Mechanics of Hydraulic Fracturing E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 753

Veröffentlichungsjahr: 2022

Beschreibung

Mechanics of Hydraulic Fracturing Comprehensive single-volume reference work providing an overview of experimental results and predictive methods for hydraulic fracture growth in rocks Mechanics of Hydraulic Fracturing: Experiment, Model, and Monitoring provides a summary of the research in mechanics of hydraulic fractures during the past two decades, plus new research trends to look for in the future. The book covers the contributions from theory, modeling, and experimentation, including the application of models to reservoir stimulation, mining preconditioning, and the formation of geological structures. The four expert editors emphasize the variety of diverse methods and tools in hydraulic fracturing and help the reader understand hydraulic fracture mechanics in complex geological situations. To aid in reader comprehension, practical examples of new approaches and methods are presented throughout the book. Key topics covered in the book include: * Prediction of fracture shapes, sizes, and distributions in sedimentary basins, plus their importance in petroleum industry * Real-time monitoring methods, such as micro-seismicity and trace tracking * How to uncover geometries of fractures like dikes and veins * Fracture growth of individual foundations and its applications Researchers and professionals working in the field of fluid-driven fracture growth will find immense value in this comprehensive reference on hydraulic fracturing mechanics.

Leseprobe

Mechanics of Hydraulic Fracturing

Experiment, Model, and Monitoring

Edited by

Xi Zhang

China University of Geosciences

Wuhan, China

Bisheng Wu

Tsinghua University

Beijing, China

Diansen Yang

Wuhan University

Wuhan, China

Andrew Bunger

University of Pittsburgh

Pittsburgh, PA, USA

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Library of Congress Cataloging‐in‐Publication DataNames: Zhang, Xi (Geologist), editor.Title: Mechanics of hydraulic fracturing : experiment, model, and monitoring / edited by Prof Xi Zhang, China University of Geosciences, Prof. Bisheng Wu, Tsinghua University, Prof. Diansen Yang, Wuhan university, Prof. Andrew Bunger.Description: First edition. | Hoboken, NJ, USA : Wiley, 2023. | Includes bibliographical references and index.Identifiers: LCCN 2022030857 (print) | LCCN 2022030858 (ebook) | ISBN 9781119742340 (hardback) | ISBN 9781119742418 (adobe pdf) | ISBN 9781119742456 (epub) | ISBN 9781119742487 (oBook)Subjects: LCSH: Hydraulic fracturing.Classification: LCC TN871.255 .M43 2023 (print) | LCC TN871.255 (ebook) | DDC 622/.3381–dc23/eng/20220907LC record available at https://lccn.loc.gov/2022030857LC ebook record available at https://lccn.loc.gov/2022030858

Cover Design: WileyCover Image: © Studio One-One/Getty Images

List of Contributors

Andrew P. BungerDepartment of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA, USAandDepartment of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, USA

Z. R. ChenCSIRO Energy, Clayton South, Victoria, Australia

A. R. CrudenSchool of Earth Atmosphere & Environment, Monash University, Clayton, Victoria, Australia

B. DamjanacItasca Consulting Group, Minneapolis, MN, USA

Emmanuel DetournayDepartment of Civil, Environmental, and Geo‐Engineering, University of Minnesota, Minneapolis, MN, USA

C. DetournayItasca Consulting Group, Minneapolis, MN, USA

E. V. DontsovResFrac Corporation, 555 Bryant St., #185 Palo Alto, CA, USA

D. I. GaragashDepartment of Civil and Resource Engineering, Dalhousie University, Halifax, Nova Scotia, Canada

D. GunaydinDepartment of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA, USA

Y. HanAramco Services Company: Aramco Research Center, Houston, TX, USA

R. G. JeffreySCT Operations Pty. Ltd., Wollongong, New South Wales, Australia

E. A. KaninProject Center for Energy Transition and ESG, Skolkovo Institute of Science and Technology (Skoltech), Moscow, Russian Federation

Olga KresseSchlumberger, Houston, TX, USA

Brice LecampionGeo‐Energy Laboratory, Gaznat Chair on Geo‐Energy, Ecole Polytechique Fédérale de Lausanne, EPFL‐ENAC‐IIC‐GEL, Lausanne, Switzerland

