A numerical simulation model of The Geysers geothermal field, California, has been developed using reservoir and production data provided by UNOCAL, Calpine, NCPA and CCPA. The model was originally developed by UNOCAL and was based primarily on data from the Unocal-NEC-Thermal (U-N-T) lease areas; the purpose of this study was to extend the model by incorporating data from other field operators and to use the re-calibrated model to forecast future production trends.

The similarities and differences ill reservoir engineering in the geothermal and petroleum industries are not familiar to many. This unfamiliarity frequently leads to aberrant perception of the risks and rewards of geothermal development in the minds of developers and financiers who are accustomed to the petroleum industry but are new to geothermal. This paper is a comparative survey of the state-of-the-art of reservoir engineering in the two industries.

The Geysers geothermal field, located in Lake, Sonoma, and Mendocino Counties, California is the largest developed geothermal system in the world. Electric power generation started at The Geysers in 1960 with a 12 MW (gross) plant. The total installed capacity in the field peaked in 1989 at 2,043 MW. As more and more power plants were built during the 1980s and net mass withdrawals increased, reservoir pressures at The Geysers declined, eventually resulting in steam shortfalls and declining generation levels. This net withdraw is due to the fact that geothermal power plants at The Geysers typically lose about 70% to 80% of produced mass to evaporation in cooling towers, with the balance of mass being returned to the reservoir through injection of steam condensate.