The Geysers geothermal project operated by Union Oil Company of California remains the worldís largest single supplier of geothermal electricity, despite having passed its peak output. The Union-NEC-Thermal (U-N-T) project supplies an installed capacity of around 900 MW, and peak output exceeds 600 MW. In the past decade the project has evolved in response to both resource and market changes. This paper reviews some of those changes and notes their effects on resource performance and
management.

In 1987, the U-N-T project was base-loaded and at the peak of its capacity, providing steam to a fully regulated electric power generator at a fixed price
well below the electric marketís average. In 1997, the project sells steam into an increasingly competitive electricity market under a variable load
contract, using a combination of fixed and variable prices to keep Geysers generation attractive. Daily and weekly load cycling are common.

Flow rate and pressure trends since 1987 show the influences of several developments. These include major power plant modifications, the change from base to variable plant loading and increased injection of surface water. Traditional decline analysis can lead to potentially misleading conclusions when applied to Geysers production data, especially those
reported since 1991. The behavior of the reservoir corresponds well to Unionís forecasts based on previously described simulation methods. These methods have been validated, and used successfully
in selecting power plant and facilities investments.

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