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220 Chapter 2 Geothermal ResourceBase Assessment 2.3.2 Crustal stress 2.3.3 EGS geology Data on the state of stress are shown in Figure 2.9 (Zoback and Zoback, 1991; Zoback et al., 1991). All stress regimes are represented in the conterminous United States. The stress regime is extensional in areas such as the Basin and Range and the Gulf Coast; and compressional in parts of the eastern United States and locally in the state of Washington. Strikeslip stresses are also typical of large areas such as along the transform plate in California. However, there are still large areas that are not wellcharacterized; detailed resource evaluation in these areas will have to include stress studies. There is not enough information to determine the optimum stress regime for EGS geothermal development. In Australia, the planned development in the Cooper Basin is in a highly compressive regime with geopressured conditions (Wyborn et al., 2005); while, at the Soultz area in Europe, the stress regime is extensional (Elsass et al., 1995). Because the stress regime determines drilling strategies (see Chapter 6); and because, in opening fractures, the most favorable ones are along the direction of maximum shearing stress, it is important to have information on regional stress direction and magnitude in the planning of EGS geothermal development. Much of the thermal energy resides in “basement” rocks below the sedimentary section. Because basement is usually defined as areas of metamorphic or igneous rocks, the composition and lithology of “basement” is actually extremely variable. The basement lithology below the sedimentary cover, where present, is as complicated as the surface exposures. While the generic description “granite” is used in this report, the lithology is not exactly specified. Quantification of the most favorable rock composition and structure for EGS development remains to be done. Most of the experimental EGS sites have been in granite (in a strict geologic sense), because of the expected homogeneity of the rock type. In fact, there may be situations where layered rocks might be equally or more favorable because the orientations of fractures might be easier to predict and the rock types may be more extensively fractured. From a more practical point of view, the lithology also affects the heat flow in the form of its radioactive content and the resulting heat flow. As has already been described above, areas of high radioactivity will have higher heat flow and so may have higher temperatures, all other factors being similar. Some of the EGS resource resides in the sedimentary section, however. In general, as depth and temperature increase, the permeability and porosity of the rocks decreases. So, at depths of 3+ km and temperatures of 150+°C, the rocks are similar to basement in permeability and porosity. In many areas of the country, there is extensive drilling for gas at depths where temperatures are well within the EGS range because the gas deadline is on the order of 200+°C. In many of these areas, the rocks are “tight” and must be fractured to produce commercial quantities of gas (Holditch, 2006). In fact, much of the gas resource remaining in the United States is related to these types of formations. Examples are the Cretaceous sandstones in the Pieance Basin, Colorado (Mesa Verde and Wasatch Formations), and the East Texas Jurassic section (Bossier, etc.). These sandstones are “granitic” in bulk composition but still have some intrinsic porosity and permeability. Modeling by Nalla and Shook (2004) indicated that even a small amount of intrinsic porosity and permeability increases the efficiency of heat extraction, so that these types of rocks may be better EGS hosts than true granite. Thus, there is a natural progression path from the deep hot gas reservoir stimulation and production to EGS reservoir development in both technology and location. It seems likely that these areas might be developed early in the EGS history, because of the lower reservoir risk than in unknown or poorly known basement settings.PDF Image | Geothermal Resource Base Assessment
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