Naturally Fractured Reservoirs

Roberto Aguilera

DEDICATION

Dedicated to my wife, Maria Ester; my daughter, Maria Silvia, and, my sons, Roberto Federico and Carlos Gustavo.

Acknowledgements

I wish to thank the Society of Petroleum Engineers, the Petroleum Society of CIM, the Society of Professional Well Log Analysts, the American Association o j' Petroleum Geolo-gists, Western Atlas, Schlumberg"r, Halliburton, Gulf PubliShing Co., l'ennWell Publishing Co., and Petm/ellln Engineer for pe rmission to draw material from their publications. In addition, I express my gratitude to the various authors and organ izations that have pub-li shed material on the subject of natllTally fractured reservoirs.

Although I am the only person responsible for the final fo rm of this book, I would like to thank Dr. H.K. van Poollen and Dr. G.R. Pickett, r.i.p., for their help and encouragement during the devdopment of some of the concepts and techniques presented here. Me. Alonso Marin, r.i .p., provided valuable help and guidance during my early years as it petro-leum engineer.

During the last 16 years I have been teaching courses on the subject of naturally frac-tured reservoirs all over the world. 1 thank my students for their questions and suggestions that have led to this second edition of my book.

Since 1984 I have been privileged to cooperate with AAPG presenting a course on frac-tured reservoir analysis with Dr. David Steams, Dr. Melvin Friedman, and Dr. Ronald Nelson . Chapter 1 of this book dealing with geologie aspects reflects many of the geolog-ical techniques I have learned with Dave, Melvin. and Ron. However, the responsibili ty for the way in which these techniques are presented is only mine.

Last, but not least, I wish to thank my wife, Maria Ester; my daughter Maria Silvia, and my sons, Roberto Federico and Carlos Gustavo, for their patience and understanding dur-ing the long evenings, weekends anti hOlidays needed for the preparation of this book.

vii

Preface .~.

Since the publication of the first edition of my book on naturally fractured reservoirs in 1980, the science and art of evaluating these types of reservoirs has advanced at a rapid pace. ,

The first edition included in a single book many subjects that are usually discussed in separate volumes. There were nine chapters of the first edition that included geologic aspects, drilling and completion methods, log interpretation, well testing, fractured shales, primary and secondary recovery, numerical simulation, case histories, and eco llOinic ('val uatlons.

Initially I attempted to include the same nine chapters in this second edition. However, I quickly realized that due to the many advances in the field, this was not going to be fea -sible. Consequently, this second edition includes only six Chapters dealing with geologic aspects, drilling and completion, formation evaluation by well log analysis, tight gas rcser-voirs/ case histories, and economic evaluations and reserves. Presently I an1 working on a second volume covering other aspects of naturally fractured reservoirs.

Chapter 1 deals with geOlogic aspects of naturally fractured reservoirs, reasons for gen-erations of fractures including tectonic, regional, contractional and surface-related fra c-tures, migration and accumulation of petr61eum, direct and indirect SOurces of informa-tion, fractured reservoirs in varioll s lithologies, how to avoid \valking a'vvay fr01u a commercial fractured reservoir due to an lInpropcr evaluatiOn, and tilt! importance of in-situ stresses on the study of naturally fractured reservoirs.

Chapter 2 reviews some important drilling and completion concepts for deviated holes. Since most fractures at depths of interest arc verti cal to subvcrticial, deviated and horizon-tal wells probably stand better ch~nccs of l'il1ding hydrocarbons than vertical wells. The advantages and disadvantages of open-hole vs. perforated completions are reviewed. Key

. elements a~sociatcd with hydraulic fracturing and aci dizing of naturally fractun:~d reser-voirs are also discussed. '

Chapter 3, which focuses on formation evaluation by well log analysiS, examines the use of many conventional and specialized well log curves in the qualitative and quantitative evaluation of naturally fractured reservoirs. The iinportance of electric and sonic imaging tools is discussed in detail. Special techniques dealing with the porosity exponent III and the water saturation exponent Ii allow quantitative estimates of porosity and watcr satu-ration in matriX, fractures and the combined matrix-fractures system. The effect of lithol-ogy variations and shalincss is rcvicwttl a.s well as logging of horizontal wells: The uncer-tainty of calculating hydrocarbons-in-place in fracture media and the effect of miscallibrated logs is also analyzed.

