Development of the Critical Gas Saturation

Abstract

Introduction The critical or threshold gas saturation is a specific property of the porous medium and internal fluids. property of the porous medium and internal fluids. Compared with the experimental research on the subject, I the theory of gas saturation development to its critical value and the significance of this growth on reservoir performance have received little attention. For a volumetric reservoir - i.e., one with negligible water influx and negligible change of rock porosity with internal fluid pressure change, and with porosity with internal fluid pressure change, and with water production and expansion also considered zero - the usual procedure to compute gas saturation development is the material balance equation: Bo - Boi + Bg (Rsi- Rs) Np=------------------------ N,..............(1) Bo + Bg (Rp - Rs) with average reservoir pressure below the bubble-point pressure and Sg less than Sgc. In Eq. 1, 1 Np Rp = --- RsdNp.........................(2) Np 0 In words, only the solution gas has been produced, since the gas saturation remains insufficient for gas flow. The recovery percentage is obtained from the material balance equation; the gas saturation is obtained from the recovery percentage. Eq. 1 is an involved integral equation of a type unknown to me. Np is a self-dependent variable with the limit of the integral in the denominator - Eq. 2. In Ref. 2, Eq. 1 is not presented in the form shown but as a numerical solution procedure that simplifies the presentation. In fact, this numerical solution is an everyday affair for the practicing engineer. An alternative formula is presented. presented. JPT P. 907