page_title: The Geysers
section_title: Geothermal development
hierarchical_section_title: The Geysers / Geothermal development
caption_attribution_description: English: Drilling the AMINOIL geothermal well at Lincoln Rock, Sonoma County, California, USA
context_page_description: The Geysers is the world's largest geothermal field, containing a complex of 18 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains approximately 72 miles north of San Francisco, California.
  Geysers produced about 20% of California's renewable energy in 2019.
context_section_description: The Geysers is the world's largest geothermal field spanning an area of around 30 square miles (78 km²) in Sonoma, Lake and Mendocino counties in California, centered in the area of Geyser Canyon and Cobb Mountain. Power from The Geysers provides electricity to Sonoma, Lake, Mendocino, Marin, and Napa counties. It is estimated that the development meets 60% of the power demand for the coastal region between the Golden Gate Bridge and the Oregon state line. Unlike most geothermal resources, the Geysers is a dry steam field which mainly produces superheated steam.
  Steam used at The Geysers is produced from a greywacke sandstone reservoir, capped by a heterogeneous mix of low permeability rocks and underlain by a silicic intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 miles (6.4 km) beneath the surface, and greater than 8 miles (13 km) in diameter.
  The first geothermal wells drilled in Geyser Canyon were the first in the Western Hemisphere. The first power plant at the Geysers was privately developed by the owner of The Geysers Resort and opened in 1921, producing 250 kilowatts of energy to light the resort. In 1960, Pacific Gas and Electric began operation of their 11-megawatt plant at the Geysers. The original turbine lasted for more than 30 years and produced 11 MW net power.
  By 1999 the steam to power extraction had begun to deplete the Geysers steam field and production began to drop. However, since October 16, 1997, the Geysers steam field has been recharged by injection of treated sewage effluent, producing approximately 77 megawatts of capacity in 2004. The effluent is piped up to 50 miles (80 km) from its source at the Lake County Sanitation waste water treatment plants and added to the Geysers steam field via geothermal injection. 
  In 2003, the City of Santa Rosa and Calpine Corporation partnered on constructing a 42-mile pipeline that became known at the Santa Rosa Geysers Recharge Project (SRGRP). Since 2003, SRGRP has delivered approximately 11 million gallons per day of tertiary treated wastewater to replenish The Geysers’ geothermal reservoir.
  In 2004, 85% of the effluent produced by four waste-water treatment plants serving 10 Lake County communities was diverted to the Geysers steam field. Injecting treated water into the Geysers field increases the amount of power that can be generated.
  The injection of wastewater to the Geysers protects local waterways and Clear Lake by diverting effluent which used to be put into surface waters, and has produced electricity without releasing greenhouse gases into the atmosphere.
caption: Drilling a geothermal well, 1977 (USGS)
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