The Geysers Geothermal Field, an Injection Success Story

Abstract

The Geysers Geothermal field, the largest geothermal field in the world, is about 100 km north of San Francisco, California. The field started production in 1960 with a 12 MWe power plant. By 1987, steam production peaked at 112 billion kg, generating approximately 1,500 MWe (installed capacity =2,043 MWe). A rapid decline in production ensued. At that point the cumulative mass replacement rate (i.e., the fluid re-injection rate) was only about 25%, resulting in reservoir dry-out and superheat. Without additional recharge, only about 33% of the recoverable heat-energy could be extracted. Hence, with injection, a major heat mining operation could start. However, there was no water except for the cooling tower recoveries and seasonal streams.
For many years, Lake County and the City of Santa Rosa (Sonoma County) had been looking for avenues to dispose their treated effluent. Since The Geysers was in need of water and the county and city needed an effluent disposal outlet, a unique public-private collaboration began. In 1997, Lake County constructed a 42 km long pipeline to transport 1.01 million kg of secondary treated effluent per month to The Geysers for injection, which resulted in additional steam. This prompted Santa Rosa and other municipalities in Sonoma County to construct a similar pipeline. By the end of 2003, the Santa Rosa pipeline was completed, resulting in an additional 1.25 million kg of tertiary treated effluent to The Geysers every month. The current mass replacement from both pipelines and other sources is about 85% of production. This has resulted in sustained steam production, a decrease in non-condensable gases, improved electric generation efficiency, and lower air emissions. The additional electricity generated as a result of these two pipelines is about 155 MWe per year. The Geysers has become the largest heat mining operation in the world. By December 2008, The Geysers had produced 2,394 billion kg of steam, and injected 954 billion kg of fluids, resulting in a net mass replacement of 39.9%. Locally this success story is called “Flush to Flash”.