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8.2.6 Land subsidence Chapter 8 Environmental Impacts, Attributes, and Feasibility Criteria between 3035 times the surface area for a flash or binary plant, on either a per MW or MWh basis. The nuclear plant occupies about seven times the area of a flash or binary plant. The land use for geothermal plants having hypersaline brines is about 75% greater than either simple flash or binary because of the large vessels needed to process the brine. 8.2.7 Induced seismicity EGS plants are expected to conform more closely to the conventional geothermal flash and binary plants because of the relatively benign chemical nature of the circulating fluids. See Section 8.2.11 for further discussion of land use. If geothermal fluid production rates are much greater than recharge rates, the formation may experience consolidation, which will manifest itself as a lowering of the surface elevation, i.e., this may lead to surface subsidence. This was observed early in the history of geothermal power at the Wairakei field in New Zealand where reinjection was not used. Subsidence rates in one part of the field were as high as 0.45 m per year (Allis, 1990). Wairakei used shallow wells in a sedimentary basin. Subsidence in this case is very similar to mining activities at shallow depths where raw minerals are extracted, leaving a void that can manifest itself as subsidence on the surface. After this experience, other geothermal developments adopted actively planned reservoir management to avoid this risk. Induced seismicity in normal hydrothermal settings has not been a problem because the injection of waste fluids does not require very high pressures. However, the situation in the case of many EGS reservoirs will be different and requires serious attention. Induced seismicity continues to be under 89 active review and evaluation by researchers worldwide. Annual workshops have been held recently to discuss current results (see, e.g., Majer and Baria, 2006). Most of EGS geothermal developments are likely to be in granitictype rock formations at great depth, which may contain some waterfilled fractures within the local stress regime at this depth. After a geothermal well is drilled, the reservoir is stimulated by pumping highpressure water down the well to open up existing fractures (joints) and keep them open by relying on the rough surface of the fractures. Because the reservoir is kept under pressure continuously, and the amount of fluid in the formation is maintained essentially constant during the operation of the plant, the usual mechanism causing subsidence in hydrothermal systems is absent and, therefore, subsidence impacts are not expected for EGS systems. The process of opening fractures can occur in a sliding manner by shear failure or in extensional manner by tensile failure. In either case, acoustic noise is generated during this process. This acoustic noise is referred to as microseismic noise or events. The acoustic noise is monitored during the stimulation process as an EGS reservoir management tool to see how far the stimulation has opened the reservoir in three dimensions (Batchelor et al., 1983; Baria et al., 1985; Baria and Green, 1989; Baria et al., 1995; Baria, 1990; Baria et al., 2005; Baria et al., 2006). This is analogous to tracking a submarine through acoustic noise patterns. The microseismic monitoring pinpoints how the pressure waves are migrating in the rock mass during the reservoir creation process. In the EGS systems studied to date (see Chapter 4) shear failure has been the dominant mechanism.PDF Image | Geothermal Environmental Effects
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