Porous Rock Fracture Mechanics

with Application to Hydraulic Fracturing, Drilling and Structural Engineering

Amir Shojaei, Jianfu Shao

• Publisher: Woodhead Publishing
• Year: 2017
• Print ISBN:
  9780081007815
• eBook ISBN:
  9780081007822
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Porous Rock Failure Mechanics: Hydraulic Fracturing, Drilling and Structural Engineering focuses on the fracture mechanics of porous rocks and modern simulation techniques for progressive quasi-static and dynamic fractures. The topics covered in this volume include a wide range of academic and industrial applications, including petroleum, mining, and civil engineering. Chapters focus on advanced topics in the field of rock's fracture mechanics and address theoretical concepts, experimental characterization, numerical simulation techniques, and their applications as appropriate. Each chapter reflects the current state-of-the-art in terms of the modern use of fracture simulation in industrial and academic sectors. Some of the major contributions in this volume include, but are not limited to: anisotropic elasto-plastic deformation mechanisms in fluid saturated porous rocks, dynamics of fluids transport in fractured rocks and simulation techniques, fracture mechanics and simulation techniques in porous rocks, fluid-structure interaction in hydraulic driven fractures, advanced numerical techniques for simulation of progressive fracture, including multiscale modeling, and micromechanical approaches for porous rocks, and quasi-static versus dynamic fractures in porous rocks. This book will serve as an important resource for petroleum, geomechanics, drilling and structural engineers, R&D managers in industry and academia. - Includes a strong editorial team and quality experts as chapter authors - Presents topics identified for individual chapters are current, relevant, and interesting - Focuses on advanced topics, such as fluid coupled fractures, rock's continuum damage mechanics, and multiscale modeling - Provides a 'one-stop' advanced-level reference for a graduate course focusing on rock's mechanics

Part I

Introduction

Outline

1 Application of rock failure simulation in design optimization of the hydraulic fracturing

1

Application of rock failure simulation in design optimization of the hydraulic fracturing

A. Ghassemi, University of Oklahoma, Norman, OK, United States

Abstract

Energy production from unconventional geothermal and petroleum resources relies on reservoir stimulation by hydraulic fracturing. Unconventional reservoir stimulation involves multiple fractures possibly propagating in both tensile and shear mode or propagation. Therefore, application of rock fracture mechanics and advanced modeling are necessary for improved understanding of the process and its optimization. This paper presents an overview of hydraulic fracturing conceptual models and rock failure mechanisms followed by a series of numerical simulations and analyses to provide some insight into various phenomena observed in hydraulic fracturing of unconventional reservoirs. In particular, two-dimensional and three-dimensional examples are used to illustrate the impact of hydraulic fracture interactions with each other and with discontinuities and highlight the role of rock anisotropy and heterogeneity and mixed-mode stimulation in relation to permeability change.

Keywords

Hydraulic fracturing; hydroshearing; rock anisotropy; shear slip; stimulated volume; stress shadow; wing cracks

1.1 Introduction

Extraction of unconventional geothermal and petroleum resources is made possible by reservoir stimulation to provide access a large volume of rock with a network of fractures for fluid flow and/or heat exchange. For petroleum resources development, stimulation is often accomplished by multiple hydraulic fracturing of horizontal wells, whereas stimulation of enhanced geothermal systems has usually relied upon injection-induced shear slip on preexisting fractures and their coalescence. Maximizing facture permeability over the reservoir lifetime is the desired goal, and thus modeling and analysis of the stimulation process are called upon to guide the practice. The hydraulic fracturing process presents a complex mathematical problem that involves the mechanical interaction of multiple propagating fractures, fluid diffusion into the rock mass, heat transfer between the fluid and the rock, and hydraulic fracture interactions with natural fractures. Therefore, application of rock fracture mechanics and advanced modeling are necessary for improved understanding of the process and its optimization. Numerical modeling can be used to predict fractures trajectories, dimensions, and the induced stresses in the reservoir for the interpretation of seismicity and refraction analysis. This paper presents an overview of hydraulic fracturing conceptual models and rock failure mechanisms followed by a series of numerical simulations and analyses to provide some insight into various phenomena observed in hydraulic fracturing of unconventional reservoirs. In particular, two-dimensional (2D) and three-dimensional (3D) examples are used to illustrate the impact of hydraulic fracture interactions with each other and with discontinuities and highlight the role of rock anisotropy and heterogeneity and mixed-mode stimulation in relation to permeability change.

