Study on enhancing natural gas recovery by CO2 injection in water-bearing heterogeneous carbonate reservoirs

The technology of switching to CO 2 in natural gas reservoirs to enhance natural gas recovery is highly recognized for its ability to store carbon dioxide and improve gas recovery rates. However, the application of this technology in carbonate reservoirs with edge and bottom water presents two main challenges: the significant heterogeneity of carbonate rocks and the pronounced intrusion of edge and bottom water. This study investigates the mechanisms of Enhanced Gas Recovery and the basic laws of gas-water seepage in reservoirs through laboratory physical simulation experiments. Using core samples with varying permeability, high and low permeability conditions were simulated to characterize the heterogeneity of carbonate reservoirs. The experimental results indicate that: 1. After switching to CO 2 , high-permeability cores have better energy replenishment effects compared to low-permeability cores, with the recovery rate of high-permeability cores improving by 8.2 % compared to low-permeability cores; 2. The injected carbon dioxide can effectively push back the formation water that has intruded into both high and low permeability cores, releasing natural gas that was previously trapped by the formation water, thus alleviating the adverse effects of water intrusion that lead to low recovery rates; 3. Under different injection pressures, the recovery degree of high-permeability cores is consistently higher than that of low-permeability cores; when the pressure recovers to 100 %, the recovery rate of high-permeability cores reaches its highest level at 76.97 %, which is 30.79 % higher than that of low-permeability cores. Therefore, CO 2 -EGR technology demonstrates significant potential in controlling water intrusion and improving recovery efficiency in carbonate gas reservoirs. • CO 2 injection enhances gas recovery in high-permeability cores to 76.97 %, 30.79 % higher than low-permeability cores. • CO 2 edisplaces intruding formation water and releases trapped gas, alleviating edge- and bottom-water effects. • CO 2 -EGR shows strong potential for water control and recovery enhancement in heterogeneous carbonate gas reservoirs.