This paper examines on the effect of the reservoir rock permeability on gas injection by using reservoir simulation. This task will be performed by using reservoir simulation software (Eclipse). This injection interacts with CO2 to create conditions favorable for oil recovery. The main target of this project is to investigate the effect of the reservoir rock permeability on gas injection and the optimum injection rate to get the optimum recovery. The problems statement of this study is: As the oil and gas in a formation is produced, the hydrocarbons remaining in the reservoir may become trapped because the pressure in the formation has lessened, making production either slow dramatically or stop altogether. Climate change refers to long-term shifts in temperatures and weather patterns. Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures. Examples of greenhouse gas emissions that are causing climate change include carbon dioxide and methane. The result of effect of the reservoir rock permeability on gas injection by using reservoir simulation shows that with the increase in the permeability of reservoir rock, the rate of gas production increases. The greater the permeability of rocks, the rate of water production increases, which is a direct method relationship between water production and permeability. We note after this evaluation that the cumulative oil, water, and gas production increases with the increase in rock permeability................. Keywords: ............Reservoir Rock Permeability: Gas Injection: Reservoir Simulation “ECLIPSE Software”.