1.2 Reservoir stimulation by hydraulic fracturing of horizontal wells

Horizontal well stimulation usually involves creating multiple fractures along the wellbore using different well completion techniques. These multiple fractures generate large contact areas with the reservoir and also increase reservoir permeability. The multistage fracturing of a single or multiple horizontal wells is usually carried out either in simultaneous or sequential manner. In simultaneous fracturing, the multiple fractures are initiated and propagated at the same time (Fig. 1.1), whereas in sequential fracturing the fractures are created from one cluster after another in a well, usually by keeping the previously created fractures either propped or pressurized with fluid (Rodrigues et al., 2007; Soliman et al., 2008). Simultaneous or sequential fracturing is carried out in multiple wells and is called the “zipper” fracturing technique.

Figure 1.1 Schematics of (A) simultaneous fracturing (single stage) of a lateral, (B) conventional zipper fracturing of multiple wells, (C) modified zipper fracturing of multiple horizontal wells. Stages are performed from toe to heel of a lateral. Note that figures are not depicting any interactions or stress effects. The fracturing can be carried out simultaneously or sequentially as depicted in the 2D images. Perforations are created in the direction N–S (parallel to maximum principal stress direction).

The “zipper-frac” approach aims to create a network of long and closely spaced hydraulic fractures. Zipper fracturing also has operational benefits when multiple wells are stimulated from the same pad. Instead of simultaneous fracturing of parallel wells, stimulation can also be performed in a sequential manner that is called sequential zipper fracturing. In this method, an adjacent lateral is stimulated while restricting flow back from the stimulated fractures in another lateral. Fig. 1.1 illustrates the difference in stimulation design between simultaneous and sequential zipper fracturing. Over 50% of current shale plays employ zipper fracturing technique for operational benefits (Jacobs, 2014). In sequential stimulation of a single well a few hours are spent on wireline operations (setting plugs and perforating) before continuing on to the next stage. In sequential zipper fracturing, these operations can be performed on a neighboring lateral while stimulating is done on another lateral. Even though most operators have adopted the zipper fracturing technique for its operational efficiency, improved production has been observed in a few cases (Jacobs, 2014). A modification of “zipper” fracturing termed “modified zipper” fracturing (MZF) also has been discussed (Rafiee et al., 2012) based on stress analysis. In the MZF method, the stimulation stages in neighboring laterals have an initial offset between them. Sesetty and Ghassemi (2015) investigated the total stimulated rock volume by fracture networks obtained from conventional and MZF and concluded that theoretically, both methods of fracturing yielded almost equal stimulated reservoir volume. However, it was pointed out that in MZF, fracture turning due to stress shadow effects in the overlap region is more conducive to reactivation of preexisting natural fractures, potentially increasing complexity and the stimulated volume.

Stresses changes in the vicinity of hydraulic fractures mainly depend on the net fluid pressure inside the fracture and the fracture geometry (Warpinski and Branagan, 1989; Ge and Ghassemi, 2008; Ghassemi et al., 2013; Safari and Ghassemi, 2014). These induced stresses in the region surrounding the hydraulic fractures are termed the “stress shadow.” The effect is a more dominating in the case of multiple closely-spaced fractures in which mechanical interactions among fractures may restrict or terminate propagation of some of the fractures (El Rabba, 1989). The stress shadowing could lead to reduction of fracture aperture, increase the risk of proppant screen-out, and fracture reorientation due to altered stress conditions. The effect of stress shadowing when closely spaced multiple hydraulic fractures are created parallel to each other is of major interest for numerical simulation of multistage fracturing. The spacing between the fracture surfaces, net fluid pressure, and in-situ stress contrast (i.e., the difference of the maximum and minimum horizontal stresses) play an important role in the mechanical interaction between the fractures (Tarasovs and Ghassemi, 2014; Sesetty and Ghassemi, 2015; Wong et al., 2013). Therefore, optimization of the fracture spacing is critical in the multistage fracturing, both from technical and economical point of view, because it affects both fractures creation and subsequent well productivity. A fully coupled numerical model capable of simulating multistage fracturing can be a vital tool in understanding the effects of various parameters such as well spacing, fracture spacing, and fracture offset. The numerical models need to consider the relevant rock failure and fracture propagation mechanisms involved in the hydraulic fracturing concepts.

1.3 Hydraulic fracturing conceptual models

Hydraulic fracturing numerical models have been developed on the basis of specific conceptual models. Early hydraulic fracturing numerical models (e.g., Carter et al., 2000; Ouyang et al., 1997; Yew, 1997; Lee et al., 1994; Clifton and Wang, 1991; Clifton and Abou-Sayed, 1981; Vandamme and Curran, 1989; Cleary and Wong, 1985; Wiles and Curran, 1982; Abe et al., 1976) were dev...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• List of contributors
• Preface
• Introduction
• Part I: Introduction
• Part II: Coupled Fluid Structural Deformation and Fracture Mechanisms in Porous
• Part III: Progressive Fracture
• Part IV: Advanced Topics and Future Prospects
• Index