Dynamic Well Testing in Petroleum Exploration and Development

By Huinong Zhuang, Yongxin Han, Hedong Sun and Xiaohua Liu

Dynamic Well Testing in Petroleum Exploration and Development, Second Edition, describes the process of obtaining information about a reservoir through examining and analyzing the pressure-transient response caused by a change in production rate. The book provides the reader with modern petroleum exploration and well testing interpretation methods, including their basic theory and graph analysis. It emphasizes their applications to tested wells and reservoirs during the whole process of exploration and development under special geological and development conditions in oil and gas fields, taking reservoir research and performance analysis to a new level.

This distinctive approach features extensive analysis and application of many pressure data plots acquired from well testing in China through advanced interpretation software that can be tailored to specific reservoir environments.

• Presents the latest research results of conventional and unconventional gas field dynamic well testing
• Focuses on advances in gas field dynamic well testing, including well testing techniques, well test interpretation models and theoretical developments
• Includes more than 100 case studies and 250 illustrations—many in full color—that aid in the retention of key concepts

• Language: English
• Publisher: Elsevier Science
• Release date: May 12, 2020
• ISBN: 9780128191637

Huinong Zhuang

Zhuang HuiNong, a professor and senior engineer, graduated from Peking University in 1962. He took part the research of development program in its early stage of Daqing Oilfield after graduation and then since 1965 he served in Shengli Oilfield and was interested in oil/gas well test. In 1980’s took charge and operated interference tests and pulse tests in an oilfield in carbonate reservoir successfully; during this period he invented the interpretation type curves for interference well test in dual porosity reservoirs and applied these type curves in field practice; took charge of research of downhole differential pressure gauge and applied these gauges in data acquisition in fields and consequently won the invention award from China Nation Science and Technology Committee and was present the First International Meeting on Petroleum Engineering in Beijing in 1982,and his paper was published in the JPT. Since 1990 he served Research Institute of Petroleum Exploration and Development of CNPC, was concerned with the exploration and development of several large- or medium-scale gas-fields in China and has been doing dynamic performance research in about recent 20 years. Now is serving SPT Energy Group Inc. as its chief geologist. He has been devoting himself to dynamic performance analysis and well test for more than 40 years.

Book preview

Dynamic Well Testing in Petroleum Exploration and Development

Second Edition

Huinong Zhuang

Yongxin Han

Hedong Sun

Xiaohua Liu

Table of Contents

Cover image

Title page

Copyright

Preface

About the author

Chapter 1: Introduction

Abstract

1.1 The purpose of this book

1.2 Role of well test in gas field exploration and development

1.3 Keys of well test analysis

1.4 Characteristics of modern well test technology

Chapter 2: Basic concepts and gas flow equations

Abstract

2.1 Basic concepts

2.2 Gas flow equations

2.3 Summary

Chapter 3: Gas well deliverability test and field examples

Abstract

3.1 Gas well deliverability and AOFP

3.2 Three classical deliverability test methods

3.3 Treatment of deliverability test data

3.4 Parameter factors influencing gas well deliverability

3.5 Short term production test combined with modified isochronal test in gas wells

3.6 Stable point LIT deliverability equation

3.7 Production prediction in development program design of gas fields

3.8 Discussion on several key problems in deliverability test

3.9 Summary

Chapter 4: Analyzing gas reservoir characteristics with pressure gradient method

Abstract

4.1 Pressure gradient analysis of exploration wells in the early stage and some field examples

4.2 Calculation of gas density and pressure gradient under formation conditions

4.3 Pressure gradient analysis during development of a gas field

4.4 Some key points in pressure gradient analysis

4.5 Acquisition of dynamic formation pressure after a gas field has been put into development

Chapter 5: Gas reservoir dynamic model and well test

Abstract

5.1 Introduction

5.2 Pressure Cartesian plot—pressure history plot

5.3 Pressure semilog plot

5.4 Log-log plot and model graph of pressure and its derivative

5.5 Characteristic diagram and field examples of transient well test in different types of reservoirs

5.6 Summary

Chapter 6: Interference test and pulse test

Abstract

6.1 Application and development history of multiple well test

6.2 Principle of interference test and pulse test

6.3 Field examples of multiple well test in oil and gas field research

6.4 Summary

Chapter 7: Coalbed methane well test analysis

Abstract

7.1 Coalbed methane well test

7.2 Flow mechanism and well testing models in a coalbed

7.3 Injection/falloff well test method for coalbed methane wells

7.4 Analysis and interpretation of injection/falloff test data

7.5 Summary

Chapter 8: Gas field pilot production test and dynamic description of gas reservoir

Abstract

8.1 Pilot production test in unusual lithologic gas fields in China

8.2 Dynamic description in development preparatory stage of JB gas field

8.3 Short term production test and evaluation of gas reservoir characteristics in KL-2 gas field

8.4 Tracing study on gas reservoir dynamic description of SLG gas field

8.5 Dynamic description of YL gas field

8.6 Dynamic description of DF offshore gas field

8.7 Dynamic description of Longwangmiao—Carbonate gas reservoir in Moxi block of Anyue gas field, Sichuan Basin

8.8 Dynamic description of fractured tight sandstone gas reservoir with ultra high pressure in Keshen gas field, Tarim Basin

8.9 Dynamic description of Tazhong No.1 fractured vuggy carbonate gas reservoir

8.10 Dynamic description of Xushen volcanic gas reservoir

8.11 Summary

Chapter 9: Well test design

Abstract

9.1 Procedure of well test design and data acquisition

9.2 Key points of simulation design of transient well test for different geologic objectives

Nomenclature [with China statutory units (CSU)]

Appendix A: Commonly used units in different unit systems

Appendix B: Unit conversion from China statutory unit (CSU) system to other unit systems

Appendix C: Formulas commonly used in a well test under the China statutory unit system

C.1 Formulas in log-log plot analysis

C.2 Formulas in semilog pressure analysis

C.3 Gas flow rate formulas

C.4 Gas well deliverability equations

C.5 Pulse test formulas (by Kamal)

C.6 Other common formulas of gas wells

Appendix D: Method for conversion of coefficients in a formula from one unit system to another

D.1 Conversion of gas flow rate formula

D.2 Conversion of dimensionless time formula

References

Index

Copyright

Elsevier

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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Preface

Since 2013, when the first edition of this book was published by Elsevier, we have been working on dynamic description research of some large and medium sized gas fields in China. In the past decade, additional breakthroughs were made in gas exploration and development within the country. Deep, low permeability to tight, complex carbonate and volcanic reservoirs emerged gradually to become the new reserves contributors, but their development was more challenging. In this circumstance, the role of the gas reservoir dynamic description became increasingly prominent. While participating in the preliminary appraisal, development planning, and subsequent dynamic research for these complex gas reservoirs, we extended the dynamic description technique to various lithologic gas reservoirs. Thus this technique was made more applicable and practicable. It can help ensure the scientific, predictable, and economic development of gas fields.

