From 1955 to 1962, approximately 40 wells have been drilled in 13 California thermal areas. Twenty-four of the wells were drilled in the three areas which at present seem to have the greatest potential for production of natural steam: The Geysers, Sonoma Co.; Casa Diablo, Mono Co.; and the Salton Sea area, Imperial Co.

In light of data from these three areas, three fundamental problems of geothermal power development can be considered: (a) preliminary evaluation of a thermal area; (b) locating exploratory wells; and (c) estimating steam reserves, Preliminary evaluation of an area is usually based on natural surface heat flow. By drilling wells in a thermal area, however, the heat flow may be increased from 3 to more than 100 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 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 a sealed petroleum reservoir.

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