Analysis of production and reservoir performance at the CASA Diablo geothermal project

The Mammoth-Pacific geothermal project at Casa Diablo has been in production since January, 1985. The plant generates 7-8 MW of electric power using a binary system supplied by geothermal fluid production from four wells that produce about 3500 GPM of 340º F, low salinity geothermal fluid. The wells produce from a fault/fracture system that is apparently continually recharged from a deep "reservoir" with no significant drawdown in pressure or decline in flow rate over the 2 year period. Prior to the start of production a series of well tests were conducted to determine the pumped flow capacity of the original four wells and to determine reservoir properties from pressured drawdown and build-up analysis. Since the start of operations a continuous record of production rate, flowing bottom-hole pressure, and temperature has been maintained. The well tests and production records have been evaluated to determine the nature of the reservoir and reservoir permeability and other properties. This paper presents the results of that evaluation