Naturally Fractured Reservoirs
Dedi cated to my wife, Maria Ester; my daug hter, Maria Silvia , and, my sons, Roberto Federico and Carlos Gustavo.
I wish to thank the Society of Petroleum Engineers, the Petroleum Society of CIM, the Society of Professional Well Log Analysts, the American Associati o n o j' Petroleum Geologists, Western Atlas, Schlumberg"r, Halliburton, Gulf PubliShing Co., l'ennWell Publishing Co., and Petm/ellln Engineer for pe rmission to draw material from th eir publication s. In addition, I express my gratitude to the various authors and organ izati ons that have publi shed material o n the subject of natllTally fra ctured reservoirs. Altho ugh I am the only perso n 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 petroleum engineer. During the last 16 years I have been teaching courses on the subject of naturally fractured 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 fractured reservoir analysis with Dr. David Steams, Dr. Melvin Friedman, an d Dr. Ronald Nelson . Chapter 1 of this book dealing with geologie aspects reflects many of the geological techniques I have learned with Dave, Melvin. and Ron. However, the responsibility 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 daught er Maria Silvia, and my sons, Roberto Federico and Carlos Gustavo, for their patien ce and understanding during the long evenings, weekends anti hOlidays needed for the preparation of this book.
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, fracture d 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 rcservoirs/ 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 generations of fractures including tectonic, regional, contractio nal and surface-related fra ctures, migration and accumulation of petr61eum, direct and indirect SOurces of information, 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 horizontal wells probably stand better ch~nccs of l'il1ding hydrocarbons than vertical wells. The advantages and disadvantages of open-hole vs. perforated compl etions are reviewed. Key . elements
with hydraulic fracturing and aci dizing of naturally
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 watc r saturation in matriX, fractures and the combined matrix-fractures system. The effect o f lithology variations and shalincss is rcvicwttl a.s well as logging of horizontal wells: Th e uncer-
tainty of calculating hydrocarbon s-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 multilayered adding a great deal of complexity to the evaluation. The chapter covers both fra c. 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 sandstones, 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 inj ection
Preface schemes (w\\ter, gas, polym€rs, C02, steam), subsidence, rapid pressure decline, strong gravity scgr~gation with counterflow, fold and fault-related fractures, vertical communication through fractures, slanted and horizontal wells, and retrograde condensation. Chapter 6 discusses some of the most important aspects of economic analysis. The optimum 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 guidelines based on my experience for estimating oil and gas reserves in naturally fractured reservoirs.
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 Method s Hydraulic Fracturing Acidizing Explosive Fracturing References
1 1 1 4
7 9 11
14 14 20 24 30 32 42 50 61 68 77 77
77 85 91
97 97 97 103 104 124 137 148 173 177 177
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
181 183 184 186 188 189 189 190 191 192 194 201 201 202 203 203 205 207 209 210 210 211
215 215 218 219
225 226 231 251 257 259 259 264 267 268 268 280 285 286 287 289 291
300 300 302 306
Contents xiii Geothermal Reservoirs Miscalibration of Logs General Remarks References "..
TIGHT GAS RESERVOIRS
Fractured Shales Worldwide Distribution of Black Sh ale Oil and Gas Occurre nce in Fractured Shales Remote Sensing Surveys GeophYSical Investigatio n Physical Characterization Geochemical Cha racterizatIon 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
306 306 309 309 315
315 315 316 321 322 322 323 323 327 327 328 342 343 345 365 369 379
CHAPTER 5 CASE HISTORIES Fractured Sandstones· Fractured Carbona tcs Fractured Ch erts . Fractured Shales Fractured Basement Tight Gas Reservoirs Naturally Fractured Reservoirs StatiSlics Reiereliccs
ECONOMIC EVALUATION AND RESERVES
Com pound 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
383 400 443 447 447 448 450 457
461 464 464 . 465 466 466 468 469 470 470 482 482
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
487 489 490 492 494 494 496 499
Ge olo gic As pec ts
Many of presen t-day produc ing natura lly fractur ed reservoir~ have been accide ntally discovere d when lookin g for some other type of reservoir. Some years ago McNa ughton and Garb (1975) estima ted that ultima te recovery from produc ing fractur ed reservoirs would surpas s 40 billion stock tank barrels of oil (STllO). Today I firmly believe that this figure was very conser vative. I am con vinced that there are signifi cant volum es of hydroc arbons that have been left behind pipe as undisc overed , or behind plugge d and aband oned wells or becaus e of vertica l wells that have not interce pted vertica l fractur es. Figure 1-1 shows the locatio n of some impor tant natura lly fractur ed reservoirs. They are found over the world, in all types of litholo gies and throug hout the geolog ic stratigraphi c colum n. This is demon strated in Chapt er 5 dealin g with Case Histories.
WHA T IS A NATU RAL FRACTURE? A natura l fracture is a macro scopic planar discon tinuity that results from stresses that exceed the ruptur e streng th of the rock (Stearns, 1990). Anoth er definit ion proVided by Nelson (1985) is as follows: itA reservo ir fracture is a natu!a lly occurr ing Inacro scopic planar discon tinuity in rock due
to deform ation or physic al diagen esis."
WHA T IS A NATURALLY FRACTURED RESERVOIR? A natura lly fractur ed reservo ir is a reservoir which contai ns fractur es created by mothe r nature . These natura l fractures can have a positiv e or a negativ e effect on fluid flow. Open uncem ented or partial ly minera lized fractures might have, for examp le, a positiv e effect on oil flow but anegativ e effect on water or gas flow due to coning effects. Totally minera lized natura l fractur es might create perme ability barriers to all types of !low. This in turn might genera te small compa rtment s within the reservoir that can lead to unecon omic or margin al recove ries. In my opinio n a1l reservoirs cuntai n a certain alnoun t of natura l fracturing. However, froIn a geolog ic and a reservo ir engine ering point of view, 1 regard as natura lly fractured
reservoirs only those where the fractures have an effect, either positiv e or negativ e, on fluid tlow as sugges ted by Nelson (1985).
REQUIREMENTS FOR HYD ROC ARB ON ACC UMU LATI ON In genera l, a petrole unl reservo ir consist s of source rock, reservo ir rock, seal rock, trap, and fluid
conten t. Source rock, Or source enviro nment , is believe d to be respon sible for the origin of petroleum. Most geolog ists believe that the origin of petrole um is organi c, related mainly to vegetables which were altered by pressure, temper ature, and bacteri a. Some geolog ist (Hunt et aI, 1992), howev er, believe that the origin of petrole um is igneou s and indica te that oil rises from depth in graniti c shield terrain s of the world.