| Abstract |
A model of the Geysers reservoir consistent with existing geological data has been calibrated against 30 years of production and pressure history. Three principal assumptions in the model were that natural recharge and discharge could be ignored, that 98% of the initial fluid mass-in-place was in the liquid phase within the range of drilled depths, and that temperatures increased with depth according to a vapor-static saturation gradient from 0 to 8,000 feet below sea level, and then according to a boiling-point-depth gradient from 0,000 to 12,000 feet below sea level in a zone which was superheated. Reservoirpropertiesand production history in non-UNOCAL leaseholds were not well-constrained because of a lack of publicly available data. Steam from UNOCAL-L. NEC-Thermal ( U-N-T) leases is used to supply 1,103 MW of installed generating capacity built by PG&E, while the gross installed capacity of The Geysers is 2,043 MW. The model was constructed with a uniform Cartesian grid consisting of 32 x 15 x 6 cells each 2,000 feet on a side. The long axis of the model was aligned northwest-southeast roughly parallel to the regional geologic strike. A double-porosity formulation was used and 90% of the initial fluid mass-in-place was contained as liquid within the rock matrix. Pressure losses associated with wells on U-N-T and DWR-Bottlerock leaseholds were calculated individually in the model but the production from wells of other operators was lumped together within grid blocks. matched for the period 1957 to 1987 by comparing measured reservoir pressures extrapolated to mean sea level against simulated values extrapolated to the same datum from the layer with the The model was history-/ highest fracture permeability. The model was used to forecast steam production from U-N-T leaseholds, and the results compared favorably with observed production for the period 1987 to 1989. Over the next 10 years the model predicted that U-N-T steam production would decline to about 8 million lb/hr. |