A simulation study to verify Stone's simultaneous water and gas injection performance in a 5-spot pattern

Water alternating gas (WAG) injection is a proven technique to enhance oil recovery. It has been successfully implemented in the field since 1957 with recovery increase in the range of 5-10% of oil-initially-in-place (OIIP). In 2004, Herbert L. Stone presented a simultaneous water and gas injection technique. Gas is injected near the bottom of the reservoir and water is injected directly on top at high rates to prevent upward channeling of the gas. Stone's mathematical model indicated the new technique can increase vertical sweep efficiency by 3-4 folds over WAG. In this study, a commercial reservoir simulator was used to predict the performance of Stone's technique and compare it to WAG and other EOR injection strategies. Two sets of relative permeability data were considered. Multiple combinations of total injection rates (water plus gas) and water/gas ratios as well as injection schedules were investigated to find the optimum design parameters for an 80 acre 5-spot pattern unit. Results show that injecting water above gas may result in better oil recovery than WAG injection though not as indicated by Stone. Increase in oil recovery with SSWAG injection is a function of the gas critical saturation. The more gas is trapped in the formation, the higher oil recovery is obtained. This is probably due to the fact that areal sweep efficiency is a more dominant factor in a 5-spot pattern. Periodic shut-off of the water injector has little effect on oil recovery. Water/gas injection ratio optimization may result in a slight increase in oil recovery. SSWAG injection results in a steady injection pressure and less fluctuation in gas production rate compared to WAG injection