**Appendix 1**

prediction of the performance. Instrumentation, both downhole and on-surface observes the initial response of the system and compares it with the prediction. This triggers a feedback signal to modify the design input to move the performance closer to the one desired. This

Although the writer knows of no such Fracture Network Engineering system currently in

Expectations for higher living standards of a rising world population, and the associated demand for Earth's resources of energy, minerals and water, lead inevitably to greater focus

This focus includes the need to develop improved technology to develop these resources, and a better understanding of the nature of the subsurface environment as an engineering material. Earthquakes and dynamic releases of energy are a daily reminder that on the global scale, Earth is critically stressed, and constantly trying to adjust seeking to achieve a condition of minimum

On going for many, many millions of years, such adjustments have resulted in the heteroge‐ neous assembly of blocks of rock bounded by essentially planar surfaces; fault, fractures and similar 'discontinuities' varying in scale from tectonic plates and continents down to micron

Some of these volumes are critically stressed; others are far from a critical condition. National

Although Earth Resource Engineering activities may be kilometers in extent, they are smallscale within the larger Earth context. Subsurface engineering in a critically stressed region can be a much different challenge than in a stable region. It is important to assess the initial conditions carefully for each case, and especially where fluid injection is a main component of

The sub-surface is opaque in several ways. Details of the key features that can control the response to an engineering activity in the sub-surface are often unknown. Problems are datalimited. This is particularly the case when the engineering is based on deep borehole systems,

Although operating in ways that may appear complex, the response of the subsurface to stimulation does obey the laws of Newtonian mechanics, and it is clear that pre-existing natural discontinuities have a major influence on how the subsurface responds to engineered changes. The advent of powerful computers and developments in numerical modeling provide a potentially major tool to help develop better-informed strategies of subsurface engineering. Used interactively in close conjunction with instrumentation, both downhole and surface

maps of seismic hazards provide evidence of this heterogeneity on a larger scale.

as in hydraulic fracturing and related fluid injection technologies.

iteration continues, changing progressively towards the performance desired.

operation, many of the components are available and it is time to start.

**13. Conclusions**

and even nanometers.

a project.

on resources of the subsurface.

72 Effective and Sustainable Hydraulic Fracturing

potential energy for the entire system.

#### **Earth resources engineering**

In 2006, the US Academy of Engineering introduced the term 'Earth Resources Engineering' to replace 'Petroleum, Mining and Geological Engineering' in recognition of the broader range of engineering activities and concerns associated with use of the subsurface. The new title, it is hoped, will also stimulate important synergies between the various disciplines involved. Mining and civil engineers, for example, have direct three-dimensional access to the subsurface not available to colleagues in other subsurface activities. This access provides a major oppor‐ tunity to conduct research and gain understanding of the mechanics of subsurface processes under actual in-situ conditions, as exemplified by Jeffrey et al. (2009), see Figure A1-1.

**Figure A1-1.** The restless Earth. Earth Resource Engineering activities are all confined to a very shallow part of the 40 km -700 km thick Earth's solid crust (lithosphere). Deepest borehole ~ 12 km; mine ~ 4km. Rock stress increases verti‐ cally σv ~ 27MPa/km; laterally σh~ (0.5- 3.0).σv: Pore water pressure p = 10 MPa /km; temperature increase ~25°C /km depth.

Study of slip on active faults is a good example.

"The physics of earthquake processes has remained enigmatic due partly to a lack of direct and near-field observations that are essential for the validation of models and concepts. DAFSAM15 proposes to reduce significantly this limitation by conducting research in deep mines that are unique laboratories for full-scale analysis of seismogenic processes. The mines provide a 'missing link' that bridges between the failure of simple and small samples in laboratory experiments, and earthquakes along complex and large faults in the crust. There is no practical way to conduct such analyses in other environment. To unravel the complexity of earthquake processes, this project is designed as integrated multidisciplinary studies of specialists from seismology, structural geology, mining and rock engineering, geophysics, rock mechanics, geochemistry and geobiology. The scientific objectives of the project are the characterization of near-field behavior of active faults before, during and after earthquakes". 16See also http://www.iris.edu/hq/instrumentation\_meeting/files/pdfs/IRIS\_Johnston.pdf

engineering is illustrated by the fact that currently more than 60% of the world's energy is delivered via a borehole. The Deepwater Horizon accident in the Gulf of Mexico in April 2010 provides a sober example of the consequences of error. In summary, hydraulic fracturing and related stimulation technologies are likely to see application to an increasing range of subsur‐ face engineering challenges. HF2013, the first International Conference for Effective and

The consequences of disturbing a pre-stressed rock medium are illustrated by examining the rock coring operation. Figure A2-1 shows the stress concentrations in a rock core in a brittle

s

t

/

0.50 0.25

D

s<sup>t</sup> /s<sup>o</sup> 0.0

> 1.0 0.5

s

o

s

R

t

/

s

Maximum

o

= 0.05

D/R = 0.1 D/R = 0.2

s<sup>t</sup> /s<sup>o</sup> 0.00

Fractures and Fracturing: Hydraulic Fracturing in Jointed Rock

http://dx.doi.org/10.5772/56366

75

0.50 0.25

s<sup>t</sup> /s<sup>o</sup> 0.0

> 1.0 0.5

0.00 /

s

t

s

Maximum

o

= 0.1

D/R = 1 D/R = 2

s s

Maximum Maximum

<sup>o</sup> <sup>o</sup>

= 0.5 = 1.0

s s

<sup>t</sup> <sup>t</sup>

/ /

Sustainable Hydraulic Fracturing, is very timely.

**Effect of coring in pre-stressed rock**

0 0

s

t

=

=

s s

t i c

o

s

s

o

s s

c c

s

o

> n/m i.e., 0.2

2 4

Numerical results Best fit curve

Core Depth / Core Radius, D/R

tensile strength

induced tension in core compressive strength

in-situ horizontal stress

**Figure A-2.1.** Tensile stress concentrations induced in a brittle rock during coring.

Potential for core damage during

0.2

0.4

0.6

coring operation

s s t o /

Max. tensile stress in core

Let

n

=

m

=

[m ~ 0.5] Then core damage occurs if >

[n ~ 0.1]

Let

Far-field stress ,

**Appendix 2**

Petroleum engineers can now reach depths in excess of 6 km and have developed advanced drilling control technologies that allow precise access to locations extending horizontally to more than 10-15 km from a single vertical hole (see Figure 2).

**Figure A1-2.** Schematic illustration of directional drilling for petroleum production.

These and related developments are stimulating interest in application of borehole technolo‐ gies to other areas of subsurface engineering, including the development of less-invasive mining technologies, i.e., borehole extraction of minerals. Some applications, e.g., where crystalline rocks are involved, are contingent on the development of significantly lower-cost drilling technologies. The critical dependence of society on reliable and economic subsurface

<sup>15</sup> DAFSAM -Drilling Active Faults in South African Mines.

<sup>16</sup> http://www.icdp-online.org/front\_content.php?idcat=460

engineering is illustrated by the fact that currently more than 60% of the world's energy is delivered via a borehole. The Deepwater Horizon accident in the Gulf of Mexico in April 2010 provides a sober example of the consequences of error. In summary, hydraulic fracturing and related stimulation technologies are likely to see application to an increasing range of subsur‐ face engineering challenges. HF2013, the first International Conference for Effective and Sustainable Hydraulic Fracturing, is very timely.
