Coupled Effects of Injection Pressure and Coal Moisture on Gas Pressure and Concentration Distribution during Gas...

The combined use of hydraulic stimulation and gas-injection-enhanced drainage is an important technical approach to improve gas recovery from low-permeability coal seams. However, the coupling effect between residual water after hydraulic measures and the gas injection pressure on gas migration and distribution remains unclear. On the basis of a deep coal-rock stimulation and dynamic seepage simulation experimental platform, a large-scale 500 mm cubic water-bearing briquette coal sample was prepared. N₂ displacement of CO₂ experiments were conducted under different injection pressures (0.6 and 1.0 MPa) and coal moisture contents (4 and 6%). The spatiotemporal evolution of the gas pressure field and the concentration field was systematically investigated. The results show that along the gas flow direction, the pressure rise and CO₂ concentration decline exhibit good synergy and hierarchical response. In the lateral direction, the farther from the injection port, the more delayed the pressure response and the slower the CO₂ concentration decline. In the vertical direction, water migrates downward under gravity, significantly reducing the displacement efficiency in the lower part. Increasing the injection pressure effectively alleviates the inhibition of water on gas transport, especially in the early and middle stages of the displacement. Increasing the moisture content delays the pressure rise and concentration decline and prolongs the displacement cycle. The coupling effect exhibits significant directional differences. The combined effect of injection pressure and water content on gas pressure shows a "1 + 1 > 2" synergistic enhancement, whereas their effect on CO₂ concentration decline shows "1 + 1 < 2" mutual inhibition. This reflects the fundamental difference in the sensitivity of the pressure field and concentration field to water. This study reveals the coupled evolution mechanism of gas pressure and concentration fields during gas injection displacement in water-bearing coal seams, providing a theoretical basis for optimizing injection parameters and improving the gas drainage efficiency.