Numerical Simulation of Gas Injection to Increase CH ₄ Production on the Basis of Gas–Water Two-Phase Flow

Gas injection to increase coalbed methane (CBM) production has triple benefits. In this paper, a coupled heat-water-force flow model is established to discuss the influence of different injected gases on enhancing the extraction of CBM, and the effects of mixture injection and injection temperature on gas production are analyzed. The results show that the competitive adsorption phenomenon is significant near the gas injection well, the temperature is higher during CO₂/N₂ gas injection, and CO₂ injection reduces the permeability of coal seam, while N₂ injection helps to increase the permeability. N₂ enhances gas production mainly through high-speed scour, dredging cracks, and displacement, while CO₂ relies on the competitive adsorption mechanism. In short, the effect of N₂ gas injection is better than that of CO₂, but the single injection of N₂ has the problem of premature breakthrough. Compared with single gas injection, mixed gas injection can effectively delay the breakthrough time, reduce temperature fluctuations, and help restore the permeability of the coal seam, thus improving the safety of mining. In the mixed gas injection process, with the increase of the injection temperature, the temperature of the coal seam rises, the repair effect of N₂ is enhanced, and the methane production is increased. Therefore, in practical application, adopting mixed gas injection and appropriately increasing the injection temperature is conducive to increasing the CBM production.