Vapor-dominated geothermal reservoirs such as Larderello, Italy, and The Geysers, California, are fundamentally water-short systems. This is apparent from the relatively low pressures in the natural state, typically about 35 bars at loo0 m depth, which is much below hydrostatic pressures. It is also apparent from the high heat content of the produced fluids, typically superheated steam. Large-scale production from these systems has caused reservoir pressures and well flow rates to decline, leading to an underutilization of installed electrical generating capacity. Clearly, these :eservoirs are beginning to run out of fluid, while heat reserves in place are still enormous. (At Larderello, th-re is a long-term trend of increasing formation temperatures in the upper productive zones of the reservoir; Sestini, 1970). Reinjection of colder waste waters, or injection of waters from a source other than the geothermal reservoir, is the primary means by which dwindling fluid reserves can be replenished.

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