Z. LeiDepartment of Hydraulic Engineering, Tsinghua University, Haidian, Beijing, China

Mengli LiKey Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai, ChinaandDepartment of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai, China

Dong LiuGeo‐Energy Laboratory, Gaznat Chair on Geo‐Energy, Ecole Polytechique Fédérale de Lausanne, EPFL‐ENAC‐IIC‐GEL, Lausanne, Switzerland

John NapierDepartment of Mining Engineering, University of Pretoria, Hatfield, Pretoria South Africa

A. A. OsiptsovProject Center for Energy Transition and ESG, Skolkovo Institute of Science and Technology (Skoltech), Moscow, Russian Federation

Panos PapanastasiouDepartment of Civil and Environmental Engineering, University of Cyprus, Nicosia, Cyprus

A. PeirceDepartment of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada

Lei TaoState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China

S. T. ThieleHelmholtz Institute Freiberg for Resource Technology, Helmholtz‐Zentrum Dresden‐Rossendorf, Freiberg, Germany

Shouceng TianState Key Laboratory of Petroleum Resources and Prospecting, University of Petroleum (Beijing), Beijing, China

M. TorresItasca Consulting Group, Minneapolis, MN, USA

Haizhu WangState Key Laboratory of Petroleum Resources and Prospecting, University of Petroleum (Beijing), Beijing, China

Xiaowei WengRetired, Schlumberger, Houston, TX, USA

Bisheng WuDepartment of Hydraulic Engineering, Tsinghua University, Beijing, China

Diansen YangSchool of Civil Engineering, Wuhan University, Wuhan, China

J. YangInstitute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, China

Xi ZhangFaculty of Engineering, China University of Geosciences, Wuhan, ChinaandSCT Operations Pty Ltd, Wollongong, Australia

Fengshou ZhangKey Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai, ChinaandDepartment of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai, China

Guangqing ZhangCollege of Petroleum Engineering, China University of Petroleum‐Beijing, Beijing, China

Yun ZhouState Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei, China

Dawei ZhouCollege of Petroleum Engineering, China University of Petroleum‐Beijing, Beijing, China

Haiyan ZhuState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China

Foreword

For about 15 years until his retirement from the Commonwealth Scientific and Industrial Research Organization (CSIRO) in 2015, Rob Jeffrey led a group of scientists, technical officers, students, and academic visitors, all engaged in hydraulic fracturing research. Under Rob's direction, the CSIRO Hydraulic Fracturing (CSIRO HF) group, located in Melbourne, was a vibrant center, unique in the diversity of its activities that involved field testing, laboratory experiments, theoretical modeling, and numerical simulations. Many authors of this book have been associated with the CSIRO HF group, and their contributions are tributes to Rob's inspired leadership.

A permanent focus of the CSIRO HF group was to support the development of hydraulic fracturing as a means to precondition a rock mass for cave inducement of ore bodies and coal mine goafs. This application of hydraulic fracturing to mining was Rob’s brainchild, an invention foreshadowed by the experience and expertise Rob gained in the United States while working, first, for the mining industry and later for the petroleum industry in research and development. His patented technique is now being used routinely in mines in several countries, including Australia, Chile, and Indonesia. Preconditioning by hydraulic fracturing has a profound economic impact on hard rock mines relying on block caving and on longwall coal mines, in some cases ensuring the economic viability of these operations.

One of Rob's outstanding achievements at CSIRO was the establishment of a world‐class laboratory dedicated to hydraulic fracturing research with a large polyaxial cell as its centerpiece. A particular point of pride for Rob was the heavy‐duty equipment used to conduct field tests. The lab was the birthplace of innovative experiments; some were conducted in glass or polymethyl methacrylate (PMMA) to enable visualization of the fracture evolution and photometric measurements of the aperture field to test computational models, while others explored the interaction of a hydraulic fracture with discontinuities or with a free surface, to cite a few. Although experiments in the field or in the lab are Rob’s passion, he was always enthusiastically supportive of theoretical and computational work.

