Numerical simulation of gas injection for upstructure drainage

Abstract

This study describes the development and application of a two-dimensional numerical model capable of simulating a gas injection project for upstructure oil drainage. The partial differential equations are approximated by the finite difference method. The numerical model simulates the fully implicit three phase flow of oil, water and gas. A simultaneous solution of the pressure and saturation distributions is done by matrix inversion. In an areal simulation, dynamic pseudo relative permeabilities, calculated from a vertical cross-section, are used to account for the gravity effects in the third dimension. A variable bubble point technique, which allows for the mixing of the different bubble point oils, is used to properly simulate the gas-oil solubility effects. Results from the simulation runs have shown that the most important controllable variable in an attic oil recovery project is the volume of gas that is injected during each cycle. The maximum gas injection volume can be calculated by applying the Buckley-Leverett theory to a down dip oil flood in an inclined reservoir. The simulation runs also showed that for a wide range of rates if the maximum gas volume was not exceeded, the up or down structure migration of gas was independent of injection rate.