On this basis, some critical parts in the first edition have been revised. The book covers almost all types of unusual lithologic gas fields discovered in China in the past 30 years. In Chapter 8, the dynamic description is further illustrated with some examples, such as the deep grain shoal carbonate gas reservoir represented by the Longwangmiao gas reservoir in the Moxi block of the Anyue gas field of the Sichuan Basin (by Xiaohua Liu); the deep and ultradeep fractured tight sandstone gas reservoirs represented by the Keshen gas field in the Tarim Basin; the ultradeep fractured vuggy complex carbonate gas reservoirs represented by the Tazhong No. 1 gas field (by Hedong Sun), and the volcanic gas reservoirs represented by the Xushen gas field in the Songliao Basin (by Yongxin Han). Moreover, some defects in the first edition have been corrected and all maps/figures were redrawn.

We appreciate Wen Cao, Lianchao Jia, Ruilan Luo, and all other colleagues for their assistance in writing and proofreading this book. We also appreciate Elsevier and Petroleum Industry Press for their contributions to this publication.

This book is a summary and refinement of the authors’ research. It reflects the integration, advances, and upgrading of gas reservoir engineering theory and field practice. We hope this publication can contribute to the development of complex gas reservoirs.

About the author

Huinong Zhuang, professorate senior engineer, graduated from Peking University in 1962. After graduation he took part in research on the development program of the Daqing Oil Field in its early stages, and after 1965 he served in the Shengli Oil Field, where his interest was in oil/gas well testing. In the 1980s he took charge of and successfully operated interference tests and pulse tests in carbonate reservoirs; during this period he invented the interpretation type curves for interference well tests in double porosity reservoirs and applied these types of curves in the field. He managed research on downhole differential pressure gauges and applied these gauges in data acquisition in the field, and consequently won the invention award from the China National Science and Technology Committee and was present at the First International Meeting on Petroleum Engineering in Beijing in 1982; his paper was published in the Journal of Petroleum Technology. Since 1990 he has served at the Research Institute of Petroleum Exploration and Development (RIPED) of PetroChina, has been concerned with the exploration and development of several large or medium scale gas fields in China, and has carried out dynamic performance research. He has devoted himself to performance analysis and well testing for about 60 years.

Yongxin Han is a Senior Reservoir Engineer of the Research Institute of Petroleum Exploration and Development (RIPED) of PetroChina and Deputy Director of the Department of Gas Field Development, and a member of the Society of Petroleum Engineers (SPE). Since he graduated from Daqing Petroleum Institute in 1989, he has worked for RIPED, specializing in pressure transient analysis, production data analysis, and dynamic gas reservoir description. He has participated in the exploration and development of several large or medium scale gas fields in China and completed more than 1000 gas well interval dynamic performance studies over the past 30 years. He holds a BS degree in reservoir engineering from Daqing Petroleum Institute of China, and MS and PhD degrees in reservoir engineering from China University of Geosciences (Beijing). He has copublished 6 books and over 30 papers in peer reviewed journals and conference presentations on well testing and gas reservoir evaluation and development.

Hedong Sun, PhD, Society of Petroleum Engineers (SPE) member and professorate senior engineer, earned his PhD from Xi’an Jiaotong University in 2004. Since 2004, he has been a research engineer in the Research Institute of Petroleum Exploration and Development (RIPED) of PetroChina. Hedong has 23 years of reservoir engineering experience with a focus on well test and production data analysis. He has published over 50 papers in peer reviewed journals and SPE conferences. He is an author of three books published by Elsevier.

Xiaohua Liu, PhD, is senior reservoir engineer with the Research Institute of Petroleum Exploration and Development (RIPED) of PetroChina. She has 25 years of work experience in natural gas field development research and has been involved in some of China’s major gas field development programs and reservoir engineering. Her focus is combining well short term prefracture pressure buildup with long term performance and geology to propose production optimization. She has coauthored 3 books and over 25 papers in peer reviewed journals and conference presentations.

Chapter 1

Introduction

Abstract

Well testing is systems engineering and includes well test design, pressure data acquisition and interpretation, and dynamic description of the oil/gas well and the oil/gas reservoir. This book combines all of these to illustrate the process of operation and the methods of well testing, and provides many field examples to validate them.

This chapter introduces the parameters possibly obtained from a well test and the problems commonly met in the field that can possibly be resolved by well testing and its study in different stages, from exploration to development of the fields. This study of gas reservoirs can be called dynamic description of gas reservoirs. Geophysical prospecting, geology-well logging, and this study are three pillar technologies, and this study has its own special and irreplaceable functions.

In addition, this chapter introduces the new idea of dynamic description of a gas reservoir on the basis of well test studies, including studies on deliverability of gas wells, dynamic models, pressure gradient distribution, and deciphering of geological information from gas reservoirs, which enables reservoir engineers interested in this new idea to start off by gaining knowledge of the characteristics of reservoir structure and the law of deliverability decay of the well and/or the gas reservoir.

