of various types of deposifion, various theories of paraffin deposition, and remedial measures in use were gathered. A brief rex iexx of some relevant common teiTns was also discussed. An understanding of what an expert system is, how it works and situations \\ hen it should be used have been achieved. A few expert system shells, an important component of expert system, have been reviewed. An expert system based computer system has been envisioned and a detailed requirement document has been prepared. The information gathered from the literature survey has been analyzed for possible extraction of information to construct an expert system (particularly its Knowledgebase). A statistical method to identify suitable remedial measures for paraffin deposition has been devised. A database was also designed to make use of the abox e said statistical method. A commercial survey was conducted to populate the database in an attempt to help build a knoxvledgebase. 5.2 Conclusions The two most vital components of Expert Systems are: (1) Knoxx ledge base and (2) Inference Engine. The construction of the knowledgebase requires a structured understanding of the subject matter. Stand-alone Inference Engines are axailablc in the market.

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The analysis of the informafion gathered by literature surxex' revealed that there is abundance of theoiies on the subject of paraffin deposition. Hoxvever, it lacks sufficient comparafive study of various remedial measures, consistent/ordered information. The problem of selecfing the best remedial measure for paraffin deposition is an ideal situafion where Expert Systems can be applied. However, the cun^ently ax ailable informafion is not adequate to construct the knowledgebase module of an Expert System.

5.3

Recommendafions

A consultafion with multiple human experts is required to come up xx ith a structured understanding of the subject. Such understanding is of paramount importance for construcfion of the knowledgebase. Informafion sharing among various groups is also essential for building the knowledgebase. The service companies must reveal, at least the generic chemical formulation of their proprietary products. The operating companies must share data on effectiveness of various products on their crude oils. A large database of laboratory tested effectiveness of various inhibitors on different types of crude oil need to be built. This will help construct the much needed knowledgebase.

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REFERENCES

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4.

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