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Review of EGS and Related Technology – Status and Achievements Chapter 4 45 While the Fenton Hill experience demonstrated the technical feasibility of the HDR concept by 1980, none of the testing carried out at Fenton Hill yielded all the performance characteristics required for a commercialsized system. Three major issues remained at the end of the project as constraints to commercialization: (i) the demonstration of sufficient reservoir productivity with highproductivity fracture systems of sufficient size and thermal lifetime to maintain economic fluid production rates (50 to 100 kg/s per well pair at wellhead temperatures above 150°C), (ii) the maintenance of these flow rates with sufficiently low pumping pressures, and (iii) the relatively high cost of drilling deep (> 3 km) wells in hard rock. Drilling costs become the dominant economic component in lowgrade, low gradient EGS resources (see Chapter 6). In certain geologic situations, controlling water losses will be important, as it can have negative economic and environmental impacts. Initially, the Los Alamos team and others tried to adapt techniques from oil well stimulation using hydraulic fracturing to produce idealized vertical “pennyshaped” fractures formed in a rock mass that behaves as an isotropic, homogeneous continuum where the minimum stress is in the horizontal direction. The implication of creating a reservoir in such a medium was that the most likely effect of water injection under high pressures would be to create a new fracture by tensile failure, thus forming the required surface area needed for heat mining (Smith et al., 1975; Kappelmeyer and Rummel, 1980; Duffield et al., 1981; Kappelmeyer and Jung, 1987). After several years of active field work, some researchers recognized that EGS reservoirs probably consisted of 3dimensional networks of hydraulically activated joints and fractures. These fissure systems contribute to the connection between injection and production boreholes, rather than just one – or even a series of – artificially created hydraulic fractures (see, for example, Batchelor, 1977; and Armstead and Tester, 1987). By the early 1980s, research at various sites (Pine and Batchelor, 1984) confirmed that the creation of new hydraulic fractures was not the dominant process; but that the shearing of natural joints favorably aligned with the principal directions of the local stress field was a more important mechanism. These joints could be completely or partly sealed in their natural state. They fail in shear, because fluid injected under pressure reduces the normal stress across them, but only marginally affects the magnitude of the shear stress. The shearing mechanism allows frictional slippage to occur before tensile failure, i.e., there will be a component of shearing ahead of any hydraulically fractured zone (Baria et al., 1985; Baria and Green, 1989). Shearing of the fractures increases their aperture through selfpropping on the naturally rough surfaces. The realization that shearing on existing joints constitutes the main mechanism of reservoir growth has been one of the most significant outcomes of the international research projects. This has led to a basic change in how researchers interpret the evolution of the structure of an EGS reservoir, as a result of hydraulic pressurization. It has led to a departure from conventional oil field reservoir development techniques (which emphasize discrete hydraulic fracturing as a means of stimulation) toward a new technology related to the properties of any jointed rock mass that is subjected to a particular anisotropic stress regime. Because all formations that have been investigated so far have some sealed or partly sealed fractures (even those in continental shield areas with very low stresses), we can stimulate fracture networks and have them stay open using pumping pressures just over the critical pressure for shear failure. It should also be mentioned that, in every case, connections between the wells follow multiple paths. Every well has a number of flow entry and exit points. These occur at various depths, and their relative importance can change as a function of different pressure regimes.PDF Image | Review of EGS and Related Technology
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