Some members of the former CSIRO HF group have moved to academic positions in China, Europe, and North America, but they remain engaged in hydraulic fracturing research. With this migration, the theoretical expertise and know‐how for innovative experimental work gained in the CSIRO lab has been kept alive and further flourished. This is one of Rob’s enduring legacies.

On a personal note, I am indebted to Rob for giving me the wonderful opportunity to work in his group. It was initiated at the ARMA Vail Symposium in 1999, when Rob asked me “would you be interested to spend some of your sabbatical in Melbourne ….” This led to a long‐term collaboration and a chance to discover Australia on multiple occasions.

Emmanuel Detournay, University of Minnesota, Minneapolis, USA

Preface

This volume of contributed chapters comes from the network of researchers working on mechanics of hydraulic fracturing in theory, experiments, and applications. Many of these contributors comprised the participants in a series of Hydraulic Fracturing Summits held from 2001 to 2015. These workshops provided a venue for lively debates and effective exchanges of ideas. The success of these Summits is attributed to Rob Jeffrey and Emmanuel Detournay, who initiated this series of annual meetings and fostered the community in their collaborations. They recognized the scientific significance of this fast‐growing field prior to the surge of unconventional oil and gas reservoir stimulation, and through this workshop mentored a generation of hydraulic fracturing researchers.

This volume is devoted to celebrating the 70th birthday of Rob Jeffrey, who was the program leader at Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia, when three of the editors worked there. Rob is one of the pioneers for the application of hydraulic fracturing in coal seam gas reservoir stimulation and the foundation of the knowledge on hydraulic fracture growth in naturally fractured rocks. During his forty years working in hydraulic fracturing field, he launched many important research directions, being among the first to work in areas such as fracture network formation, novel fracture monitoring methods, mine‐through mapping of full‐scale hydraulic fractures, and use of hydraulic fracturing to modify rock mass properties for both block caving and longwall coal mining. All of his work brought together theoretical and experimental methods in order to address new issues with rigor and pragmatism. In recognition of Rob's outstanding contributions both to the field and to their careers, his Hydraulic Fracturing Summit colleagues and others are happy to contribute this book that is in his honor.

This volume contains 17 chapters, covering a wide range of relevant research topics and important application areas. The editors wish to thank every contributor for their diligence and careful choice of topics. The chapters will provide the first comprehensive story on these experimental and field monitoring results, including comparisons of several numerical methods with industrial applications. This book will benefit a wide spectrum of readers, ranging from newcomers seeking an efficient orientation to the field to seasoned experts looking for a unique set of references on some of the most important issues within the hydraulic fracturing community over the past two decades.

The editors are grateful to those anonymous reviewers of the book proposal for their positive comments and constructive suggestions and to the many volunteer reviewers who spent tremendous amount of time and effort in the evaluation of each chapter. We also owe much to Dr. Rituparna Bose, Layla Harden, and Stefanie Volk at Wiley, who are aware of the rapid‐growing knowledge and social impact of hydraulic fracturing, and along with other editorial team members supporting this book. Without their vision, enthusiasm, and diligent efforts, this book would not have been possible.

Xi Zhang, China University of Geosciences,Wuhan, China

Bisheng Wu, Tsinghua University,Beijing, China

Diansen Yang, Wuhan University,Wuhan, China

Andrew Bunger, University of Pittsburgh,Pittsburgh, PA, USA

1Hydraulic Fracture Geometry from Mineback Mapping

R. G. Jeffrey

SCT Operations Pty. Ltd., Wollongong, New South Wales, Australia

KEY POINTS

Details of full‐size hydraulic fracture geometry are only available from mapping of mined fractures.

Hydraulic fractures mapped in naturally fractured coal, sandstone, and in crystalline rock show many similarities that result from interactions with natural fractures and shear structures.

1.1 Introduction

Hydraulic fractures have been mapped during mining in a range of rock types and in a variety of geologic and in situ stress settings. Mapping has occurred for fractures placed into clay and soil [1], welded tuff [2], coal ([3–5]), andesite [6], and other crystalline and metamorphic rocks in porphyry ore bodies [7–9].

