| Abstract |
The United States Geological Survey (USGS) has been conducting a series of geothermal resource assessments of the United States, and an important component of the ongoing work is evaluating the resource potential associated with the application of Enhanced Geothermal Systems (EGS) technologies, which involve the creation of productive geothermal reservoirs in low permeability rock units. Whereas conventional geothermal resources are formed due to hydrothermal fluid circulation that results from the convergence of high temperatures and high permeability, typically fracture permeability produced as a result of recent or active faulting, the exploitation of EGS resources involves the augmentation or creation of permeability in situ. Consequently the presence of elevated temperatures at drillable depths is the dominant factor controlling the quality of the resource, provided stimulation of the host rock is technically viable. For the 2008 provisional USGS assessment of EGS potential in the western United States, we calculated EGS resource values for those portions of the crust where estimated temperatures exceed 150 °C at depths less than or equal to 6 km. In this report we summarize the series of thermal modeling studies that formed the basis for the EGS assessment, studies which included evaluation of existing models, acquisition and interpretation of new data, development of alternative models for the distribution of temperature at depth in the crust, and formal analysis of the uncertainties associated with models for temperature at depth. |