What is a geothermal production well?
A geothermal production well produces fluid heated by the natural heat of the earth. Geothermal fluids may be steam or hot water and in California have total dissolved solid concentrations of up to 250,000 parts per million (about seven times above sea water). Very hot geothermal fluids may be used for electrical power generation. Cooler (but still quite hot) geothermal fluids are used for projects such as space heating, aquaculture, snow melting, food processing, dehydration, and hot tubs and spas.
How many wells are there and how much water is produced?
There are about 470 producing steam wells and 230 high-temperature, hot-water wells in 10 high-temperature geothermal fields in California. In 2009, over 328 billion kilograms of water was produced from these wells. In addition, there are several hundred low-temperature geothermal wells in the state for which the Division has no records.
How is a geothermal well drilled?
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 first anticipated steam entry. Then high-pressure air is used. Mud would plug the fractures and pore spaces in the formation and much less steam would be produced.
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.
How is a geothermal well constructed?
High-temperature and Steam-dominated Reservoirs
After a well is drilled, often beyond 5,000 feet, 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 produced fluids.
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, very expensive titanium-alloy casing is needed for all the production wells because the geothermal fluids are so corrosive.
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 polluted surface waters from contaminating shallow, water-bearing zones. The standards for this type of well resemble those for other water wells
Who regulates the wells?
The Division regulates all high-temperature geothermal wells on private and state lands. The U.S. Bureau of Land Management regulates all high-temperature geothermal wells on federal lands, except for wells on U.S. Navy and Marine bases, which are regulated by the Department of the Navy.
How are the wells permitted?
All drilling, reworking, and abandonment operations for geothermal production wells on private and state lands require a permit from the Division. For exploratory wells, an environmental study is required under the California Environmental Quality Act (CEQA) and the Division will act as the lead agency.
Division engineers monitor all production wells to ensure they are operated properly and that well casings remain sound. Most well sites are inspected annually.