Coal seams are typically fractured for the purpose of stimulating production of seam gas (coalbed methane), either for commercial use of the gas or to improve drainage of gas from the coal before mining. Approximately 50 hydraulic fractures have been mined and mapped in detail in coal seams in the United States and Australia ([3, 4, 10, 11]; Jeffrey et al. 1993). In comparison, fractures mapped in other materials (soil and rock) total less than 20, with 10 of these located in porphyry copper and gold orebodies ([6, 7, 9]; containing intrusive monzonites and metamorphosed volcanic sediments. These fractures were placed as part of investigations into fracture geometry expected to be produced by hydraulic fractures used to precondition the orebody in advance of mining. The fractures placed into welded tuff were part of early research into hydraulic fracture growth [2] and the soil fractures were part of a study of fracturing for waste remediation [1].

By comparing the fracture geometry mapped in these different natural materials, common and disparate features of the fractures are highlighted. To help with the comparison, dimensionless groups that have been shown to be important in hydraulic fracture growth are calculated or estimated for each mapped fracture. There is an extensive body of work using experimental, analytical, and numerical methods to investigate interactions of hydraulic fractures with bedding and natural fractures and faults. This paper limits itself to the mapped geometry exposed by mining of full‐size hydraulic fractures and presents a comparison of features found in coal, sandstone, and stronger metamorphic or igneous rocks.

1.2 Summary of Mapped Fracture Geometries

Selected hydraulic fractures that were placed in coal, sandstone, and hard rock are described in this section. For each fracture the treatment parameters, rock properties, and in situ properties are listed.

1.2.1 Fractures in Coal

Two fractures placed into coal seams will be described, one located in Australia as described by Jeffrey et al. (1993) and one located in the United States [10]. The fracture descriptions include details of the treatment and site characterization, including rock mechanical properties and in situ stress data.

1.2.1.1 DHM‐7 Fracture

As part of a program to better understand hydraulic fracture stimulation of coal, well DHM‐7 was fractured using linear gel with sand proppant [10]. DHM‐7 was drilled and completed open hole through the Blue Creek coal seam in Alabama. The well was located over the Oak Grove Mine in the Warrior Basin. Table 1.1 summarizes the parameters of the site and the treatment parameters. It is difficult to measure σH in coal because the cleat and natural fractures make overcoring impractical and the determination of fracture initiation is difficult when using a microfrac stress test. A value for σH that is larger than the vertical stress has been selected because the mine back mapping revealed no development of propped fractures at right angles to the direction of the main vertical fracture branch.

Room and pillar mining exposed the fracture in the coal rib around the sides of two pillars and along the coal rib nearest the well (Figure 1.1). A vertical and horizontal propped fracture (T‐shaped) was found with the vertical part consisting of a number of parallel strands. The vertical fracture extended for more than 30 m to the north of the well. A horizontal propped fracture, extending over the vertical fracture, was located at the coal‐roof rock interface and was mapped in detail (see Figure 1.2). The fracture was not mined to the south, but based on the area and fracture widths mapped, the propped fracture mapped on the north side of well DHM‐7 was estimated to contain approximately 75% of the sand proppant injected [12].

The treatment pumped into DHM‐7 used a non‐crosslinked guar‐based gel fluid that was injected at an average rate of 8.3 barrels/min (0.022 m3/s). A thick resin‐coated sand system was pushed into the near‐well part of the fracture on the morning following the main treatment. This resin coated sand was used to test its ability to stabilize the wellbore region. It is designed to retain 80% of the sand's permeability after curing. Much of the vertical fracture exposed nearest the wellbore, at location A in Figure 1.1, was filled with this resin‐coated sand. Samples of this propped fracture were excavated and taken from the mine for later analysis and display. No resin‐coated sand was found at the next exposure at location B. Mapping of the horizontal fracture was done at the detail level represented in Figure 1.2 along all propped exposures.

Table 1.1 DHM‐7. Coal, well, and treatment parameters.

