Pressure drawdown and build-up analyses in geothermal reservoirs

Abstract

Classical well analyses were performed using Systems, Science and Software's Quasi-Active Geothermal Reservoir simulator (QUAGMR). QUAGMR solves the equations of heat flow and unsteady Darcian fluid flow in one, two, or three space dimensions. The computer model is capable of handling a single-phase liquid-water system or a multiphase liquid/vapor system. Pressure drawdown and build-up well-tests are performed to acquire permeability data for the reservoir. Classical well-test interpretation assumes that only a single phase is flowing during the test. This assumption can lead to dramatic errors in the permeability data if, in fact, there is multiphase flow during the test. Several calculations were performed which demonstrate these errors in interpretation. A suite of four problems is presented to illustrate these results. One and two-dimensional problems were formulated with single and multiphase flow during the well-test. In the single phase problems, very good agreement with the classical theory was obtained. However, in the multiphase problems, the classical interpretation led to gross errors in the permeability data. These results indicate the need for a simulator which will handle multiphase flow in a geothermal system. Without this capability, errors in the interpretation of well flow data could lead to mismanagement of the reservoir engineering of the geothermal field.