Applied Concepts in Fractured Reservoirs

John C. Lorenz, Scott P. Cooper

• Publisher: Wiley-Blackwell
• Year: 2020
• Print ISBN: 9781119055860
• eBook ISBN: 9781119055983
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

A much-needed, precise and practical treatment of a key topic in the energy industry and beyond, Applied Concepts in Fractured Reservoirs is an invaluable reference for those in both industry and academia

Authored by renowned experts in the field, this book covers the understanding, evaluation, and effects of fractures in reservoirs. It offers a comprehensive yet practical discussion and description of natural fractures, their origins, characteristics, and effects on hydrocarbon reservoirs. It starts by introducing the reader to basic definitions and classifications of fractures and fractured reservoirs. It then provides an outline for fractured-reservoir characterization and analysis, and goes on to introduce the way fractures impact operational activities.

Well organized and clearly illustrated throughout, Applied Concepts in Fractured Reservoirs starts with a section on understanding natural fractures. It looks at the different types, their dimensions, and the mechanics of fracturing rock in extension and shear. The next section provides information on measuring and analyzing fractures in reservoirs. It covers: logging core for fractures; taking, measuring, and analyzing fracture data; new core vs. archived core; CT scans; comparing fracture data from outcrops, core, and logs; and more. The last part examines the effects of natural fractures on reservoirs, including: the permeability behavior of individual fractures and fracture systems; fracture volumetrics; effects of fractures on drilling and coring; and the interaction between natural and hydraulic fractures.

• Teaches readers to understand and evaluate fractures
• Compiles and synthesizes various concepts and descriptions scattered in literature and synthesizes them with unpublished oil-field observations and data, along with the authors' own experience
• Bridges some of the gaps between reservoir engineers and geologists
• Provides an invaluable reference for geologists and engineers who need to understand naturally fractured reservoirs in order to efficiently extract hydrocarbons
• Illustrated in full color throughout
• Companion volume to the Atlas of Natural and Induced Fractures in Core

PART 1
Understanding Natural Fractures: Fracture Types, Dimensions, and Origin

1.1 Introduction

We begin this text with a short discussion of basic fracture nomenclature in order to provide a common understanding and framework for the rest of the volume. Nomenclature would seem to be a rather stodgy lead‐off topic, but the whiplash provided by the variety of fracture terminology in the literature (e.g. Pollard and Aydin, 1988; Lorenz and Cooper, 2019) should prevent the discussion from being overly dull and boring. A nomenclature discussion provides a basis for melding data, observations, and concepts from the laboratory, outcrops, and theory, as well as for following the arguments of papers written by different authors.

All fracture types do not have equal effects on a reservoir, so the nomenclature section is followed by descriptions of the range of fracture dimensions and by illustrations of the characteristics that must be accounted for when attempting to model fluid flow through a naturally fractured reservoir. This discussion focuses on shear and extension fractures, which are the most common fracture types in hydrocarbon reservoirs, but it will include some of the other kinds of fractures found in reservoirs. We will also explore the basic mechanics of fracturing rock in shear and extension since that knowledge is useful in extrapolating the limited fracture data obtained from a wellbore into the three‐dimensional volume of the reservoir, and can sometimes be used to infer the characteristics or even the presence/absence of fracture systems in a reservoir prior to drilling.

Fracture characteristics depend on the stress conditions at the time of fracturing as well as the mechanical properties of the host rock, which in turn depends on the basic composition, sedimentary heterogeneity, and diagenetic history of the rock. Therefore, we will discuss the variations of fracture characteristics as controlled by lithology, principally the gross‐scale differences inherent in fracturing limestone, sandstone, and shale or mudrock. We will also examine what happens when a fracture forms within one stress system and is later reactivated within a reorganized stress system, forming a compound fracture.

Open fracture apertures in a reservoir provide high‐permeability pathways that can be enhanced by dissolution and/or restricted by mineralization, so fracture apertures and modifications to fracture apertures are also discussed. Finally, the origins and effects of closely spaced fractures in fracture corridors are examined.

These descriptions and discussions from Part 1 of this volume and will provide the basic understandings necessary to move on to Part 2, Measuring and Analyzing Fractures in Reservoirs, and Part 3, The Effects of Natural Fracture on Reservoirs.

