Reservoir engineering studies of the Gladys McCall geopressured-geothermal resource; Final report

Transient pressure analysis techniques have been used to evaluate the performance of the Gladys McCall geopressured-geothermal reservoir. A fault-controlled aquifer influx model has also been developed to account for pressure support observed during both reservoir depletion and recovery phases. The Gladys McCall No. 1 well was drilled and completed in the lower Miocene geopressured sandstones under the US Department of Energy geopressured-geothermal research program. The well was shut in October 1987 after producing over 27 MMstb of brine and 676 MMscf gas since October 1983. Eight pressure transient tests were conducted in the well. Analysis of transient pressure data provided a quantitative evaluation of reservoir characteristics, including: (a) formation transmissibility and skin, (b) the size and possible shape of the main producing reservoir, and (c) characteristics of the pressure support mechanism. The pressure behavior of 1983 Reservoir Limits Test (RLT) suggested that the Gladys McCall reservoir might have a long narrow shape with the well located off-center. An elongated numerical model developed accordingly was able to reproduce the pressure characteristics shown in the test. During both the reservoir production and shut-in periods, pressure buildup tests indicated some degree of external pressure support. Aquifer recharging was believed to be the main source. Based on reservoir material-balance calculations, an aquifer influx model was derived from a conceptual model of water leakage through a partially sealing fault into the reservoir under steady-state conditions. Moreover, a match of the pressure history required that the conductivity of the fault be a function of the pressure difference between the supporting aquifer and the reservoir