Parameter

Value

Units

Description

σ

H

>8.1

MPa

Maximum horizontal stress

σ

h

6.2

MPa

Minimum horizontal stress

σ

v

8.1

MPa

Vertical stress

P

o

<3

MPa

Pore pressure (estimated)

k

1.2

md

Permeability, millidarcy

E

4000

MPa

Young's modulus of coal

ν

0.3

Poisson's ratio of coal

Q

0.022

m

3

/s

Injection rate

μ

25 × 10

−9

MPa s

Apparent fluid viscosity, at 170 s

−1

Depth

331.3

m

Depth to top of Blue Creek seam

r

0.108

m

Wellbore radius

The mapped geometry in DHM‐7 and other cases presented below rely on the proppant to mark the fracture path. The hydraulic fracture typically extends beyond the proppant, especially when less viscous fluids are used. Hydraulic fractures cannot in general be found or mapped if they do not contain proppant. The distribution of the proppant, especially in a horizontal fracture, depends on both the fracture width and the fluid velocity field. Proppant transport in horizontal fractures is an area of study that has received little attention, primarily because horizontal fracture growth is thought to be a rare occurrence at depths greater than approximately 300 m. The hydraulic fractures described below that were placed into the orebody at Northparkes at a depth of 580 m were horizontal. T‐shaped fractures are common in stimulation of coal and better models that can deal with horizontal fracture growth and the associated proppant transport problem would be welcome.

1.2.1.2 DDH 190 Fracture

An uncased HQ‐size cored borehole (DDH 190) was drilled through the German Creek coal at Central Colliery in Queensland and was hydraulically fractured using a borate crosslinked hydroxypropyl guar gel fluid. The site was characterized by undertaking well testing, stress measurement, core testing, and fracture testing before the main fracture treatment. Table 1.2 summarizes the site parameters relevant to the treatment as given by Jeffrey et al. [5].

Mapping of the fracture during and after development of the roadways in this area of the mine revealed a vertical fracture in the coal that extended into the roof rock (Figure 1.3). The fracture trace in the roof rock (Figure 1.3a) was primarily a single fracture, but interactions with natural fractures resulted in the formation of some offsets and short parallel branches. The vertical fracture trace in the coal at the north side of 13 cut‐through (Figure 1.3b) was typical of other vertical sections mapped at this site, consisting of a single fracture that interacted with bedded and sheared coal. The 150 mm‐thick mid‐seam shear zone (mssz) runs through much of the German Creek seam and is composed of sheared coal, with particles ranging from clay size to a few centimeters in size. The mssz is softer and weaker than the coal above and below it. This hydraulic fracture and others mapped in this coal seam often developed an offset across the mssz.

Figure 1.1 A plan view of the propped hydraulic fracture mapped at the DHM‐7 site is shown in the top drawing while three mapped vertical fracture sections exposed at the coal rib are shown in the lower drawing. Mapping of the horizontal fracture occurred along the ribs of the roadways where the fracture was exposed near the roof (

Source: Boyer et al. [12]/Gas Research Institute).

Figure 1.2 Seven linear feet of horizontal fracture mapped along the north side of the coal pillar

(Source: Boyer et al. [12]/Gas Research Institute).

The location of this portion of the horizontal fracture is indicated in Figure 1.1 and labeled as station 1 through 6. The entire horizontal fracture was mapped at this level of detail and the numbers in Figure 1.1 along the pillar boundary indicate the average propped width in hundredths of inches (e.g. 30 represents 0.30 in.).

Table 1.2 DDH 190 site parameters.

Parameter

Value

Units

Description

σ

H

>4

MPa

Max horizontal stress, in roof

σ

h

2.5 1.9

MPa MPa

Min horizontal stress in coal Min horizontal stress in roof

σ

v

4.5

MPa

Vertical stress

P

o

1.08

MPa

Pore pressure

k

4.2

md

Permeability, millidarcy

E

2 000 25 000

MPa MPa

Young's modulus, coal Young's modulus, roof rock

ν

0.35 0.13

Poisson's ratio, coal Poisson's ratio, roof rock

Q

0.002 5

m

3

/s

Injection rate

μ

610 × 10

−9

MPa s

Apparent fluid viscosity at 170 s

−1

Depth

193.5

m

Depth to top of German Creek seam

r

0.048

m

Wellbore radius

Figure 1.3 Looking down on propped fracture trace (a) in sandstone roof rock at 13 cut‐through. Vertical section (b) showing propped fracture exposed on north coal rib of 13 cut‐through near borehole DDH 190

(Source: Jeffrey et al. [5]/Coalbed Methane Association of Alabama).

