Model Studies of the Depletion of Two-Phase Geothermal Reservoirs

Abstract

Abstract The simulator SHAFT79 of Lawrence Berkeley Laboratory has been used to study the depletion of different types of geothermal reservoirs. Investigations of idealized systems include effects of gravity and fluid injection. Pressure decline is analyzed as a function of cumulative production. The main conclusions are as follows. The well-known p/Z method for estimating fluid reserves is not applicable to two-phase geothermal reservoirs. There is a strong tendency toward spatially uniform boiling. This causes a pressure decline, which allows in many cases estimates of the total reservoir volume and of the total heat content of the reservoir rock. Propagation of a boiling front through a deep water table, as a consequence of fluid production, gives rise to a peculiar pattern of pressure decline. This may allow prediction of the distance of the water table from the producing wells and of the vertical thickness of the water zone, thereby giving important clues to estimating fluid reserves. The pressure effects of injection of colder fluid depend strongly on (one- or two-) phase conditions in the reservoir, injection rate, and absolute permeability. Average pressure actually may decline in two-phase reservoirs rather than increase as a result of injection.