Keywords

Modern well test; Well test model; Graphical analysis; Systems engineering; Gas reservoir dynamic description; Dynamic deliverability; Dynamic reserves

Outline

1.1The purpose of this book

1.1.1Well test: A kind of system engineering

1.1.2Well test: Multilateral cooperation

1.1.3Writing approaches of this book

1.2Role of well test in gas field exploration and development

1.2.1Role of well test in exploration

1.2.2Role of well test in predevelopment

1.2.3Role of well test in development

1.3Keys of well test analysis

1.3.1Direct and inverse problems in well test research

1.3.2How to understand direct problems

1.3.3Describing gas reservoirs with well test analysis: Resolving inverse problem

1.3.4Computer aided well test analysis

1.4Characteristics of modern well test technology

1.4.1One of the three key technologies of reservoir characterizations

1.4.2Methods of gas reservoir dynamic description

1.1 The purpose of this book

The modern well test has been around since the beginning of the 1980s. In China, during the implementation of reform and opening up policies, modern well test methods, interpretation software, and advanced test instruments, tools, and equipment were introduced almost simultaneously. Looking back at the advances made since the early 1980s, it is very exciting to see that new developed knowledge and techniques have been applied successfully in the discovery, development preparation, and development operation of many major gas fields in China. However, it should also be noted that application of the modern well test sometimes and in some places is still not good enough and needs to be improved further.

The well test today is very different from that of three or four decades ago. Just as in all other fields, due to the application of computers and advances in science and technology, engineers today seldom make calculations manually; well test analysts and reservoir engineers no longer frequently look up complicated formulas in well test books and perform tedious computations with calculators; the results can be obtained easily by simply selecting some menu items of software.

But does this mean that well test work has become much easier? The answer is no; on the contrary, as research activities go further, the well test does not become easier, but faces greater challenges.

First of all, well test analysis is required to provide not only simple parameters such as reservoir permeability, but also more detailed information about reservoirs, such as their types and boundary conditions, and ultimately to deliver a dynamic model of gas wells and gas reservoirs—that is, a dynamic model reflecting the conditions of the gas well and the gas reservoir truly and correctly, which can be used in gas field evaluation and performance forecasting.

In China, there are many reservoir types, so well test analysis becomes much more difficult. As far as the reservoir type is concerned, there are sandstone porous reservoirs, fissured reservoirs and fractured vuggy in carbonate rocks, biothermal massive limestone reservoirs, and irregularly distributed block shaped reservoirs in volcanic rocks; as far as the planar structure of a reservoir is concerned, there are well extended, uniformly distributed large area formations, fault dissected reservoirs with complicated boundaries, and banded lithologic reservoirs formed by fluvial facies sedimentation; as far as the fluid type is concerned, there are common dry gas reservoirs, condensate gas reservoirs, and gas cap gas reservoirs with oil rings and edge water or bottom water; and as far as reservoir pressure is concerned, there are gas reservoirs with normal pressure coefficients, extremely thick gas reservoirs with super high pressure, and underpressured gas reservoirs. As indicated, these reservoirs are richly varied, which has undoubtedly brought about new challenges to well test analysts and reservoir engineers.

Moreover, the quality of pressure data nowadays is no longer as it was in the early 1980s. At that time, pressure data were acquired by mechanical pressure gauges and the number of pressure data points read out from a pressure chart would be about 100 or even fewer than that. The results interpreted from such pressure data are not only simple, but also will not be controversial. Today, however, the number of data points acquired by electronic pressure gauges is usually as many as 10,000, or even 1,000,000; they consist not only of the pressure buildup interval but also the pressure whole history, including all flow and shut in intervals during the testing. Even very slight differences, if any, between the well test interpretation model obtained from analysis and the actual conditions, that is, the tested reservoir and the tested well, will be shown at once in the verification process during interpretation so that no careless error is allowed.

1.1.1 Well test: A kind of system engineering

Therefore, we can say that the well test today no longer merely means several formulas and simple calculations, but rather is a kind of systems engineering that includes several parts as follows:

1.Timely proposing of appropriate test projects by those persons in charge of exploration and development.

2.Creating an optimized well test design.

3.Acquiring accurate pressure and flow rate data onsite.

4.Interpreting acquired pressure data by well test interpretation software and integrating geological data and test technique; performing reservoir parameters evaluation.

5.Providing dynamic descriptions of gas wells and gas reservoirs by integrating the pressure and production history data acquired during production tests of gas wells.

6.Creating new well test models when necessary and adding them into well test interpretation software for future application.

1.1.2 Well test: Multilateral cooperation

The work listed previously should be carried out by different departments; each of them is associated with others, and each one affects the final results:

1.Only when leaders of the competent authorities have thoroughly recognized the important role of well test data in describing gas reservoir characterization and guiding development of the gas field can they arrange test projects in a timely manner and provide financial support for such projects to be executed.

2.Only by conducting optimized designs can we get better results with less effort and acquire pressure data that can explain and resolve our problems.

3.The acquisition of pressure data is usually done by service companies. The test crew of the service company, although working pursuant to the contract, should recognize what good data are and how to meet design requirements. The well test supervisor must check data before acceptance according to the design requirements, to ensure the success of data acquisition.

4.Data analysis will ultimately demonstrate the application value of the test results. In this book, such analysis is summarized as a dynamic reservoir description, which means using dynamic data acquired in gas wells, such as pressure and flow rate, as the main basis to evaluate the gas production potential of gas wells, while at the same time providing a description of geological conditions within the gas drainage area that affect gas deliverability and its stability, including reservoir structures, reservoir parameters, boundary distribution, and dynamic reserves controlled by this individual well, thereby guiding deliverability planning and development plan design for the gas field. This is usually accomplished through collaboration between performance analysts and reservoir engineers. Furthermore, only when such analysis results have been approved by the competent authorities can they play their due roles.

As part of the study of gas reservoirs from different perspectives or different individual positions, the purpose of this book is to explain how to jointly comprehend well test data and understand gas reservoirs for their proper development.

1.1.3 Writing approaches of this book

The approaches adopted in writing this book are as follows:

1.The application of well test methods aims not only at gas wells but also at gas reservoirs. Analyzing well test data should have the gas field or the gas reservoir in mind: it is in fact the goal that the author strives for.

