Advanced Decline Curve Analysis in Vapor-Dominated Geothermal Reservoirs

The Fetkovich decline curve analysis method was extended for application to vapor-dominated geothermal reservoirs to estimate the permeability-thickness product (kh) from a well's transient production response. The analytic dimensionless terms for pressure, steam flowrate, decline rate, and decline time were derived for saturated steam using the real gas pseudopressure and customary geothermal production units of pounds-mass per hour. The derived terms were numerically validated using "Geysers-like" reservoir properties at permeabilities of 1, 10, and 100 mD and at a range of initial matrix liquid saturations from 0 to 100%. The rate-time responses collapsed onto a single set of curves using the derived dimensionless terms, validating the derived dimensionless equations. This collapse was accomplished by including the effective compressibility of a boiling liquid or by an alternative formulation considering an apparent time. This technique was applied to actual field production data at The Geysers, California, the world's largest developed geothermal, vapor-dominated reservoir. The production data for over 100 wells in the southeast Geysers were analyzed and the permeability-thickness products determined by application of the derived analytic dimensionless terms and the Fetkovich production decline type curve.