Gas EOR processes can have microscopic displacement efficiency as large as 100%, but suffer from poor sweep of the reservoir. Injected gas has tendency to segregate at the top of the reservoir due to density difference and gravity. Water-alternating-gas injection was proposed by Caudle and Dyes (1958) to reduce the effect of adverse mobility ratio. Stone (2004) proposed a new injection scheme (sometimes called “modified SWAG”) in which water and gas are injected simultaneously from parallel horizontal wells, with gas being injected from the bottom of the reservoir and water is injected from top from a site directly above the gas well. The water injected from the top impedes the vertical flow of gas, allowing it to move horizontally before the gas segregation happens. This gives a deeper penetration of gas before gravity segregation than simultaneous co-injection water and gas from the same well (“SWAG”). A recent study by Jamshidnezhad et al. (2010) examined the performance of the “modified SWAG” process in 3D and an MSc thesis (Mahalle, 2013) followed up on this work. The first study found that the gas injection was non-uniform in nature, even in homogeneous reservoirs. The more-recent MSc thesis (Mahalle, 2013) appears to show that the non-uniform injection behavior may have been a result of poor grid refinement near the injection well. It was suggested to further investigate the effect of grid-block size on non-uniform nature of gas injection. The MSc thesis also shows that if the instability develops, it depends on complex interactions among the grid blocks along the well: the injection rate of one segment of the well depends on that in its neighbors, and the instability grows along the well from one end to the other. This project extends the earlier studies, first, by examining the effects of grid refinement near the well. We conclude that grid refinement around the injection well doesn’t seem to stop non-uniform injection and the non-uniformity of gas injection is not a simulation artifact of using grid blocks that are too large. Increasing gas-injection rate correlates with increasing non-uniformity of gas injection. Decreasing gas saturation exponent (ng) in Corey’s 2-phase model and decreasing gas viscosity (?g) leads to more-uniform injection. When there is no connection between neighboring grid blocks along the gas injection well, the gas injection behavior is more uniform. We further examine the effect of gas flow on hydrostatic pressure and conclude that hydrostatic pressure is involved in the non-uniform behavior of gas injection. The results from this study indicate that the non-uniformity in gas injection is a result of coupling of various factors, such as, gas saturation, gas relative permeability, gas injectivity, effect of gas flow on hydrostatic pressure, and effect of adjacent grid blocks. When coupled together they form the self-reinforcing cycle leading to non-uniform behavior of gas injection, with most of the gas being issued from one end of the well. Segmenting the well into multiple segments helps ensure that gas issues from most of the perforations along the gas-injection wells and thus ensures better sweep.