2.Establish a graphical analysis method. The basis of the graphical analysis method is utilizing fundamental flow theories. Create a set of model graphs of the pressure curve and establish organic connections between flow characteristics in reservoirs and well test curve characteristics so that interpreters can take a quick look, that is, to understand reservoir conditions quickly and conveniently from measured well test curves.

3.Analysis of many field examples is another important feature of this book. This book introduces field examples of well test analysis applications not only to gas well studies but also to gas field studies; not only successful cases are examined but also some failed ones, from which some lessons are drawn, experiences summarized, and ultimately successes achieved through such continuous experiences.

4.Although some basic formulas are introduced in one chapter, this book will neither explain how to apply them in calculation nor derive them. This book is written for those who understand these formulas and shows how to make interpretations using well test interpretation software. This book will help readers grasp the correct interpretation and analysis methods, especially the research methods for gas fields. Regarding the derivation and application of these formulas, some very good monographs are available for reference (Jiang and Chen, 1985; Liu, 2008).

Therefore, this book is a good reference for well test analysis applications. Readers are herein expected, with the help of this book, to comprehend the essence of well test analysis, to acquire and apply well test data properly, and then to contribute a reliable description to the development of gas fields. It is the purpose of this book to help readers understand the well test comprehensively, make use of the well test properly, and establish and confirm dynamic models of gas fields correctly with the powerful means of the well test.

1.2 Role of well test in gas field exploration and development

The well test is indispensable in the exploration and development of gas fields. During the entire process, starting from when the first discovery well in a new gas province is drilled, to verification of reserves of the gas field, and to the whole history of its development and production, the well test plays very important roles in many aspects, such as confirming the existence of gas zones, measuring the deliverability of gas wells, calculating the parameters of the reservoir, designing the development plan of the gas field, and providing performance analysis during development. In fact, none of these tasks mentioned can be done without a well test. Table 1.1 indicates in detail the roles of a well test during the different exploration and development stages.

Table 1.1

1.2.1 Role of well test in exploration

1.2.1.1 Drill stem test of exploration wells

After discovering a potential structure in a new prospect exploration area, the first exploration wells are drilled. During drilling, the show of gas and oil (SG&O) may be discovered by gas logging or logging while drilling. At this moment, it is not certain whether the SG&O really means that those hydrocarbon zones are the zones with commercial oil/gas flow. In order to be certain, a drill stem test (DST) needs to be run. If the zones have quite high productivity during the DST, a further test for measuring their pressures and flow rates and a transient test for estimating their permeability and skin factor should be done.

High gas productivity of an exploration well foretells the birth of a new gas field, and flow rate and pressure data acquired in a DST are the direct evidence of the birth (Table 1.1).

1.2.1.2 Exploration well completing test

Further verification of the scale and gas deliverability of the gas field is generally carried out by well completing tests. These tests are usually run zone by zone when an exploration well has penetrated the target beds and well completion with casing or other modes has been done. At this moment, the borehole wall of the well is solid, the test conditions are fairly mature, and there is enough time for testing so that various parameters of the reservoir can be estimated more accurately. Different flow rates can be selected for the deliverability test so that the initial absolute open flow potential (AOFP) qAOF of the well can be calculated.

For some low permeability reservoirs, such as gas reservoirs in Carboniferous and Permian systems in the Ordos Basin, a commercial flow rate is not obtained by just the ordinal perforation completion, and it is necessary to recomplete the well by undertaking strong stimulation treatments such as acidizing and/or fracturing. In this situation, reestimating the skin factor and fracturing index is very important (Table 1.1).

Sometimes the expected gas production rate of a tested well or a tested reservoir cannot be obtained during the test after perforation; this may mean that the gas saturation is very low or there is no gas at all in the reservoir, but it is also possible that the permeability of the reservoir is so low and/or that the reservoir near the wellbore was damaged so seriously that the gas cannot flow from the reservoir to the wellbore. Distinguishing the real reasons for low production rates is extremely important for evaluation of the reservoir.

Skin factor is an important parameter indicating if a gas producing well has been damaged. Importance should be attached and much attention must be paid to every tested zone in which a transient pressure test can be run, especially to those zones with high permeability and low pressure, penetrated with dense drilling fluid and a long soak time, because those zones are probably damaged so seriously that their productivities are reduced too much. In this case, acidizing should be done to remove or reduce the damage; if the permeability of the tested zone is known from the well test to be very low, say less than 0.1 md, fracturing may be necessary to improve its productivity.

Whether the tested zone needs to be stimulated, and the effects of the stimulation treatment, are both identified by the well test.

1.2.1.3 Reserves estimation

Once data on production rate, reservoir pressure, and permeability of an exploration well have confirmed the birth of a gas field, estimation of reserves of the field should be commenced.

Several issues worth noting in reserves estimation

Volumetric methods are commonly used now for calculating reserves based on static data provided by geophysical prospecting, logging, and core analysis. Then the analogy method is applied to estimate the recoverable reserves by using a given recovery factor.

However, it has been discovered from practice in recent years that there is a serious risk in estimating reserves depending only on static data; the following problems, at the least, need to be noted:

1.Reserves calculated by the volumetric method are erroneous for fissured reservoirs with group and/or series distributed fissures.

Fissured reservoirs with group and/or series distributed fractures herein mean the ancient buried hill typed fissured reservoirs with heterogeneously distributed fractures; their special character is that the oil or gas is stored in the fissure system with areal and group and/or series distributed fractures. Also, some local regions of the system have very high permeability, and the matrix rock is very tight: that is, in well test terms, a double media with very high storativity ratio ω, whose value can be as high as 0.3–0.5.

This kind of reservoir can be best identified by the shape of the well test curves:

•The shape of pressure buildup curves, especially of a pressure derivative curve, is often very strange or unusual: it often has no obvious radial flow portion; it goes up and down steeply and so shows sharp fluctuations, and then approaches a trend of abrupt updip at a later time.

•The pressure drawdown curve declines rapidly, and the bottom hole pressure cannot build up to its original value after shutting in.

