Simulation and Prediction of CO2 Injection Characteristics in Carbonate Gas Reservoirs

Abstract Understanding the flow and transport characteristics and mechanisms of CO2 in water-saturated carbonate gas reservoirs is crucial for enhancing CH4 recovery and CO2 sequestration. We developed a multiphase multicomponent numerical model for gas and water phases, which considers the multiple transport mechanisms between CO2-CH4 and CO2-water and couples the chemical reactions of CO2 in water-saturated carbonate porous media. After validating the model with analytical solutions, we investigated the effects of CO2 injection density, in-situ CH4 density ratio, and initial fluid saturation on CO2 flow characteristics and porous media properties. The results indicate that the fluid density difference between different components controls their transport mechanisms: smaller density differences lead to higher concentration diffusion intensity between components, while larger density differences result in viscous flow with convective transport characteristics. The dissolution effect of CO2 on carbonate rock enhances the flow capacity of the porous medium for different components and provides greater storage space for CO2 sequestration. Fluid saturation is a key factor affecting the transport characteristics of CO2 in porous media. Due to the mutual solubility of CO2 and water, water accelerates the flow of CO2 in the porous medium, leading to premature CO2 breakthrough and negatively impacting CH4 recovery.