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Library of Congress Cataloging‐in‐Publication data has been applied for
ISBN: 9781119055860 [hardback]
Cover Design: Wiley
Cover Image: © John C. Lorenz and Scott P. Cooper
For my family and mentors; Mom and Dad, Sean, Ryan, Karen, John, Alvis, and Laurel. It has been an amazing journey, Thanks!
Scott P. Cooper
For Nancy and Ned, Margaret and Norman, who were part of a generation with ideals, principles, and an appreciation for critical thinking; and for Alex who arrived later.
John C. Lorenz
Foreword
Applied Concepts in Fractured Reservoirs is a much‐needed precise and practical treatment of a key topic in the energy industry and beyond. The subject of natural fractures and their impact on hydrocarbon reservoirs has been a mutual interest that I have shared with the senior author of this book, John Lorenz, for decades. In my view this will become a standard reference for geoscientists and engineers working on fractured reservoirs, for example reservoir engineers, geophysicists, geologists, and rock mechanics practitioners. It will take its place among the many other publications by the authors already addressing related issues. The importance of the book lies in the fact that it addresses what is probably the most pervasive feature of rocks: the tendency to break under natural or man‐made stresses. The authors put this in an applied context for all involved in exploration and development in the industry and in academia. In that context the book is well organized and clearly illustrated in an easy to grasp collection of applications for fracture studies, for example their impact on reservoir petrophysics, their influence on drilling, and production engineering.
The book is balanced in that it introduces the reader to basic definitions and classifications of fractures and fractured reservoirs at the outset. It then proceeds by outlining a workflow for fractured reservoirs characterization and it goes on to introduce the way fractures impact operational activities. The book allocates a considerable section to discussing the impact of natural fractures on hydraulic fracturing. In my opinion such impact is not fully understood and including it in the book is a timely approach to raise questions, stimulate thoughts, and shed some light on different experimental explanations. The ability to predict the outcome when natural fractures interact with hydraulically stimulated/induced fractures in a reservoir is a challenge not yet fully achieved. Advancement in this area of hydrofracturing is a crucial step in making hydrofracturing more efficient and safer.
John Lorenz and Scott Cooper, who are accomplished researchers and consultants, have produced a valuable resource on the subject of fractured reservoirs, a publication which complements previous texts, and takes the topic to a broader, up‐to‐date, applied level and scope.
Mohammed S. Ameen (Ph.D., DIC, FGS) Principal Professional in Geomechanics, Emerging Unconventional Assets Department, Saudi Aramco, Dhahran, Saudi Arabia
Preface
This book is a companion to our previous Atlas of Natural and Induced Fractures in Core, moving on from the basic recognition of fracture types described in that volume, which must be the foundation of any fracture study, to explanations of how those fractures form, how they are measured, how they can be assessed, and how they affect reservoirs.
This volume is the summary of decades of experience with industry doing applied fracture studies. It brings together numerous fracture‐related topics that are not collected elsewhere. We hope that it will be useful to both academia and industry, and that it is not in the vein of the apocryphal third‐grader doing a book report on penguins, who concluded that “This book tells me more than I want to know about penguins.”
Acknowledgements
Numerous people have contributed to this effort, providing reviews, materials, references, photographs, and insights. We would specifically like to acknowledge and thank Mohammed Ameen, Bruce Hart, Connie Hawkins, Nate Gilbertson, Ron Nelson, Ahmed Ouenes, David Schechter, Joe Simonson, and Norm Warpinski.
Much of the original material used to illustrate this volume has come from detailed, unpublished industry studies, and we thank those companies for allowing us to use the material, sanitized for publication. We would also like to acknowledge all of the astute people who over the years have worked in this amazing field of study and whose published papers were used throughout this text.
We would also like to thank our wives, who, knowing better than we did the size of the undertaking, said “Sure, why not?” when asked if we might carve the needed time from family commitments.
Introduction
One can't begin to write a textbook without self‐indulgently explaining how important the subject matter is and how readers therefore absolutely need to know the material. Fortunately, the importance of understanding the origins, characteristics, and effects of natural fractures in hydrocarbon reservoirs is becoming widely accepted, so it is enough for us to note that technology has continued to extend a recognition of the presence and importance of natural fractures in many reservoirs once thought to be un‐fractured. Moreover, the increasing use of horizontal and deviated wellbores is providing unique and invaluable information on the close fracture spacings exhibited by many fracture systems, even in relatively undeformed strata.
It was not always so. The default conceptual model of a reservoir before image logs and extensive coring, even in deformed strata, rarely included natural fractures, and there were few data points to indicate otherwise. Cored fractures used to be relatively rare because of the wide fracture spacings typical of the thicker reservoir units most commonly cored, and because vertical core has a low probability of capturing vertical fractures, the most common type in many reservoirs. For example, a vertical, 4‐inch (10 cm) diameter core cut vertically through a fractured bed where fracture spacing averages 40 inches (about 1 m) has only a 10% probability of sampling the fracture population (see Lorenz, 1992). The absence of vertical fractures in vertical core used to be accepted as proof that a reservoir was not fractured, but that is like saying that there are no mosquitos about on a summer's evening picnic because you have not been bitten yet.
This absence of good subsurface data, and the slowly maturing study of geomechanics prior to the 1980s, did not support the presence of fractures in the subsurface below about 2000 ft. Statements such as “At depth… most joints are generally closed…” (Heck, 1955) were widely accepted, and even the experts at the respected rock mechanics laboratory of the Exploration and Production Research Division of the Shell Development Company would write that “It is, of course, inconceivable that an open crack could exist at depth…” (Griggs and Handin, 1960). Thus, those fractures that were