Injection of Water Above Gas for Improved Sweep in Gas EOR: Nonuniform Injection and Sweep in 3D

Abstract

Summary In gas-injection enhanced oil recovery (EOR), simultaneous injection of water and gas from parallel horizontal wells with water injected from the upper well [sometimes called modified simultaneous water and gas (SWAG) injection], can give deeper penetration of gas before gravity segregation than SWAG from the same well. Most previous studies of this process were limited to 2D, in which the injection rate is uniform along each well. In 3D, we find that gas injection can be far from uniform, even in homogeneous reservoirs—which are the focus of this study. If gas injection is nonuniform in homogeneous reservoirs, it surely would be so in heterogeneous reservoirs. In some cases, there is an inherent instability in uniform injection along the gas well, even as the water well continues to inject nearly uniformly along its length. In our results, the uniformity of gas injection increases with increasing total injection rate and decreasing vertical distance between gas- and water-injection wells. The instability leading to nonuniform injection depends on the relation between gas saturation and gas relative permeability; we speculate that effects of gas flow on hydrostatic pressure, and therefore on gas-injection pressure, may also play a role. We find that in some cases, lateral movement of gas from the injection point partially mitigates the effects of nonuniform gas injection. Injecting gas from separate, independent segments along the horizontal well improves sweep somewhat by increasing the number of points at which gas exits the well.