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

CO2 injection is critical for enhancing shale gas recovery, yet the competitive adsorption mechanisms between CO2 and CH4 in fracture systems remain unclear. This study integrates nuclear magnetic resonance (NMR) experiments with numerical modeling to investigate CO2/CH4 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 CO2 injection model was developed, incorporating fracture and competitive adsorption effects. Results demonstrate that CO2 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 CO2 storage capacity by 9.10%. Field implementation should prioritize enhancing fracture development, followed by increasing the CO2 injection rate and high-permeability streak permeability, to maximize storage performance. This study provides a theoretical and experimental foundation for optimizing stratified CO2 injection into shale reservoirs.