American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 45th Annual California Regional Meeting of the Society of Petroleum Engineers of AIME, held in Ventura, Calif., April 2–4, 1975. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Geothermal energy is being used throughout the world to heat commercial and residential buildings, to provide hot water for spas, and for the production of electricity. Geothermal resource areas occur primarily in areas of geologically recent volcanism, mostly near the edges of continental plates which are either moving apart (spreading centers) or overriding one another (subduction zones). The western United States, and California in particular, is blessed with numerous geothermal areas. The spreading center known as the East Pacific rise passes through the Gulf of California into passes through the Gulf of California into Southern California and exits the continent near the northern end of the state. There are a dozen or so prime geothermal areas in the state, including the productive Geysers field in Sonoma County and the imminently productive Long Valley and Imperial Valley areas in Mono and Imperial Counties. At the present time the geothermal industry is in its infancy. Throughout the world, most of the geothermal energy produced is used for spas and space heating. The need for electric power provides the main thrust of present power provides the main thrust of present worldwide exploration for geothermal energy, but it is believed that the direct utilization of low-temperature geothermal waters will increase markedly. In the United States, the emphasis of geothermal exploration is for the production of electricity. The world's largest electricity producing geothermal installation is at The producing geothermal installation is at The Geysers in Northern California. It is this country's only commercially productive geothermal field and is currently producing 516 megawatts, about 1.5% of the state's needs. Projected estimates of ultimate capacity range Projected estimates of ultimate capacity range above 5,000 megawatts. With normal development of The Geysers and other promising areas in the state, geothermal energy resources may be producing more than 4% of the state's electrical producing more than 4% of the state's electrical energy needs by 1990. DEFINITION Geothermal energy is the natural heat of the earth which can be extracted in the form of hot water and/or water vapor (steam). It is presently being used worldwide and in California presently being used worldwide and in California to generate electrical power, heat commercial and residential buildings, and to provide hot water for spas.