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The main objective of this study is to increase oil production at “X” field, located in South Sumatra, Indonesia which has been declining every year. Water-alternating-gas (WAG) injection is a method that could improve macroscopic efficiency and microscopic efficiency by combining gas injection and water injection. This study used a reservoir simulation to: 1) evaluate and design WAG parameters such as gas-water ratio, gas injection rate, and number of cycles; 2) compare WAG injection to water injection and gas injection; 3) compare the injection gas between hydrocarbon gas and CO2 gas. CMG was used for each of the predetermined combinations of the chosen parameters, getting the cumulative oil production in the well as a function of operational period of the well. The results of this study predict that Water-Alternating Gas injection method results in 15.57% incremental oil recovery and is better compared to the gas injection and water injection. The result of this study also shows that incremental oil recovery of hydrocarbon gas injection is 1.2% higher than incremental oil recovery of CO2 gas injection. In addition, the MMP of CO2 is 1339 psia and MMP of hydrocarbon is 940 psia. Combination of several parameter will be used in design optimization to determine the optimum design to be applied is a water-alternating-gas (WAG) injection method by using hydrocarbon gas with 2777.78 BPD water injection rate, 0.15 MMSCFD gas injection rate, 1:2 WAG ratio, and 3 number of cycles.