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

N-2 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 N-2 injection, a mathematical coupling model of N-2 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 N-2 injection on the CH4 extraction rate was investigated using this model. Results indicate that the action mechanism of N-2 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 N-2 injection and less pronounced coal permeability increase. A larger initial coal seam permeability results in more favorable N-2 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 N-2 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.