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Rock permeability in high-temperature geothermal systems

Economic development of high-temperature geothermal systems for electrical power generation requires the production, through wells, of large volumes of steam or hot water. However, one of the principal cost factors in drilling a geothermal well stems from the loss of drilling fluid from the well bore to the formation. Both fluid production and fluid loss result from permeable zones in the rock. Fluid-loss zones in the upper parts of wells are caused by both inter-granular and fracture permeability. Reservoirs for high-temperature fluid production are formed by fracture systems where, typically, fractures result from faulting. Faults are generally modeled as consisting of core and damaged zones. The core is the area where most of the strain is accommodated. It is separated from the protolith, or undeformed rock, by a damaged zone that includes joints, which have little or no offset, as well as faults with lesser offset than the core. In nongeothermal zones, the core is often of lower permeability than the damage zone due to the accumulation of crushed rock. However, within the geothermal system, the core appears to have the highest permeability. Brittle rocks are preferred lithologies as the hosts of geothermal reservoirs, whereas rocks that behave in a ductile manner form system boundaries or barriers to fluid flow within the reservoir. Evidence suggests that the bottoms of geothermal systems are formed by the transition from brittle to ductile behavior that is a function of rock type, temperature and strain rate. A model is presented here that describes the changing permeability in different zones of a geothermal system.