The lifetime of a geothermal power plant is strongly dependent on the fracture spacing and fracture permeability of the geothermal reservoir. This conclusion is based on solution of three coupled linear rate equations for the time dependence of the average temperature of the rock, pore fluid and fracture fluid. By averaging temperature over the entire reservoir and assuming all the fluid is reinjected, effects such as drawdown are ignored, and an upper bound for the reservoir heat content and average wellbore temperature is determined as a function of time. Heat transfer from the rock to the fluid by conduction both in fractures and in pores and by the flow of fluid through the pores is accounted for in this model.

Several approaches to the problem of thermal depletion of a geothermal reservoir are possible. A volumetric estimate of heat in place may be based on an estimate of the volume of fluid available (Towse, 1975). Heat obtained from the rock may be estimated and included in the thermal resource estimate. An estimate for the resource life time is then made by assuming some fraction of the thermal resource is recoverable (White and Williams,1975). A complete numerical simulation of the coupled heat-flow and mass-flow equations for specified well and reservoir geometries can be attempted. Numerical simulations are complex and often involve so many adjustable parameters that it is often difficult to gain insight into a problem from such a solution. Our approach is to idealize the problem (see e.g., Bodvarsson, 1972) and consider only the essential aspects of the heat and mass transport as discussed below. In this way, a model is obta ned which involves a small number of parameters and provides insight into the reservoir behavior during production.

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Copyright 1975, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.