The fracture at DDH 190 extended into the lower stress roof rock with only 12% of the proppant injected estimated to be accounted for by the propped fracture in the coal seam. The mapping clearly shows the trace of the propped hydraulic fracture in the roof and in the coal, but does not reveal if the fracture was growing primarily laterally or vertically at the sections mapped. Modeling of this fracture suggests upward growth of 7 m into the roof rock at the borehole [13].

1.2.2 Fractures in Hard Rock

Hydraulic fracturing is used in mining to induce caving and to precondition rock for caving [7]. More recently, preconditioning has been used in areas of high stress as a means of reducing the potential for the occurrence of large, damaging seismic events [14]. A total of nine fractures have been mined and mapped at four metalliferous mine sites in Australia [7, 9, 15] and Chile [6]. The fractures described by Jeffrey et al. [9] will be compared to the fractures placed into coal seams.

1.2.2.1 Northparkes E48 Mapped Fractures

Six hydraulic fractures were placed ahead of a tunnel at 580 m depth at the E48 Northparkes mine as part of a mine‐through experiment [9]. The fractures were propped with colored plastic and sand and were monitored by microseismic and tiltmeter arrays. The mapped hydraulic fractures at Northparkes consist of nearly horizontal segments with offsets at intervals along them produced as the fracture grew into and along dipping veins, natural fractures, and shear zones. Fracture branches and sub‐parallel propped sections were also mapped, making up 10–15% of the total fracture extent. The rock mass at the site is naturally fractured, containing approximately five natural fractures per meter. Additional details of the site, fracturing, and mine‐back can be found in Jeffrey et al. [9]. Table 1.3 lists site and rock parameters. The stress directions given in Table 1.3 are based on overcoring data measured near the fracture site. The minimum stress was nearly vertical with the maximum stress oriented nearly horizontal and directed approximately east–west.

Table 1.3 Site and rock parameters for Northparkes E48.

Parameter

Value

Units

Description

σ

H

40

MPa

Max horizontal stress, 290

0

, 8

0

dip

σ

h

22

MPa

Min horizontal stress, 22

0

, 11

0

dip

σ

v

15

MPa

Vertical stress, 165

0

, 76

0

dip

P

o

<1

MPa

Pore pressure

k

0.005

md

Permeability, millidarcy

E

50 000

MPa

Young's modulus

ν

0.2

Poisson's ratio

Q

0.007 5

m

3

/s

Injection rate

μ

610 × 10

−9

1 × 10

−9

MPa s MPa s

Apparent crosslinked gel viscosity at 170 s

−1

Viscosity of water

Depth

580

m

Depth below surface

r

0.048

m

Wellbore radius

Figure 1.4 Hydraulic fractures mapped along a tunnel at the Northparkes E48 mine

(Source: Jeffrey et al. [9]/Society of Petroleum Engineers).

The initiation point of each fracture is indicated along the borehole which is drawn with a purple line.

The hydraulic fractures were mapped along the E48D102 tunnel as it was driven. The fractures were horizontal with steps along their path often forming where they interacted with natural fractures and shear structures. These offsets were large enough to increase the average dip because the fracture trace following a stair‐step pattern. Figure 1.4 shows an overview of the five fractures that were propped with colored plastic and mapped along the sides of the tunnel.

Fractures 6 and 7 were placed using crosslinked guar gel while fractures 5, 8, and 9 were placed using water. The colors of the points mapped along each fracture shown in Figure 1.4 correspond to the colors of the plastic proppant used. The grid lines shown are the mine coordinates in meters. The initiation point of each fracture is indicated by a black square symbol and the borehole is shown by the line connecting these symbols. Figure 1.5 contains a more detailed sketch of Fracture 8, which contained the largest offset or step mapped in any of the fractures at this site. The injection interval, which was in the borehole E48D102, is shown. This interval is located approximately 2.5 m out of the plane of the fracture trace shown because the borehole was drilled along the centerline of the tunnel. Fracture growth is likely to have been semi‐radial from the injection interval and should not be visualized as occurring purely along the fracture trace. Fracture 7 contained an offset similar in size to the one shown in Figure 1.5 and a series of smaller offsets (but consisting of 200–400 mm steps) were mapped at one exposure of Fracture 5. Therefore, at this site the crosslinked gel did not produce more planar fractures compared to the water driven fractures.

