Replacement–Displacement Effects During CO2/N2-Enhanced Coalbed Methane Recovery for CH4 Mitigation and CO2 Storage

CO2/N2-enhanced coalbed methane recovery (ECBM) offers a potential route to improve coalbed methane production, reduce CH4 emissions, and couple gas drainage with low-carbon coal development. However, the relative roles of adsorption-controlled replacement and pressure-driven displacement under deep stress conditions remain insufficiently resolved. Here, CO2 and N2 injection experiments were conducted under different vertical stresses to quantify the evolution of gas flow, breakthrough time, increase in coal gas content, replacement–displacement ratios, and injection efficiency. Increasing stress compressed the pore–fracture network, reduced gas transport capacity, and delayed breakthrough of the injected gas. CO2, because of its strong adsorption affinity, remained dominated by replacement throughout the injection process. Higher stress enhanced CO2 retention in coal and therefore its potential storage capacity, but it also weakened sustained CH4 recovery by restricting transport. In contrast, N2, which adsorbs weakly, rapidly shifted to displacement-dominated recovery after breakthrough. Although high stress delayed the formation of connected displacement pathways, N2 maintained high injection efficiency. These results show that stress controls the dominant ECBM mechanism by regulating adsorption retention, seepage transport, and displacement outflow. The findings provide a mechanistic basis for selecting injection gases and designing low-carbon ECBM strategies in deep coal seams.