•In most of this kind of gas well, the water content ratio rises quickly after water breakthrough; the pressure buildup curve will become more complicated early on if there is condensate oil in the reservoir.

2.Estimation of recoverable reserves in lithologic gas reservoirs formed by fluvial facies deposition.

In the 1990s, many studies in the world showed that the recovery efficiency of some low permeability gas reservoirs formed by fluvial facies deposition is quite low. Further studies discovered that the existence of lithologic boundaries hinders the improvement of the recovery efficiency under the conditions of a normal well pattern (Junkin et al., 1995). It is possible to improve the recovery efficiency of this kind of reservoir by drilling infill wells.

3.Integral reservoir characteristics shown by pressure distribution.

If all gas wells in a gas field are located in an integral connected reservoir, when measuring their initial reservoir pressures and converting them from the measured depths into corresponding elevation depths, the relation of the initial pressures of these gas wells with the depths will be consistent with the pressure gradient measured in any single gas well in the reservoir. The overall characteristics of the gas field can be determined by such a simple principle.

If a gas field is an integral reservoir, the calculation of reserves is typically fairly simple; if not, the causes therein must be found and analyzed, combined carefully with its geologic characteristics and reflected in reserves estimation. Sometimes, the poor accuracy of tested pressure data brings difficulty to analysis and identification, or even makes the research insignificant. Therefore, paying more attention to this study and acquiring raw test data properly are undoubtedly the basis for all evaluation work. However, if the formations in the same horizon drilled by an exploration well are indeed not in the same pressure system, reserves estimation must be evaluated further.

Role of well test method in reserves estimation

During the exploration stage, well test data cannot be used directly in reserves calculation, but can supplement or correct it to a certain extent, including:

1.Providing deliverability as a basis of reserves calculation.

The evaluated original gas in place of gas reservoirs means reserves, under the condition that the flow rates of the gas wells meet the commercial flow standard. Whether the wells meet this standard or not must be evaluated by a well test. Sometimes the zones near the borehole have been damaged seriously during drilling and/or completion; therefore, the value of skin factor S of this type of well is very high and the flow rate is rather low, even very low. However, it met the commercial gas flow standard after stimulation treatment for eliminating the damage. Also, whether a gas well has been damaged and how much its absolute open flow potential is after stimulation must both be determined by the well test.

2.Providing characteristic coefficient of stabilized production for double porosity reservoirs.

Geologic studies very often regard all carbonate reservoirs containing fractures as double porosity but do not distinguish such a double porosity reservoir from homogeneous sandstones in reserves analysis. This special term of double porosity was suggested by Barenblatt et al. (1960) when he was studying the mathematical model of well tests for fissured reservoirs, and a flow model graph was also given by him. Barenblatt proposed two parameters: storativity ratio ω and interporosity flow coefficient λ, to describe flow characteristics of this kind of reservoir. The storativity ratio ω means the ratio of hydrocarbon stored in fissures to that stored in the whole reservoir, that is, in both fissures and the matrix of it. The greater the ω, the more hydrocarbon stored in fissures. Because the hydrocarbon in fissures can flow very easily into the well and be produced, it is therefore the fissures that bring a high flow rate at the beginning of production. However, if the ω value is high, as time elapses a little further, due to little hydrocarbon being supplemented from the matrix, the deliverability will drop sharply; however, if the ω value is very low, for example, ω = 0.01 or even lower, which means more hydrocarbon is stored in the matrix, the deliverability of the reservoirs will be very stabilized.

Another parameter, the interporosity flow coefficient λ, is also very important. It means the flow conductivity of hydrocarbon from the matrix to the fissures. If the λ value is fairly high, when the pressure in fissures decreases due to the fluid that flows into the well, the fluid in the matrix will be supplemented into the fissures promptly so that the well will maintain stable production. However, if the λ value is very low, even if quite a lot of hydrocarbon does exist in the matrix, the matrix still cannot feed the fissures sufficiently for a very long time, even as long as several years after an extremely sharp drop of fissure pressure. For this reason, such reserves have no commercial value at all.

Therefore, for reservoirs with double porosity characteristics, the parameters ω and λ calculated from the well test are really very important indices for diagnosis of the stabilized production characteristics of the reserves; ω and λ can be determined only by the well test. Moreover, determination of these two parameters imposes very stringent requirements on well testing conditions, as discussed further in Chapter 5.

There are a large number of fissured carbonate reservoirs in China. Some oil/gas wells have very high deliverability in the very beginning. Encouraged by this phenomenon, field management personnel may think that they have found a gold mine. However, they may fail to analyze the roles of parameters ω and λ properly. For example, some wells start flowing at a rate of 100,000 m³ of natural gas per day, but they last only a few days and then are depleted. This is indeed a bitter lesson to be learned.

3.Providing information about planar distribution of the reservoir for reserves estimation.

If reservoirs of a gas field extend continuously on a horizontal plane, only the outer boundary must be demarcated in reserves estimation, resulting in more room for maneuvering in placing development wells. When the well spacing is quite large in the early exploration stage, an effective thickness distribution map can only be drawn by the method of interpolation with a few thickness values of drilled wells, but this map cannot reflect the true distribution characteristics of the reservoir. However, well test data, especially long-term well test data, can authentically reflect the change of extension of the reservoir. For example, the area and shape of a block oil/gas field in which the tested well is located can be confirmed by well test analysis; the distance of the gas water contact to the tested well located in a gas field with edge water can also be estimated by well test analysis. Take the JB gas field as an example: the conclusion that its Ordovician reservoirs are widespread but extremely heterogeneously was obtained from the analysis of pressure buildups and interference tests between wells run during short term production tests; these results provided powerful evidence of the planar distribution characteristics of the reservoirs, thus freeing the managers’ minds of apprehensions about the reserves ultimately passing examination and approval by the National Reserve Committee of China. This example is discussed in detail later on in this book.

4.Providing original reservoir pressure data for reserves estimation.

In addition to being related to static parameters of reservoirs such as area, thickness, porosity, and gas saturation, the reserves of a gas reservoir are also proportional to its original reservoir pressure; for overpressured gas reservoirs, the influence of the original reservoir pressure is even more prominent. Therefore, the original reservoir pressure must be determined accurately before beginning reserves estimation of a gas reservoir.