Figure 1.5 The mapped trace of fracture 8, exposed along the south side of the tunnel, at the Northparkes E48 mine‐back site

(Source: Jeffrey et al. [15]).

Details of the fracture geometry were not collected except along the sides of the tunnel and occasionally across the tunnel face and back. This is a common difficulty experienced when mapping hydraulic fractures in a commercial mine. The tunnelling or coal extraction operations take precedence over mapping. In the case of the E48 site, the tunnel is extended in approximately four‐meter intervals using a drill‐blast‐muck‐support cycle. Mapping is restricted to occur between the muck and support or support and drill steps.

1.2.3 Other Mapped Fractures

Hydraulic fractures have been placed into a range of rock and soil materials followed by mining and mapping. Warpinski et al. [16] and Warpinski and Teufel [2] describe hydraulic fractures placed into volcanic tuff and Murdoch [1] describe fractures placed into clay and soil at shallow depth. Details of these fractures can be found in the papers cited.

1.3 Comparison of Mapped Fracture Geometries

Nondimensional parameters are useful in helping to determine the type of fracture growth to expect in different rocks and at different sites. For example, the dimensionless toughness or dimensionless viscosity can be used to determine if the treatment was carried out in the fracture toughness or fluid viscosity dominated regime [17]. Viscosity dominated growth leads to more planar fracture geometries [18].

The fracture width restrictions resulting from offsets and branches along the fracture path are potential sites for proppant bridging and the narrow width at these locations result in a higher injection pressure and slower fracture growth compared to more planar fracture conditions [19]. The growth of closely spaced fractures has been shown to be well described by several dimensionless parameters [8, 20]. Results from numerical calculations of the path of a new fracture placed next to an existing fracture were presented by Bunger et al. [20] and were compared to mine‐back and laboratory experiments in Bunger et al. [8]. The fractures mapped and described above will be compared with each other and are further categorized by calculating values for the dimensionless viscosity and differential stress parameters.

1.3.1 Dimensionless Parameters

The nondimensional parameters used are listed in Table 1.4. The physical parameters that are used in defining the nondimensional parameters are listed in Table 1.1. The nondimensional viscosity is used to determine whether the fracture growth is dominated by rock fracture toughness or by frictional losses associated with viscous fluid flow in the fracture channel [17]. The dimensionless stress is commonly used in studies of hydraulic fractures interacting with other hydraulic fractures or crossing natural fractures [8, 18, 19]. Two values of dimensionless differential stress are defined because a hydraulic fracture growing with an orientation such that it opens against σ3 will be subject to a range of differential stresses, depending on the location of the point being considered with respect to the other two principal stresses. These parameters are therefore used here as reference values in the comparisons of overall fracture growth behavior as determined by mine‐back mapping.

Table 1.4 Dimensionless groups used in comparisons.

Name

Parameter

Notes

Dimensionless viscosity

If

M

 < 0.25, regime is toughness dominated. If

M

 > 1, regime is viscosity dominated.

Dimensionless differential stress

d

13

applies to fracture growth in the direction of

σ

1

.

Dimensionless differential stress

d

23

applies to fracture growth in the direction of

σ

2

.

Table 1.5 Nondimensional parameters for three sites.

Site

M

d

13

d

23

Northparkes F8

7.7

1.7

0.5

DHM‐7

4.5

0.4

0.3

DDH 190 coal

3.3

0.8

0.6

DDH 190 roof

2000

1.5

0.7

The values of the three dimensionless parameters at each of the three mapping sites are listed in Table 1.5.

