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.

Pressure decline within The Geysers geothermal reservoir has resulted in decreased power output from existing power plants. As a means of partially arresting these ,pressure declines, several field operators and the County of Lake propose to add approximately 13,000 lpm of treated municipal waste water from several small communities into 6 injection wells in the southeastern corner of the field. Water will be carried to the geothermal field via a 50- or 6O-cm diameter pipeline from the vicinity of Clear Lake, a distance of about 45 km. The results of this study suggest that the proposed injection of effluent could help reducing the high rate of pressure and production rate declines without inducing significant geochemistry incompatibility or microseismicity.

There is a persistent perception worldwide that The Geysers field is a classic example of everything that could go wrong with geothermal power development. This presentation analyzes the history of The Geysers to dispel that perception. It concludes that this field has proven to be the most productive geothermal field discovered yet, and that it provides the best example to date of maintaining commercial viability of geothermal power generation through ingenious field management. The analysis considers both the resource behavior and the socioeconomic forces at play at this field. The presentation also forecasts the performance of this field over the next two decades and considers the lessons learned from its forty-year history.