It was required somewhere and sometime that the controlled reserves of an individual well must be calculated using data from every well test. Such a requirement is improper, for it has too much oversimplified reserves calculation from well test analysis or too many overestimated well test methods, and so no useful conclusions can usually be drawn.

When entering into a pseudo steady flow period at a medium late stage of development of a gas field, many methods can be used to check the reserves. This is discussed further in more detail later on in this book.

1.2.2 Role of well test in predevelopment

The dependence on well test data at this stage is definitely more serious.

A foreign company, for example, decided to develop a gas field in cooperation with a Chinese partner. The reserves of this gas field had been examined and verified. The company insisted on spending a year of time and much manpower and money to conduct dynamic tests and analysis on more than 10 wells. Initially, the necessity of doing so was suspected, but later it was proved to be effective. It is just this dynamic performance research that results in what has become the decisive basis for making development plans.

Many uncompartmentalized gas fields have been discovered in China in recent years—the number of them is more than the number of those ever discovered before. Performance research during the predevelopment stage is also gradually being put on the agenda. It is therefore especially important to focus on performance research based on previous experiences and lessons learned.

1.2.2.1 Deliverability test of development appraisal wells

Deliverability values of individual wells are taken as the primary basis for making development plans. The AOFP is usually used to indicate the deliverability level. The inflow performance relationship (IPR) curve is further required to be plotted from the initial deliverability analysis.

Just as is discussed in Chapter 3 of this book, several deliverability test methods are used onsite to determine the AOFP. The deliverability test methods applied in exploration and predevelopment stages are different: in the exploration stage, some simple methods, for example, the single point test method, can be used only for identifying whether the deliverability of the gas well has met the commercial gas flow standard, and for setting up the lower limit of it for reserves estimation in the predevelopment stage, however, the deliverability test is not only for accurate calculation of deliverability indices and the planar distribution of the reservoirs in the gas field, but also for finding out the long-term stability characteristics of the deliverability.

It will be introduced in Chapter 3 of this book that, for some low permeability lithologic gas reservoirs formed by fluvial facies sedimentation, because the effective drainage area controlled by an individual well is limited and the flow ability of reservoirs is poor, the transient absolute open flow potential evaluated during the early stage of exploration would be very different from the commonly referred deliverability under stable production conditions. Sometimes, such a difference could be 10 times or even larger. Some Chinese and overseas research results suggest that if the reservoirs are confirmed to be like this, a new development strategy should be adopted. In addition, those fissured reservoirs with group- and/or series-distributed fractures in buried hill gas fields obviously cannot be put into production with a conventionally designed stable production rate.

Therefore, during the predevelopment stage, a systematic and rigorous deliverability test of development appraisal wells is essential for a gas field, especially for a large uncompartmentalized gas field. Test analysis and calculations must not only give conventional initial AOFP, but also evaluate and provide dynamic deliverability indices during the production process, and even provide a proper production rate arrangement over its whole life by well test analysis and deliverability prediction conducted by software when necessary (Mattar et al., 1993).

1.2.2.2 Transient well test of development appraisal wells

Development appraisal wells in large uncompartmentalized gas fields in China are usually studied by short term production tests today. During the short term production tests, high-precision electronic pressure gauges are used to measure or monitor the bottom hole pressure (flowing pressure and shut in pressure) throughout the entire process. Such tests not only can determine the deliverability of gas wells, but also can provide shut in pressure buildup curves and the entire pressure history. Just like what is shown in Table 1.1, much important information about a gas reservoir can be obtained from the tests, such as:

1.Information about distribution of gas bearing areas and gas bearing formations in the gas fields.

2.Initial reservoir pressure pi.

3.Initial absolute open flow potential and dynamic absolute open flow potential of main gas zones, as well as planar and vertical distribution of the deliverability.

4.Effective permeability of gas zones and the relationship between effective permeability (from well test analysis) and permeability from logging analysis.

5.Information about damage of gas wells, whether acidizing and/or fracturing stimulation treatment is needed, and the skin factor after stimulation.

6.For fractured wells, estimation of the effect of the fracturing treatment and calculation of the length, permeability thickness, and skin factor of the generated fracture.

7.For double porosity reservoirs, when significant double porosity characteristic curves appear, storativity ratio ω and interporosity flow coefficient λ values are analyzed, and special properties of the reserves and stabilized production characteristics are evaluated.

8.Non Darcy flow coefficient during the production of gas wells is provided. In the design of gas field development plan, non Darcy flow coefficient D must be used whenever selecting parameters related to the relationship between flow rate and pressure drawdown. Non Darcy flow is formed due to turbulent flow near the bottom hole, and the skin due to non Darcy flow is a major part of the pseudo skin. The reasons resulting in turbulent flow are very complicated. Non Darcy flow coefficient D can only be determined by the well test, as errors are always generated when it is estimated by theoretical methods.

9.Information about reservoir boundaries can be obtained if the pressure buildup test lasts long enough. Also, if information about boundaries is obtained soon after beginning the test, it indicates the boundaries are near to the tested well.

Well test interpretation software today usually contains well test models comprising different types of boundary combinations. Furthermore, numerical well test software is able, by considering the specific geological characteristics of the gas zones, to assemble the reservoir model with proper shaped boundaries and formation parameter distributions and to provide related theoretical well test curves. Vivid descriptions of a specific tested object can be obtained by matching the theoretical well test curves with measured ones. It is especially worthy to note that such a description comes from the vivid exhibition of gas zones in the process of production and so reflects the features much closer to the reality.

10.If conditions allow, the planar and vertical connectivity of layers in the reservoir can be studied through an interference test between wells or a vertical interference test.

An interference test between wells is very difficult to run in gas fields. This is simply because the compressibility of natural gas is much greater than that of oil or water. Moreover, the permeability of gas zones is usually very low and the well spacing is large, so that a successful test often takes a long time. In the JB gas field, for instance, the interference test between well L5 and other wells lasted 10 months. This test delivered extremely valuable knowledge: it verified the interwell communication within the gas-bearing area and also revealed obvious heterogeneity characteristics.