1.4 Fracture Geometry Summary

The M value listed for the Northparkes site applies to fractures created using water, which was the case for Fracture 8, while the treatment in DHM‐7 used linear gel and a crosslinked gel was used in DDH 190. The M values below 0.25 represent a fracture regime that is toughness dominated while an M value of one or greater is viscosity dominated. All of the fractures described here fall into the viscosity dominated regime. The fracture growth regime for the part of the fracture that extended into the roof rock at DDH 190 was strongly viscosity dominated.

Natural fractures and shear zones were crossed by the hydraulic fractures at all three sites. The downward growth of the fracture at the DDH 190 site was blunted by a soft clay at the coal‐floor rock interface.

Fracture 8 at Northparkes entered a 45° dipping natural fracture and grew along it for approximately 2.5 m before exiting down dip. The exit point coincides with additional calcite mineralization in the natural fracture. This section of the structure would be stronger in shear, resulting in higher shear‐generated tensile stress and we propose that this allowed tensile fractures or wing fractures [21] to form at that location, allowing the hydraulic fracture to escape the natural fracture.

Table 1.6 provides a summary of the features that are present in the fractures at the three sites for two different resolutions. The resolution is defined as the smallest feature on the hydraulic fracture that is included in describing the fracture geometry. In Table 1.6, nonplanar means the main fracture channel is not in a single plane while branching means the fracture treatment has propped several fractures over parts of its extent that may be parallel or at an angle to the main fracture.

The propped fracture at the DHM‐7 site was T‐shaped, with each segment planar when viewed with a resolution of 1 m. When viewed in more detail (with a resolution of 0.01 m) offsets and branches are apparent as shown in Figures 1.1 and 1.2. A T‐shaped fracture is considered to be a branched fracture, because the horizontal and vertical segments are both considered as branches of the fracture channel. In Table 1.6, the DHM‐7 fracture is considered to be nonplanar and branched at the coarser resolution. T‐shaped fractures represent a case where the hydraulic fracture is diverted into the horizontal direction rather than crossing the horizontal feature and continuing to grow vertically. Horizontal fractures associated with T‐shaped fractures have been mapped to extend 10–100 s of meters without diverting back into the coal or up into the roof. The mechanics of this observed feature of these fractures has not been completely explained.

The qualitative description in Table 1.6 shows that the classification regarding planarity, branches, and offsets depends on the scale of the observation. When a more detailed view is available, the fractures at all the sites show offset, nonplanar, and branched growth. The main factor that controls the development of these features in a hydraulic Efracture is the presence of natural fractures or bedding interfaces. The fracture growth regime and the deviatoric stress, at least in the range represented by the three sites reviewed here, do not seem to result in a strong change in the fracture geometry, at least as expressed by mapping of its propped extent. Network like fracture development would be expected as the deviatoric stress is reduced toward zero, but we do not have mapped fractures representing that stress condition.

Table 1.6 Fracture geometry versus resolution of observation.

Resolution 1 m

Resolution 0.01 m

Site

Offsets

Nonplanar

Branching

Offsets

Nonplanar

Branching

Northparkes

Yes

Yes

No

Yes

Yes

Yes

DHM‐7

No

Yes

Yes

Yes

Yes

Yes

DDH 190 coal

No

No

No

Yes

Yes

Yes

DDH 190 roof

No

No

No

Yes

Yes

Yes

1.5 Conclusions

Three sites at which hydraulic fractures have been mined and mapped are described. Measured parameters are given for each site that are then used to find the dimensionless viscosity and dimensionless deviatoric stress values for the sites.

The mapped fractures at all three sites are found to be similar in the sense that, when they are studied at a detailed scale, they all contained offsets, were nonplanar, and contained branches. The common factor between all sites is that the rock (or coal) was naturally fractured. The nondimensional parameters were not found to be useful in predicting the amount of offsetting, nonplanar growth, or branching. Mapping fractures at sites with lower deviatoric stress may show a sensitivity to that parameter in predicting network such as fracture development.

References

1

Murdoch, L.C. (1995). Forms of hydraulic fractures created during a field test in Overconsolidated glacial drift.

Quarterly Journal of Engineering Geology

28: 23–35.

2

Warpinski, N.R. and Teufel, L.W. (1987). Influence of geologic discontinuities on hydraulic fracture propagation.

Journal of Petroleum Technology

39 (2): 209–220.

3