Chapter 4 is concerned with tight gas reservoiIS where production is possible in many cases thanks to the presence of natural fractures. Many of these reservoirs are also multi-layered adding a great deal of complexity to the evaluation. The chapter covers both frac-

. tured shales and tight gas sands. Geographical distribution is discussed, Various methods of well log interpretation, well test analysis and performance forecasts are reviewed in detail.

Chapter 5 pres~nts case histories of naturally fractured reservoirs around the world. Giant, modest, and non-commercial oil and gas reservoirs are reviewed, highlighting the most important features associale(l with each one of them. Fractured reservoirs in sand-stones, ca'[bonates, cherts, shales, basement and tight gas sandstones are considered ill thj s chapter. The case histories include the effect on recoveries of aqUifers, vario us injection

Ix

x Preface

schemes (w\\ter, gas, polymrs, C02, steam), subsidence, rapid pressure decline, strong gravity scgr~gation with counterflow, fold and fault-related fractures, vertical communica-tion through fractures, slanted and horizontal wells, and retrograde condensation.

Chapter 6 discusses some of the most important aspects of economic analysis. The opti-mum equilibrium between well spacing, maximum efficient rate and economic recovery is reviewed. Emphasis is placed in the economic analysis of acceleration projects as recovery from most naturally fractured reservoirs is directly related to 'actual acceleration projects. The effect of directional and horizontal wells on costs ,md recoveries is reviewed. The

chapte~ includes reserves definitions as provided by various organizations and some guide-lines based on my experience for estimating oil and gas reserves in naturally fractured reservoirs.

Contents

" ,.:

.... .

CHAPTER 1 GEOLOGIC ASPECTS What Is a Natural Fracture? What Is a Naturally Fractured Reservoir? Requirements for Hydrocarbon Accumulation Porosity Ductility and Brittleness Reasons for Generation of Fractures Storage Capacities Fracture Morphology. Permeability Mechanical Behavior of Rock Fractures ClaSSification Migration and Accun~ulation Sources of Information Mapping Fractured Trends Fractured Reservoirs Exploring fo r Naturally Fractured Reservoirs Subsurface Fracture Spacing

Imp~ct Fractures Effect of Fractures on Flow Behavior Undiscovered Naturally Fractured Reservoirs, Why and How? Practical Applications References

CHAPTER 2 DRILLING AND COMPLETION METHODS Drilling Fluids Drilling Mud Air and Gas as Drilling Fluids Directional Drilling Horizontal Drilling Completion Methods Hydraulic Fracturing Acidizing Explosive Fracturing References

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1

1 1 1 4 7 9

11 14 14 20 24 30 32 42 50 61 68 77 77 77 85 91

97 97 97 i

103 -I 104

I 124 137 I 148 173 I 177 I 177

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xii Contents

CHAPTER3 FORMATION EVALUATION BY WELL LOG ANALYSIS Sonic Amplitude Logs Variable Intensity Logs Long Spacing Sonic Shear-Wavc Splitting. Induction Log!: Dual Induction Laterolog 8 Sonic, Neutron and Density Logs Comparison of Porosity IOstimates from Different Sources Borehole Televiewer Diprneter and Fracture Identification Log (I'lL) Formation Microscanner and FulJbore Formation Microimager Spontaneous Potential Cnrrcction Curve on the CompcllSated Density Log Comparison of Shale Volume to Uranium Index Lithoporhsity Crossplot Production Index Log Temperature Log Sibilation Log Kerogen Analysis Gamma Ray Circllmferential Acollstical Log Dual Laterolog-Microspherically Focused Log P, Curve on the Litho-Density Log Sonic Porosity Greater than Neutron-Density Porosity Mud Log Velocity Ratio Plot Production Logs Fracture Plausibility Fracture Evaluation in Cased Holes Quantitative Analysis Lithology Det~rmination Estimate of Water Saturation Exponent (n) from Logs Flushed Fractures Water Saturation of the Composite System Practical Application Estimates of Water Saturation in Matrix and Frattures Fracture Completion Log Shaly Formations Nuclear Magnetic-Resistivity Comhination Lithodensity-Resistivity Combination Electromagnetic Propagation (EPT)--Resistlvity Combination Borehole Gravimeter-Resistivity Combination Effect of Variations in Water Resistivities Horizontal Wells Measnre of Uncertainty in Log Calculations Original Oll-In-Place Measure of Uncertainty Hydrocarbon Recoveries from Logs

181 181 183 184 186 188 189 189 190 191 192 194 201 201 202 203 203 205 207 209 210 210 211 215 215 218 219 221 225 226 231 251 257 259 259 264 267 268 268 280 285 286 287 289 291 300 300 302 306

Geothermal Reservoirs Miscalibration of Logs General Remarks References " ..