1.2 Nomenclature and Fracture‐Classification Systems

1.2.1 Introduction

A fracture is a mechanical discontinuity in a rock, typically planar and commonly associated with a loss of cohesion in the rock across the fracture plane. Many fractures are filled by post‐fracturing mineralization, restoring some or all cohesion across the facture plane. Natural fractures are brittle to brittle‐ductile strain‐accommodation structures that develop when the rock is subjected to a stress anisotropy greater than its strength.

This definition covers an astonishing variety of structures, and numerous authors have divided fractures into various classes with assorted names in efforts to make some sense out of them. Classification, i.e. the identification of populations within which the fractures have distinctive and similar characteristics, is the first step in assessing fracture effects on a reservoir, since all fracture types do not have the same permeability or degree of interconnectedness. Nevertheless, it's useful to remember that most classification schemes, including those for fractures, are artificial constructs, and the boundaries between classes are commonly gradational rather than abrupt.

The focus of this volume is on shear fractures and extension fractures (Figure 1.1). Extension fractures form when the opposing fracture walls move apart from each other in the direction normal to the fracture plane, whereas shear fractures form when the opposing walls move in opposite directions but parallel to the fracture plane. Both structures accommodate strain in a brittle fashion under conditions of anisotropic stress, and although there is a gradation between these two basic fracture types, they have fundamentally different characteristics and therefore have significantly different effects on reservoirs. These two fracture types comprise the majority of fractures found in hydrocarbon reservoirs.

Figure 1.1 Left: two calcite‐mineralized vertical extension fractures captured by a core cut from a marine sandstone. The fractures do not intersect in the core but have relative strikes (red bars) that will intersect in the reservoir outside the core volume. Upper right: a side view of an inclined shear fracture in the same core. Lower right: view of the calcite‐mineralized and slickenlined surface of this shear fracture. 5.25 inch (13.3 cm) diameter core; uphole is towards the top of all three photos.

We prefer not to not use the terms “joint,” an open break in the rock, or “vein,” a mineralized joint, since the two terms always seem to generate discussion and require definition during debates on the outcrop. Dictionaries in fact suggest that the term “joint” comes from the verb to join, and its original connotation referred to the location where two rock masses come together. Although “joint” may have historical precedence, “fracture” is perhaps more apt since the related verb to fracture applies more appropriately to the origin of the structure as a break in the rock mass. “Joint” and “vein” are both too broad (not distinguishing between extension and shear) and too specific (is a mineralized shear fracture a vein?) to be widely useful. Some authors (e.g. Hancock, 1985; Mandl, 2005; Weijermars, 1997) apply “joint” to any fracture discontinuity even if it has shear offset; others restrict it to those fractures that do not have offset parallel to the fracture plane and prefer the term “fault” for any structure with indications of shear. We have used the term “vein” for early diagenetic filled fractures that are insignificant to reservoir permeability (see Lorenz and Cooper, 2018a), and the term also adequately describes the short, wide, mineralized, en echelon structures found in shear zones that form in rock near the brittle‐ductile transition. However, “vein” does not serve well as a generic term.

A “fracture”‐based lexicon is simple and flexible. It is easily modified with descriptors to indicate any of the numerous and important fracture characteristics such as mode of origin, dip angle, and whether or not the fracture is mineralized (i.e. an “inclined, calcite‐mineralized shear fracture”), making it easy to understand. Most fracture types and most other nomenclatures can be fit into a fracture‐based nomenclature system (Table 1.1).

Table 1.1 Fracture nomenclature as used in this volume.

Fracture Classifications Used in this Volume
Mode of Origin	Modifiers
Extension	Mineralized, Unmineralized, and Dissolution
	Dip angle: High, Intermediate, Low
	Bed‐parallel
	Microfractures
Shear	Mineralized, Unmineralized, and Dissolution
	Dip angle: High, Interm...

Table of contents

• Cover
• Table of Contents
• Foreword
• Preface
• Acknowledgements
• Introduction
• PART 1: Understanding Natural Fractures: Fracture Types, Dimensions, and Origin
• PART 2: Measuring and Analyzing Fractures in Reservoirs
• PART 3: Effects of Natural Fractures on Reservoirs
• References
• Index
• End User License Agreement