New, Rigorous Material Balance Equation for Gas Flow in a Compressible FormationWith Residual Fluid Saturation

摘要

Modern computing power has enabled very high accuracy andefficiency in complex calculations. Thus, it follows thatreservoir engineering formulations need not be approximatesolutions, as was sometimes historically the case. Being one ofthe most widely used techniques in reservoir engineering, thematerial balance equation (MBE) for gas is an excellentexample of this. The MBE is used not only for estimating theoriginal gas-in-place (OGIP), but also for calculating thedecline in average reservoir pressure with depletion. In themajority of cases, the conventional p/Z formulation of the gasmaterial balance is satisfactory. However, certaincircumstances, which are sometimes unpredictable, demandformulations with greater accuracy. Although modifications tothe standard approach have been presented, to the authors’knowledge, there is no published gas material balanceformulation that is completely rigorous. This study presents anew, rigorous MBE for gas flow in the presence of acompressible formation and residual fluid saturation.Examples will be presented to highlight the capabilities of thenew MBE.