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 CO 2 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 CO 2 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 CO 2 -rock reaction durations to elucidate the effects of CO 2 on rock wettability. The results indicate that water invasion makes CH 4 in small pores and trapped gas in large pores difficult to produce, while the NMR T 2 signal shows a pronounced decrease in the mesopore range (1 ms < T 2 < 100 ms). CO 2 exhibits strong adsorption and diffusion capacities, which improve the pore structure, enhance rock hydrophilicity, alleviate water-lock damage, and reduce flow resistance, ultimately enhancing CH 4 production. The experimental results show that continuous CO 2 flooding and CO 2 soaking followed by flooding enhanced gas recovery by 16.65 and 26.09%, respectively, indicating that CO 2 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 CO 2 unlocks gas trapped by water intrusion and enhances gas recovery in tight formations.