Exploration and Development of Geothermal Power in California

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Report: Exploration and Development of Geothermal Power in California

From 1955 to 1962, approximately 40 wells were drilled in 15 California thermal areas for the purpose of exploring and developing natural steam to utilize for electric power generation. Twenty-four of the wells were drilled in the three areas which at present seem to have the greatest potential for the production of natural steam: The Geysers, Sonoma County; Casa Diablo, Mono County; and the Salton Sea area, Imperial County.Since June 1960, steam from The Geysers thermal area, produced at a rate of approximately 250,000 Ib/hr, has been utilized to operate a 12,500 kw generating unit. Completion of a second generating unit, now under construction, will increase the total capacity of this area to approximately 28,000 kw. Geologic mapping and interpretation of temperature and pressure data from the steam wells suggest that superheated steam is confined in a steeply dipping fracture zone by an overlying body of ground water. The fracture zone is part of a complex system of normal faults which defines a graben structure at least 5 1/2 miles long and about 1 mile wide. The density inversion represented by the steam phase underlying the water phase in the fracture zone is attributed to the thermodynamic equilibrium existing between the two phases in an open system.The Casa Diablo thermal area is located on the southwest side of a volcano-tectonic collapse structure which is approximately 23 miles long and 12 miles wide. Vertical displacement within the collapse structure may be as much as 5,000 feet on the west side and 18,000 ± 5,000 feet on the east side. Four of the tested wells, drilled to depths ranging from 570 to 1,063 feet, flow saturated steam at rates ranging between 19,000 and 69,300 Ib/hr at 7.5 to 39 psig wellhead pressure.The Salton Sea thermal area is located in the vicinity of five small volcanic domes on the southeast shore of the sea. The elevation of the sea, 240 feet below sea level, indicates that it occupies a tectonic depression. Positive gravity and magnetic anomalies suggest the presence of a large intrusive body beneath the volcanic domes. A 5,230 foot well, drilled over this anomaly in late Tertiary and Quaternary sediments, flowed 123,000 Ib/hr steam and 457,000 Ib/hr concentrated brine at 200 psig wellhead pressure.In light of the above data, and data now available from foreign projects, three fundamental problems of geothermal power development can be considered: a) preliminary evaluation of a thermal area; b) location of exploratory wells; and c) estimation of steam reserves. Preliminary evaluation of an area usually is based on natural surface heat flow. Experience to date, however, has shown that by drilling wells in a thermal area, heat flow has been increased 3 to 170 times the observed natural surface heat flow, depending on the permeability and structural characteristics of the thermal fluid reservoir, as well as the initial enthalpy of the thermal fluid. The efficiency of well location can be greatly increased by regional and local tectonic analyses based on geologic mapping and geophysical methods, including gravimetric, magnetic, resistivity, and thermal. Steam reserves and life expectancy of the field depend on rates of heat and fluid flow in an open system rather than on the more familiar condition of mechanical equilibrium associated with the more or less closed system of a petroleum reservoir.

James R. McNitt

California Division of Mines and Geology, 1963

Report Number 
Special Report 75

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Internet link for Exploration and Development of Geothermal Power in California


James R. McNitt. 1963. Exploration and Development of Geothermal Power in California. San Francisco, CA: California Division of Mines and Geology. Report No.: Special Report 75.

Related Geothermal Exploration Activities
Activities (1)

Areas (1)
  1. Long Valley Caldera Geothermal Area
Regions (0)