Effects of Water and Gas Injection Rates in WAG Flooding for a Heterogeneous Oil Reservoir: A Simulation Study

A simulation study to find the effect of water and gas injection rates to the oil recovery factor from WAG flooding was undertook. The technique was applied as a secondary recovery process and a heterogeneous reservoir model was used. The aim of the study is to determine the optimum injection rate of water and CO ₂ as well as the trend of oil recovery with varying injection rates. The dimension of the model is 5000 ft x 5000 ft x 120 ft which was divided into 20 x 20 x 7 grid blocks. The STOOIP is 68 MMSTB. Primary recovery from the reservoir was 15% of STOOIP. 5 spot injection pattern was used for this study. Water flooding when used as the secondary recovery technique produced 38% of the STOOIP. WAG flooding using WAG ratio of 1:1, 2:3 and 3:2 each produced 41.5%, 44% and 39.5% of STOOIP respectively. WAG proved to be the better secondary recovery technique with WAG ratio of 2:3 being the optimum WAG ratio. Oil recovery factor increases with the injection rate of gas and water until the optimum injection rate and then the recovery starts to decline. The optimum injection rate for water is in the range of 2000 to 2500 STB/D while for CO ₂ is about 2500 to 3000 MSCF/D. The effect of heterogeneity was felt during this simulation as oil in the low permeability layers could not be produced easily. The results of this study are good indication of the future of WAG as a secondary recovery technique. Technical viability and economic feasibility of WAG should be studied intensively while other WAG parameters should be optimized. This study strongly suggests the use of WAG as the secondary recovery technique whenever the situation permits.