High-temperature, water-dominated reservoirs
The methods and equipment used to drill geothermal wells in high-temperature, water-dominated reservoirs are very similar to those used to drill oil and gas wells. Conventional rotary drilling rigs and drilling equipment are used and drilling fluid — also called drilling mud — is circulated through the well to bring the cuttings back to the surface and to cool the well.
Because conditions in steam-dominated reservoirs differ significantly from those in water-dominated reservoirs, drilling procedures do as well. In a steam-dominated reservoir, a typical well is drilled with drilling mud to a point above the geothermal reservoir where the steam is located. Then high-pressure air is used to circulate the cuttings from the hole. If drilling mud were used instead of air, it would plug the fractures in the rock formation and reduce the steam production from the well
Low-temperature, water-dominated reservoirs
As conditions in low-temperature, water-dominated reservoirs usually are similar to those found when drilling water wells, the same equipment can be used and water can be substituted for drilling mud as the circulating fluid.
High-temperature and steam-dominated reservoirs
After a well is drilled, steel pipe, called casing, is cemented in place by pumping cement into the annulus (the space between the casing and the rock formation). The casing and cement prevent fluids in different zones from mixing with each other or with the geothermal fluids called brine.
It is important to cement all of the annular space behind the casing. Cement prevents the casing from expanding or buckling when heated and helps prevent corrosion. If the casing is cemented across the production zone, small holes are punched through the casing with a special gun-perforating device. The casing usually runs from just above the producing formation to the surface. In the Salton Sea Geothermal field located in Imperial County, very expensive titanium-alloy casing is sometime used for the production wells because the geothermal fluids are corrosive due to their high mineral and gas content.
Low-temperature, water-dominated reservoirs
Wells in low-temperature, water-dominated reservoirs are usually less than 1,000 feet deep. Typically, the well is cased and cemented to a point above the production zone to prevent surface water from contaminating shallow, water-bearing zones. The standards for this type of well resemble those for other water wells.
Geothermal Resources Council
Office of Energy Efficiency & Renewable Energy
California produces more electricity from geothermal resources than any other state.
Six California counties produce geothermal resources hot enough for electrical power generation in 2018.
California’s deepest producing geothermal well is more than 2 miles deep and located in The Geysers Geothermal field, about 60 miles northeast of San Francisco in Lake and Sonoma counties.
Presidents Ulysses S. Grant, William Howard Taft, Theodore Roosevelt, Franklin D. Roosevelt, and Gerald Ford all visited The Geysers. The first four saw it as a world-famous resort and the last as a modern geothermal field.
Drilled in Southern California’s Salton Sea Geothermal field in Imperial County, the l “Vonderahe”1 well is one of the largest and hottest geothermal wells in the world. It can produce nearly 2.2 million pounds of hot water in an hour, enough to power a 30-megawatt power plant.
Twenty-eight California counties have low-temperature geothermal resources used in commercial, direct-use projects.
Geothermal Resources Council (GRC)
Geysers Geothermal Association (GGA)
International Geothermal Association (IGA)
Geothermal Heat Pump Consortium
International Ground Source Heap Pump Association
Great Basin Center for Geothermal Energy, University of Nevada, Reno
Stanford University Geothermal Program
University of Utah Research Institute
California Energy Commission (CEC)
State Water Resources Control Board (SWRCB)
State Lands Commission (SLC)
Bureau of Land Management (BLM)
Energy Information Administration (EIA)
Office of Energy Efficiency & Renewable Energy (EERE)
United States Geological Survey (USGS)
Sandia National Labs
Idaho National Laboratory
National Renewable Energy Lab
Lawrence Berkeley National Laboratory (LBL)
Lawrence Livermore National Laboratory (LLNL)