Reserve Utilization Characteristics of the Tight Sandstone Gas Reservoir in the Qingshimao Gas Field and Gas Recovery Enhancement Through CO2 Displacement and Energy Replenishment

The tight sandstone gas reservoir in Qingshimao gas field has the characteristics of complex gas–water distribution, low reservoir quality, low porosity, low permeability and high water saturation. Single-well productivity is low, the production and pressure decrease rapidly, the stable production period is short and economically viable development remains challenging. Therefore, considering the reservoir physical properties and development characteristics of the Qingshimao area, physical experiments and numerical simulations were conducted to investigate the reserve utilization characteristics of complex tight water-bearing gas reservoirs and to evaluate the effectiveness of CO2 injection in restoring reservoir pressure and enhancing gas recovery after depletion. The results show that: (1) The movable-water saturation of Type I and Type II reservoirs ranged from 2 to 18% and 3–21%, respectively, while increasing water saturation reduced cumulative gas production and increased gas-flow resistance. Type III and Type IV reservoirs are limited by low permeability and fine pore throat. The movable-water saturation is less than 8% and 6% respectively under high water saturation conditions, and the gas–water flow is obviously limited. (2) Both continuous CO2 injection and post-injection soaking can promote residual-gas recovery after depletion. Post-injection soaking prolongs the contact time between CO2 and residual methane, whereas fractured cores exhibit more rapid pressure recovery but earlier CO2 breakthrough. (3) Pore scale and two-dimensional visualization experiments show that after CO2 injection, the pressure is transferred from the injection inlet to the production outlet, and the depleted low-pressure area is supplemented. The incremental recovery factor of the two-dimensional models after CO2 injection ranged from 22.81 to 25.28 percentage points. (4) The numerical simulation results show that permeability, water saturation, and the injection and production rates jointly control pressure restoration and gas recovery during CO2 injection. The high-permeability reservoir achieves a higher recovery factor but experiences earlier CO2 breakthrough. High water saturation and high injection and production rates will weaken the effective sweep. In field application, the layers with good connectivity and moderate water saturation should be preferred, and the injection and production rates should be reasonably controlled to reduce the risk of gas channeling. Overall, post-depletion CO2 injection can effectively restore reservoir pressure, mobilize residual methane, and enhance gas recovery in tight water-bearing gas reservoirs. The experimental results support post-depletion CO2 injection as a potential approach for improving the development performance of tight water-bearing gas reservoirs.