11.In principle, the dynamic reserves of gas wells and the gas bearing area can be predicted on the basis of these successful well tests.

The in principle here refers to the fact that the dynamic reserves predicted by the results of dynamic tests are only the reserves in the area that have been influenced by the dynamic tests, but do not contain the reserves outside this area.

If a gas well is located within a closed or nearly closed lithologic block, the reserves within the block affected by this well can be estimated by analysis of dynamic characteristics. However, data from this well mean nothing for judgment of another very closely adjacent region partitioned by the boundary.

If the well is located within part of a continuously distributed reservoir, dynamic data cannot cut the boundary of the region controlled by any adjacent wells, and therefore dynamic data can only provide information about the mutual connection of these wells.

1.2.2.3 Well test of pilot production test wells

If the pilot production test wells have already been connected to the pipeline network and so can produce continuously for several months, they can provide much richer information that can be used in the design of development plans. In particular, the dynamic models of the gas wells can be improved through pressure history verification.

1.During a long term production test, the influence of boundaries around the gas well will be gradually reflected in the decrease of bottom hole flowing pressure. The dynamic model of gas wells can be improved by verifying pressure history, adding and/or modifying boundary influences, adjusting the location and distance of the boundary to the well, and so on.

2.A perfect modified dynamic model not only verifies and confirms the formation parameters near the well but also determines the area and dynamic reserves of the area controlled by the well and so can be used for performance prediction.

3.A perfect modified dynamic model of a gas well in a constant volume block can be used to calculate the average reservoir pressure during production and the variation of dynamic deliverability indices.

1.2.2.4 Selection and evaluation of stimulation treatment

Selecting the stimulation treatment measure is a very critical element in the development plan. However, evaluating whether a gas well needs stimulation treatment and the effectiveness of such stimulation treatment can only be done by well test analysis. In some foreign countries, the field owner must, when engaging a service company to implement gas well stimulation treatment, first provide the parameters of its geology and completion and those from well test evaluation of the well, so that the stimulation measure can be designed; after stimulation treatment, in order to evaluate the effectiveness of the treatment, the owner must also request third parties, such as a well test service company or relevant consulting company, to appraise the results of the treatment by well test analysis.

1.2.2.5 Verifying reserves and creating the development plan

Only after completing the performance analysis and research mentioned earlier does the time for creating the formal development plan really come.

1.The reserves have been verified by performance research, in which parameters provided by a transient well test were used.

2.Reservoir parameters have been corrected. Permeability k, for example, is not the permeability from logging interpretation but the effective permeability; skin factor S, non-Darcy flow coefficient D, double porosity parameters ω and λ, and so on are also the parameters actually acquired from the formation. In addition, the description of reservoir boundaries is a particularly very critical condition for numerical simulation.

3.The production test history can be used to match and correct the parameters used for numerical modeling.

When the requirements mentioned here are all met, a numerical simulation study can be carried out and a practical and feasible development plan can be made.

1.2.3 Role of well test in development

Conventional well test methods can be used almost throughout the entire development process of a gas field to provide dynamic monitoring, without any difficulties brought from swabbing and so on, such as is the case in oil fields.

For a normally producing gas well, however, unless permanent bottom hole pressure gauges are used, it is obviously inappropriate to perform a well test by operations that run pressure gauges in the hole and put them out of the hole, while frequently opening and shut in the well. In fact, because the formation conditions have already been known thoroughly through early research, retesting the well is required only in the case of anomalous events happening during production of the gas well.

However, the following tests are absolutely necessary:

1.Regular monitoring of downhole flowing pressure and static pressure for inferring dynamic deliverability indices of the gas well.

2.For newly drilled adjustment wells, the basic formation parameters must be obtained from well test analysis and their initial deliverability equation must be established before putting into production (see Table 1.1).

It is noted through the aforementioned analysis that items listed at the top right corner in Table 1.1 are blank, meaning that these items are not feasible. More deepening or intensive understanding of the reservoir can only be obtained through well tests, as the gas field research is being deepened continuously. It is not practical to expect that all these parameters can be determined simply through well tests during the early stage of exploration. For example, it is impossible to determine the exact initial AOFP of an exploration well simply through short term DST; it is also impossible to do an overall analysis of boundaries or to determine the double porosity parameters of reservoirs simply through very short term well tests in the exploration wells. Even if well test analysts do give those parameters mentioned previously, such parameters are merely speculative and cannot suffice as a basis for further analysis. However, as more gas wells are put into production tests or production, and as the flowing of these wells goes on and the radius of influence increases, and pressure buildup testing lasting quite a long time is carried out, the research work will continue to intensify. Some parameters, which could not be obtained previously, can and should be determined through well test analysis at this time; such parameters include initial and dynamic deliverability indices; boundary distance Lb and shape; block sizes A; double porosity parameters ω, λ; double permeability formation parameters κ; composite formation parameters Mc and ωc; non Darcy flow coefficient D; reservoir connectivity parameters ɛ and η; and the dynamic reserves of block. With this knowledge, the gas reservoir dynamic model can be established and used effectively for the performance analysis of gas zones and gas reservoirs. These are just the phased and comprehensive characteristics of a well test.

1.3 Keys of well test analysis

Well test research started in the 1930s. By the 1970–80, it evolved into the modern well test. Through advances in theoretical research on flow mechanics and continuous improvements in well test software, the role of the well test in gas field exploration and development expands and deepens continuously.

What are the key elements of well test research? What has been driving the advances and development of well test research? How does the well test serve gas field studies? All these questions are roughly answered in Fig. 1.1.

Fig 1.1 Illustration of well test research contents.

1.3.1 Direct and inverse problems in well test research

Well test research roughly resolves two types of problems: direct and inverse.

