Production Analysis for Solution Gas-Drive Reservoirs: General Variable Pressure/Variable Rate Case –Theory

Abstract Presently, analytical models for estimating reservoir parameters for solution gas-drive are restricted to either constant pressure or constant rate assumption. Thus, current models for solution gas-drive do not allow for rigorous analysis of simultaneous variations in pressure and rate. In addition, the traditional material balance time or pseudotime, which models variable pressure and/or variable rate case, is limited to single-phase flow. This paper proposes a normalized multiphase pseudotime function that is capable of modeling general variable pressure and/or variable rate data for solution gas drive reservoirs during boundary-dominated flow. In particular, we present a multiphase flow equation that incorporates this pseudotime function. This flow equation is expressed in a form that traces the rate/time harmonic depletion curve. Thus, the proposed approach allows analysts to use a single depletion curve to model constant rate, constant pressure and variable pressure/variable rate cases for solution gas drive reservoir systems. In addition, we propose a multiphase pseudocumulative function that is normalized by pseudopressure drop to permit the extension of flowing material balance method to solution gas drive. This is essential since analysis using flowing material balance method offers a better resolution than decline type curves. It also permits the computation of initial-oil-in-place. The significant contribution of this paper is the generality of the proposed model that allows the rigorous handling of variable pressure and/or variable rate case for solution gas drive reservoir systems. Thus, the proposed approach, as opposed to existing models, is not limited to production constraints. Only the appropriate equations and the methods of analyses and interpretation are presented in this paper. Illustrative examples are deferred to a subsequent writing.