Updated Methods for Estimating Recovery Factors for Geothermal Resources

Abstract

An important aspect of geothermal resource assessment methodology is the development of geothermal resource models consistent with the production histories of exploited geothermal fields. The primary method applied in past United States Geological Survey (USGS) assessments was the volume method, in which the recoverable heat is estimated from the thermal energy available in a reservoir of uniformly porous and permeable rock using a constant recovery factor, Rg, of 0.25 for the producible fraction of a reservoir's thermal energy. More recent analyses of data from the fractured reservoirs commonly exploited for geothermal energy indicate that Rg is closer to 0.1, with a range of approximately 0.05 to 0.2. In general this apparent discrepancy in Rg reflects the contrast in thermal energy recovery from complex, fracture-dominated reservoirs compared to the uniform, high-porosity reservoirs considered in the early models. Models for the recovery of heat from heterogeneous, self-similar, fractured reservoirs predict variations in Rg that provide a physically realistic basis for evaluating the production potential of both natural geothermal reservoirs and reservoirs that may be created through the application of EGS technology.