**11. Hydraulic fracturing in tight shales**

The development of inclined and horizontal drilling (see Appendix 1 - Figure A1-2) has helped stimulate intense activity to develop natural gas production from so-called tight shale, i.e., rock in which natural gas is held tightly within the very fine pore structure of the rock. Figure 15 illustrates the procedure used to stimulate these shales. The well is drilled horizontally in the gas-bearing formation, more or less in the direction of the minimum principal stress. Hydraulic fractures are generated (and propped) at intervals along the well to generate a network of connected flow paths that will allow the gas to flow to the well. Depth (i.e., extent) and spacing of the fractures should be optimized to produce the formations effectively. Bunger et al. (2012) discuss the factors in the design of an effective fracture strategy.

**Figure 15.** Staged hydraulic fracturing in a horizontal well. There may be many such wells along the horizontal well.

*Protecting the Public* 

*CanCanPotential for Induced Seismicity be Uniform across US ?* 

**N.Dakota**

Canada

**Texas**

**Figure 14.** Seismic hazard map of the United States — US Geological Survey.

**S. California**

*Seismic Hazard Varies Widely Across US Three orders of magnitude higher in S. California than in Eater n US S. California Eastern U.S.*

**New York**

**Washington D.C.** 

Fractures and Fracturing: Hydraulic Fracturing in Jointed Rock

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

67

**Figure 14.** Seismic hazard map of the United States — US Geological Survey.

numerical model of the site, preferably one that includes the influence of important uncer‐

It is sometimes asserted that the Earth's crust is everywhere close to a 'critical state of stress,' i.e., that a small change in the devatoric stress in the rock is likely to produce slip on one or more faults with associated seismic activity. The current global interest in development of major resources of natural gas, the central role of hydraulic fracturing in this development, and the public apprehension that hydraulic fracturing will 'trigger earthquakes' has led to strong opposition to fracturing, and even legislation to ban the use of hydraulic fracturing in

As illustrated by Figure 14, the seismic hazard, (i.e., probability of a damaging earthquake) varies very considerably from place to place. Thus, an earthquake of a given magnitude is 1000 times more likely to occur in Southern California than it is in the Eastern United States. The hazard is even lower in regions such as Texas, North Dakota and in the stable Canadian Shield region of the North American tectonic plate. While many earthquakes are initiated at depths considerably greater than depths where hydraulic fracturing is applied, it seems plausible to suggest that there may be less potential for fracturing to induce seismic activity in regions that have low seismic hazard. Also, as indicated by the comments of Cornet in the previous section of this paper, there is evidence that the critical stress hypothesis warrants detailed scrutiny, at least. This could have major implications for development of the world's major natural gas and EGS (enhanced geothermal systems) resources. Two recent studies, National Research Council (2012) and Royal Society – Royal Academy of Engineering (2012), have each concluded that the risk that hydraulic fracturing as used in development of energy resources would trigger significant seismic activity is small, but it would be valuable to examine the critical

The development of inclined and horizontal drilling (see Appendix 1 - Figure A1-2) has helped stimulate intense activity to develop natural gas production from so-called tight shale, i.e., rock in which natural gas is held tightly within the very fine pore structure of the rock. Figure 15 illustrates the procedure used to stimulate these shales. The well is drilled horizontally in the gas-bearing formation, more or less in the direction of the minimum principal stress. Hydraulic fractures are generated (and propped) at intervals along the well to generate a network of connected flow paths that will allow the gas to flow to the well. Depth (i.e., extent) and spacing of the fractures should be optimized to produce the formations effectively. Bunger

et al. (2012) discuss the factors in the design of an effective fracture strategy.

tainties and discussion with structural geologists familiar with the area under study.

**10. 'Critical stress state' in the Earth's crust**

66 Effective and Sustainable Hydraulic Fracturing

some countries and some States in the USA.

stress hypothesis more rigorously than has been done to date.

**11. Hydraulic fracturing in tight shales**

**Figure 15.** Staged hydraulic fracturing in a horizontal well. There may be many such wells along the horizontal well.
