Mechanisms of Water-Trapped Gas Recovery during CO 2 Injection in Tight Reservoirs: Insights from Nuclear Magnetic Resonance Analysis

Most gas reservoirs face water invasion during production, resulting in a large amount of natural gas being trapped within the formation and making it difficult to recover. Previous studies have demonstrated that CO2 injection can effectively weaken the water blocking effect and enhance gas recovery in water-invaded tight reservoirs; however, the underlying mechanisms and controlling factors remain poorly understood. Herein, we conducted CO2 flooding experiments on water-invaded, gas-trapped cores at 10 MPa and 50 °C. The entire displacement process was continuously monitored using low-field nuclear magnetic resonance (NMR) technology to quantitatively characterize the dynamic evolution of gas saturation. Meanwhile, spontaneous imbibition experiments were performed on core plugs exposed to different CO2-rock reaction durations to elucidate the effects of CO2 on rock wettability. The results indicate that water invasion makes CH4 in small pores and trapped gas in large pores difficult to produce, while the NMR T2 signal shows a pronounced decrease in the mesopore range (1 ms < T2 < 100 ms). CO2 exhibits strong adsorption and diffusion capacities, which improve the pore structure, enhance rock hydrophilicity, alleviate water-lock damage, and reduce flow resistance, ultimately enhancing CH4 production. The experimental results show that continuous CO2 flooding and CO2 soaking followed by flooding enhanced gas recovery by 16.65 and 26.09%, respectively, indicating that CO2 injection combined with a soaking period prior to production is more effective for enhancing gas reservoir recovery. This study provides insights into the mechanisms by which CO2 unlocks gas trapped by water intrusion and enhances gas recovery in tight formations.