Effect of moisture content on N₂ injection displacement efficiency of methane in bituminous coal

Hydraulic fracturing is a commonly used technique to improve coalbed methane extraction permeability, but it significantly alters coal seam water content, thereby affecting subsequent gas injection displacement efficiency. To clarify the regulatory mechanism of water content on nitrogen (N₂) injection displacement, this study used bituminous coal from Xutuan Coal Mine, Anhui Province, as samples. Coal samples with three water content states (dry, moist, and saturated) were prepared, N₂ displacement of CH₄ experiments were conducted, and the dynamic responses of tail gas composition, flow rate, and adsorption pressure were monitored. The results show that with increasing water content, the gas injection displacement rate exhibits a V-shaped characteristic of “first decreasing and then slightly increasing”, the percolation capacity generally weakens, and the CH₄ adsorption equilibrium pressure rises systematically—confirming that moisture impairs the adsorption capacity of coal samples. Comprehensive analysis reveals that free water inhibits gas migration by blocking percolation channels in the early stage of displacement, while adsorbed water accelerates CH₄ desorption by competing for adsorption sites after the middle stage. This study clarifies the dynamic mechanism of moisture affecting coal seam gas injection displacement and provides important theoretical support for optimizing gas injection stimulation strategies in water-bearing coal seams.