Mechanisms of CO 2 /CH 4 Competitive Adsorption in Fractured Shale Using NMR

CO 2 injection is critical for enhancing shale gas recovery, yet the competitive adsorption mechanisms between CO 2 and CH 4 in fracture systems remain unclear. This study integrates nuclear magnetic resonance (NMR) experiments with numerical modeling to investigate CO 2 /CH 4 competitive adsorption in fractured shale. Wire-cut cores with controlled fractures were used in online NMR displacement tests, establishing a quantitative relationship between the fracture surface area ratio and methane adsorbed capacity. A novel layered CO 2 injection model was developed, incorporating fracture and competitive adsorption effects. Results demonstrate that CO 2 enhances methane desorption, converting adsorbed methane into free and confined states, as evidenced by opposing trends in the NMR signals. Numerical simulations further reveal that a stratified injection strategy increases gas production by 11.54% compared to commingled injection. Considering competitive adsorption at a fracture surface area ratio of 0.45 also improves the CO 2 storage capacity by 9.10%. Field implementation should prioritize enhancing fracture development, followed by increasing the CO 2 injection rate and high-permeability streak permeability, to maximize storage performance. This study provides a theoretical and experimental foundation for optimizing stratified CO 2 injection into shale reservoirs.