A series of simulation runs using a compositional simulator were conducted to determine the area Of influence that results from injecting acid gas (H 2 S) for 30 years at 330 e(3) m(3) into a 75 m thick high permeability sandstone aquifer. In this study, the three-phase (gas-oil-water) equilibrium calculation was used. Initially, a 5 Township by 5 Range grid model was used to determine the area of influence; subsequently, a smaller grid model I Township by I Range was used. Boundary producers were required to properly simulate ail infinite aquifer. This study found that buoyancy and relative permeability, more than other parameters such as the perforation interval and the gas-water capillary pressure, affected the area of influence. A refined 75-layer model, one-metre thick, was constructed to check the gravity effect. The results of this refined layer model agree closely with a 13-layer model after 11 years of acid gas injection. A blowdown Simulation was also conducted to determine the time to significant water inflow from perforations at the bottom of the zone. The effect of a 50 year shut-in after 30 years of injection was also simulated.