The Geysers geothermal field is the largest developed geothermal system in the world. The total installed capacity is presently 2,056 MW although actual production in early 1994 was estimated to be only 1,200 MW(net). The inability to produce at higher generation levels is due to the significant pressure decline that has occurred within the reservoir, particularly since the mid- 1980's. In 1992, a comprehensive field-wide numerical simulation model of The Geysers field was completed, based on drilling, well-test and production data provided by four of the five major field operators. The numerical simulation model was originally developed by UNO CAL, based primarily on data from the UNOCAL-NEC-Thermal (U-N-T) lease areas, and was extended in this study to incorporate data from the other operators.