Direct and inverse problems are defined from the viewpoint of information theory. A direct problem means describing the performance of a known formation in terms of its gas production rate and reservoir pressure on the basis of flow mechanics theory, whereas resolving an inverse problem means, if the variations of gas production rates and bottom hole pressures of one or several wells in a gas reservoir during their flowing and shut in process have been measured, finding out inversely the static conditions of the gas zones, including the values of formation parameters, the structure of permeable areas in the reservoir, the planar distribution of gas zones, and so on.

This book explains the procedure of resolving these problems by well test research with the hope that readers of this book, especially those interested in participating in well test research, can correctly locate the jobs they are participating in or are interested in and straighten out the relationship between well test research and geologic research.

Early well test research failed to distinguish different types of formations or believed that all formations were homogeneous media identically. The semilog straight line analysis methods [Miller-Dyes-Hutchison (MDH) method and Horner method] invented in the 1950s found that flow will enter the radial flow stage that reflects reservoir conditions when wellbore storage disappears; in this stage, pressure variation shows a straight line on semilog paper: that is, the coordinates of pressure vs logarithm of time, and an inverse proportion relationship exists between the slope of the straight line m and the formation permeability k:

This is simply the basis of the conventional well test interpretation method that uses the well test method to determine formation parameters inversely (Miller et al., 1950; Horner, 1951).

However, measured curves are far more complicated, especially in the case of carbonate formations, multilayer formations, or formations with complex boundaries; in these cases it is often very difficult to find out proper straight line portions. Furthermore, straight line sections alone can hardly describe other characteristic parameters of the reservoir. Therefore, in the 1970s, the type curve match method was created (Agarwal et al., 1970; Gringarten et al., 1979; Bourdet and Gringarten, 1980; Earlougher, 1977).

In the early 1980s, Bourdet invented pressure derivative type curves (Bourdet et al., 1983). On this type of curve, each kind of flow in the formation corresponds to a special characteristic pattern, while each kind of flow is determined by the special geological conditions of the specific formation. Therefore, an organic connection is established between the geologic characteristics and the graphical characteristics.

So far, the combination of log-log analysis (i.e., pressure and its derivative type curve match analysis) and semilog analysis (i.e., conventional analysis method) has formed the dominant theoretical foundation of modern well test interpretation and has become the dominant analysis method of well test interpretation software. A wide variety of calculation formulas and analysis plots were used before—provided they can be integrated into the modern well test interpretation model, they can be added into the interpretation software and widely used. However, as analysts become more dependent on well test analysis software, some other methods, such as the Y-function method for judging the presence of faults, the Masket method for calculating formation parameters, and various unique point methods for conducting interference test analysis, are increasingly losing their chance of being used.

1.3.2 How to understand direct problems

The process of establishing the relationship between characteristics of the formation and those of well test plots starts from solving the direct problem. The research tasks in resolving the direct problem can be summarized in several parts, as described in the following sections.

1.3.2.1 Analyzing the formation where the oil/gas well locates and classifying it geologically

The geologic bodies across China where the gas fields locate are very complicated; their rough classification is given in Table 1.2. For easy comparison, Table 1.2 also lists typical examples of gas fields in China. In fact, there are far more types of gas reservoirs than these, and even many different types may exist simultaneously in one gas field (Wang, 1992).

Table 1.2

1.3.2.2 Classifying, simulating, and reproducing formation from the viewpoint of flow mechanics

It is seen that the generating conditions of various reservoirs are very different; if described by flow mechanics equations, they must be simplified and classified into some major categories, and the description must be used only within a certain scope. Sandstone reservoirs, for example, are usually simplified into a model of an infinitely homogeneous porous medium. Strictly speaking, the existence of this type of reservoir in nature is impossible. However, the well testing duration is limited and so the range of pressure influence is also limited; therefore, within such limited scopes of time and space, the target being studied can be considered roughly consistent with an infinitely homogeneous formation.

Based on the knowledge mentioned previously, reservoirs can be further simplified and classified from the viewpoint of flow mechanics, as shown in the following lists.

Basic medium types

•Homogeneous medium, including sandstones, fissured carbonate rocks showing homogeneous behavior, etc.

•Double porosity medium, including sandstones and carbonate rocks comprising natural fissures

•Double permeability medium, mainly means layered sandstones

These media are usually assumed to have laminar two dimensional distribution.

Bottom hole boundary conditions (i.e., inner boundary conditions)

•General completion condition of wellbore storage and skin

•Completion condition of having hydraulic fracture connecting the well hole

•Partially perforated completion conditions

•Completion conditions of horizontal wells or deviating wells

Outer boundary conditions

•Infinitely outer boundary

•Impermeable outer boundaries of single straight line or of some patterns formed by several impermeable boundaries

•Closed outer boundary: closed small faulted blocks or lithologic traps

•Heterogeneous boundaries formed by variation of lithology or fluid properties

•Semipermeable boundaries, congruent boundaries of river channels formed by fluvial facies sedimentation in different periods

•Constant pressure boundaries (in oil reservoirs only)

Assumption of fluid properties

•Oil, gas, water, or condensate gas

•Any combination of oil, gas, and water

Any assemblage of any four elements, each one of them having been selected from one of the four aforementioned conditions, constructs a physical simulation for a certain gas reservoir and reproduces the behavior of a specific gas field during the research process.

1.3.2.3 Constructing the well test interpretation model and resolving the related problem

The so called well test interpretation model should contain both a physical model and a mathematical model.

The lists given in Section 1.3.2.2 are just the descriptions of physical models. At the same time, these physical models can also be expressed in mathematical forms. For example, the flow in different types of media can be expressed by different differential equations; different boundary conditions can also be expressed by different mathematical expressions. These are the so called mathematical models.

In the 1960s, the physical models mentioned earlier were materialized during the study of well test problems. Man made sandstone bodies were built and used in the laboratory as a reduced physical micro miniature formation or model. The model was saturated with oil or water, and the flow rate change was implemented by drilling holes in the model. The pressure change at individual points on the model was measured. Such a practice, however, not only was very difficult in constructing the model and very costly, but also could hardly simulate the elastic transient process. Therefore, it was abandoned long ago.

Establishing the mathematical equations correctly is only the beginning of the process of resolving direct problems, while solving these equations is really more important.

In