Numerical Simulation of Miscible Displacement Processes in Gas Storage Reservoirs

Abstract

The Gas Research Institute has proposed a gas storage operating plan whereby an inert gas, such as nitrogen, is to be injected into a storage reservoir. This inert gas would serve a two-fold purpose. First the inert gas would act as a displacing fluid during injection/withdrawal cycles and second could also replace part of the cushion gas at the outer limits of the storage reservoir. During periods of injection and withdrawal, the convective mixing of the inert and hydrocarbon gases would be important and, during off-season shut-in periods, the mixing of the gases by physical diffusion or dispersion could be significant. In order to study this process, a reservoir simulation model was developed that can simulate the immiscible How of gas and water. The gas phase was treated as a miscible mixture of two gas components, one representing the inert and another representing the hydrocarbon gases. The model uses different PVT properties for the two gases and calculates mixture properties as a function of composition throughout the simulation. The basic model is a two-phase fully implicit reservoir simulator and contains the effects of gravity, capillary pressure, relative permeability and variable reservoir properties.