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Recoverable EGS Resource Estimates 38 the longer term. For instance, a cube of rock 1 km on a side at 200°C would contain 4 x 10 kJ of thermal energy relative to the ambient surface temperature. However, if the average reservoir rock temperature is dropped only 10°C, the heat recovered from that mass of rock would be 2.5 x 1013 kJ, leaving about 95% of the original energy in place for later exploitation. Chapter 3 3.3.2 Fractured rock volume into parts of the field that have not been exploited. This strategy has worked for hydrothermal systems and should work for EGS as well. There will come a time when old wells will be abandoned or redrilled, or new wells added. Assuming that an EGS reservoir consists of discretely fractured flow paths with average spacings of 100 m or less, then some simplications can be made. Because of the low thermal diffusivity of rock (of order 106 m2/s), most of the temperature drop in the reservoir occurs near the injection well and adjacent to the fracture faces in contact with the flowing fluid. If the reservoir rock temperature drops only 10°C on average, there would be ample energy left in the reservoir for future use with equipment designed to operate at lower temperatures, which would increase the sustainability of the resource for As discussed earlier in this chapter, following active heatmining operations, production flow and heat removal would cease, allowing rock temperatures to fully recover by conduction in less than 100 years (Elsworth, 1989). This would permit EGS energy recovery to operate sustainably into the future. To be conservative, we specified an abandonment temperature of only 10°C lower than the initial rock temperature in estimating the recoverable energy fraction. 14 While solid rock is excellent for storing heat, the rate of heat removal by conduction is slow, as a result of its low thermal conductivity. Only that fraction of the rock volume made accessible by the stimulation process can be considered part of the active reservoir where heat extraction occurs. The basic idea is to create permeability and porosity by hydraulic stimulation to open up channels for fluid to circulate through the rock, thereby shortening the rock conduction path. The transfer of heat in such a porous/fractured rock reservoir is a complex process that is not easy to model analytically. Sanyal and Butler (2005) have done sensitivity studies of the impact of various reservoir properties such as fractured volume, fracture spacing, permeability, porosity, and well configuration on the recovery fraction of heatinplace using 3dimensional finite element modeling. They varied the permeability, flow rate, fracture spacing, well spacing, injectortoproducer pattern, and fractured volume. They found that the single most important parameter affecting how much of the thermal energy that could be recovered is the fractured volume. In fact, perhaps the most important finding of their study is that the net electrical power that can be achieved from a volume of fractured rock is roughly 0.026 We/m3 (26 MWe/km3). This factor applies to a wide variety of productioninjection well arrangements (doublets, triplets, fivespots), fracture spacings (330 m), and permeability (10100 mD). The factor seems to hold constant to within about 5%. It also includes reasonable estimates for parasitic power requirements for circulating the fluid through the reservoir. Based on early field testing of EGS concepts, the geometric arrangement of the production and injection wells, to a large degree, influences the amount of rock that can be stimulated, and the accessible volume of rock that the circulating fluid contacts. EGS wells could be configured in a variety of ways: e.g., with one producer for every injector (a doublet), two producers to each injector (a triplet), or four producers to each injector (the classic fivespot pattern used in enhanced oil recovery operations). The stress regime in the rock volume will determine the fracture pattern and direction, and this will influence the optimal arrangement of injectors and producers. However, having morePDF Image | Recoverable EGS
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