Utilizing CO2 by conducting CO2 injection as a part of Enhanced Gas Recovery (EGR) for gas reservoir is favorable. The consideration for reservoir injection of CO2 lies in the composition of specific gravity from hydrocarbon components that make up the reservoir fluid is discussed on this study. CO2, with a higher specific gravity than the reservoir fluid, can help enhance the recovery of a field. The injection of CO2 into the reservoir occurs in a supercritical state, where the pressure and temperature of the CO2 match the conditions of the reservoir. This study utilizes a reservoir model from Field X, located offshore of Java Island, Indonesia. A synthetic scenario is created, preceded by model validation through history matching. The synthetic scenario involves the relocation of production wells and the addition of an injection well. Sensitivity analysis is conducted on the hydrocarbon components to generate varying specific gravity values, which play a crucial role in the study. The impact of injection rate is also considered as another factor for comparison. An increase in specific gravity leads to an increase in the initial gas in place (IGIP), with significant contributions from the heavier components ethane (C2), propane (C3), butanes (C4). However, there is an incremental decrease in recovery as specific gravity increases. Other decreases happened in the contamination of CO2 relative to C1 where the higher the specific gravity leads to lower contamination of CO2. The breakthrough time is also influenced by the specific gravity, with certain hydrocarbon components resulting in longer breakthrough times.