Numerical Simulation of Gas Injection in Oil Reservoirs

ABSTRACT Modeling of gas injection into an undersaturated oil reservoir using current simulation technology relies on the validity of assuming instantaneous equilibrium between the oil and the gas phases. This assumption makes the calculated performance highly sensitive to the size of the grid blocks. In particular, the calculated breakthrough time of the injected gas is underestimated. This is primarily due to the equilibrium assumption that requires the gas to saturate the oil in each individual grid block before flowing to the next block. A method is presented that treats the transient resolutioning of gas in oil within the framework of a practical reservoir simulation model. The inclusion of nonequilibrium mass transfer requires that the simulator include one mass balance equation for each component in each phase. A heuristic mass transfer model that governs the amount of gas transfered the oil phase is described. Parameters to the heuristic mass transfer model are determined from considerations of formation geology and the diffusion-convection characteristics of the hydrocarbon reservoir system. The results from the simulation model were compared to solutions obtained by conventional variable bubble-point models, and it was found that the performance, as estimated by the conventional simulators, is highly sensitive to the thickness of the blocks used to represent the reservoir. In order to verify the solutions obtained by the proposed model, comparisons were made to runs made with a high resolution reservoir model including both diffusion and convection effects. It was shown that the heuristic model relating the interphase mass transfer to the composition of the phases, is capable of efficiently simulating the gas injection process.