Evaluating the Volume Method in the Assessment of Identified Geothermal Resources

Abstract

In 2008 the US Geological Survey (USGS) updated the 1979 assessment of the electric power generating potential of geothermal resources in the United States associated with natural hydrothermal systems. These resources are concentrated in the states of Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming, which contain all 248 identified hydrothermal systems located on private or accessible public lands and with temperatures greater than 90 ?C in the US outside of Alaska and greater than 75 ?C in Alaska that have the potential to be exploited for electric power generation. The estimated mean electric power generation potential from identified geothermal resources in the 2008 assessment is approximately 9060 MW-electric (MWe). Recent studies have raised questions regarding the applicability of the volume method, the technique used in USGS assessments of identified resources, in the evaluation of geothermal reserves. A detailed examination of the method as applied in the 2008 assessment, an understanding of the differences between resources and reserves, and a comparison of the assessment predictions with observed power production from geothermal fields in the Great Basin, demonstrate the validity of the volume method for geothermal resource assessments when properly calibrated using field measurements and physical models.