CHAPTER 4 TIGHT GAS RESERVOIRS Fractured Shales Worldwide Distribution of Black Shale Oil and Gas Occurrence in Fractured Shales Remote Sensing Surveys GeophYSical Investigation Physical Characterization Geochemical CharacterizatIon Stimulation Interaction between Hydraulically Induced ami Natural Fractures Metamorphism Log Analysis Formation Strength Parameters Estimates of Gas-in-Place Recoverable Gas Well Testing Tight Gas Reservoirs References

CHAPTER 5 CASE HISTORIES Fractured Sandstones Fractured Carbona tcs Fractured Cherts

. Fractured Shales Fractured Basement Tight Gas Reservoirs Naturally Fractured Reservoirs StatiSlics Reiereliccs

CHAPTER 6 ECONOMIC EVALUATION AND RESERVES Compound Interest Period, Effective, Nominal, and Continuous Interest Rates Discounted Cash Flows Net Present Worth Future Worth Rate of Return Payout Time Profit to Investment Ratio Inflation Acceleration Projects Comparing Evaluation Methods Depreciation, Depletion arid Amorti zation

Contents xiii

306 306 309 309

315 315 315 316 321 322 322 323 323 327 327 328 342 343 345 365 369 379

383 383 400 443 447 447 448 450 457

461 461 464 464 . 465 466 466 468 469 470 470 482 482

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Tax Treatment Well Spacing Risk Evahlatiort Size of Helds found Monte Carlo Simulation Direct.ional and Horizontal Wells Reserves Other Definitions Guidelines for Estimating Oil and Gas Reserves References

Appendix

Index

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t

487 489 490 ~ 492 494 494 496 499 5,62 504

507

515

Ii

CHAPTER 1

Geologic Aspects Many of present-day producing naturally fractured reservoir~ have been accidentally dis-covered when looking for some other type of reservoir. Some years ago McNaughton and Garb (1975) estimated that ultimate recovery from producing fractured reservoirs would surpass 40 billion stock tank barrels of oil (STllO). Today I firmly believe that this figure was very conservative. I am con vinced that there are significant volumes of hydrocarbons that have been left behind pipe as undiscovered, or behind plugged and abandoned wells or because of vertical wells that have not intercepted vertical fractures. Figure 1-1 shows the location of some important naturally fractured reservoirs. They are found all over the world, in all types of lithologies and throughout the geologic strati-graphic column. This is demonstrated in Chapter 5 dealing with Case Histories.

WHAT IS A NATURAL FRACTURE? A natural fracture is a macroscopic planar discontinuity that results from stresses that exceed the rupture strength of the rock (Stearns, 1990).

Another definition proVided by Nelson (1985) is as follows: itA reservoir fracture is a natu!ally occurring Inacroscopic planar discontinuity in rock due to deformation or physical diagenesis."

WHAT IS A NATURALLY FRACTURED RESERVOIR? A naturally fractured reservoir is a reservoir which contains fractures created by mother nature. These natural fractures can have a positive or a negative effect on fluid flow. Open uncemented or partially mineralized fractures might have, for example, a positive effect on oil flow but a negative effect on water or gas flow due to coning effects. Totally mineral-ized natural fractures might create permeability barriers to all types of !low. This in turn might generate small compartments within the reservoir that can lead to uneconomic or marginal recoveries.

In my opinion a1l reservoirs cuntain a certain alnount of natural fracturing. However, froIn a geologic and a reservoir engineering point of view, 1 regard as naturally fractured reservoirs only those where the fractures have an effect, either positive or negative, on fluid tlow as suggested by Nelson (1985).

REQUIREMENTS FOR HYDROCARBON ACCUMULATION In general, a petroleunl reservoir consists of source rock, reservoir rock, seal rock, trap, and fluid content.

Source rock, Or source environment, is believed to be responsible for the origin of petro-leum. Most geologists believe that the origin of petroleum is organic, related mainly to veg-etables which were altered by pressure, temperature, and bacteria. Some geologist (Hunt et aI, 1992), however, believe that the origin of petroleum is igneous and indicate that oil rises from depth in granitic shield terrains of the world. 1

CoverNaturally Fractured ReservoirsDEDICATIONAcknowledgementsPrefaceContents1 Geologic Aspects2 Drilling and Completion Methods 3 Formation Evaluation by Well Log Analysis 4 Tight Gas Reservoirs5 Case Histories 6 Economic Evaluation and Reserves Appendix Index