Modeling Study of Enhanced Coal Seam Gas Extraction via N₂ Injection Under Thermal–Hydraulic–Mechanical Interactions

N₂ injection into coal seams can effectively enhance the gas flow capacity in the late stage of pumping, thereby improving the recovery rate and recovery efficiency of low coalbed methane (CBM). To reveal the thermodynamic flow coupling relationship between the reservoir and the gas phase and its transportation mechanism in the process of thermal N₂ injection, a mathematical coupling model of N₂ injection that considers the deformation of the coal seam, fluid transportation, and temperature change was established. The influence of the seepage heat transfer effect of the coal seam under the effect of N₂ injection on the CH₄ extraction rate was investigated using this model. Results indicate that the action mechanism of N₂ injection in coal seams includes increasing seepage, promoting flow, and displacing gases. A higher initial coal seam temperature results in a smaller thermal expansion and deformation of the coal skeleton during thermal N₂ injection and less pronounced coal permeability increase. A larger initial coal seam permeability results in more favorable N₂ diffusion, which strengthens the flow-promoting effect on CH₄. The effect of gas injection pressure on CH₄ recovery is greater than that of the gas injection temperature: High pressure promotes N₂ seepage, carries CH₄ flow, and increases the CH₄ diffusion effect, whereas higher temperature promotes the desorption of adsorbed gas in the coal seam and improves the recovery rate.