Competitive Adsorption–Desorption of CH 4 –CO 2 –N 2 and Synergistic Mechanism of Gas Injection Displacement in Loaded Coal

To reveal the selective storage and transport mechanisms of multicomponent gases during enhanced coalbed methane recovery by gas injection, a self-designed and improved experimental system for the dynamic testing of multicomponent gas storage and transport in coal was employed. Using Sujiagou bituminous coal from Inner Mongolia, China, adsorption–desorption experiments of CH 4 –CO 2 –N 2 mixtures and CO 2 –N 2 displacement experiments of CH 4 were systematically conducted under hydrostatic stress conditions. The effects of gas pressure, component ratio, and confining pressure on the selective storage and transport characteristics and displacement-induced flow enhancement performance were investigated. The results show that the adsorption affinity of coal for the three gases follows the order CO 2 > CH 4 > N 2, whereas the desorption capacity exhibits the opposite trend. The desorption process displays a three-stage evolution pattern: preferential desorption of N 2, subsequent desorption of CH 4, and a delayed desorption of CO 2 . Increases in both gas pressure and confining pressure weaken the selectivity of component storage and transport, and a high confining pressure severely inhibits desorption. The CO 2 –N 2 displacement of CH 4 can be divided into three stages: prebreakthrough of the displacement gases, N 2 breakthrough, and CO 2 breakthrough. The early stage of displacement relies mainly on the driving effect of N 2, while the later stage depends on the replacement effect of the CO 2 . Injection pressure and component ratio exert nonlinear regulatory effects on displacement performance: the maximum long-term cumulative displacement volume is achieved at an 80% CO 2 ratio, and a moderate confining pressure is conducive to prolonging the effective displacement period. On the basis of the existing displacement and flow enhancement index, which can comprehensively characterize the trade-off relationship between CH 4 recovery and the breakthrough rate of the displacement gases, a more comprehensive basis is provided for optimizing gas injection schemes.