A New Method of Matching Aquifer Performance and Determining Original Gas in Place

Abstract

Summary A new method is presented for determining the original gas in place (OGIP) from depletion performance data in gas reservoirs with water influx. A study of the performance of 32 gas reservoirs shows that the method gives generally better results than either the steady-state or the unsteady-state method currently used. The primary advantages of this method are that limiting assumptions about the aquifer geometry and properties are not necessary and the computation is automatic, quick, and relatively simple compared with reservoir simulation. Monthly gas production and shut-in bottomhole pressures (SIBHP’s) are used as input data. A material balance is performed to determine aquifer influx. Linear programming (LP) is used to match the aquifer behavior automatically. Then the solution is repeated for various OGIP assumptions until the optimal value is determined. With a gas-production and pressure-data history, the OGIP and an aquifer influence function (AIF) can be determined with this method and future performance can be predicted. (The AIF is a "type curve" for the aquifer.) Data from 32 offshore gas reservoirs were used to evaluate the method. Good results were obtained in most of these reservoirs. The method has useful application before, or instead of, a reservoir simulation study.