<p>Tectonic coal (coal seam) has the characteristics of three highs (geostress, gas content, gas pressure) and two lows (coal strength, permeability), and gas extraction is exceptionally difficult. Coal seam gas injection for CH₄ replacement is an important means to improve the pumping efficiency. For CH₄ flow behavior and adsorption by injecting CO₂/N₂ replacement in coal seams, experimental tests have been carried out. A thermodynamic model of gas injection replacement has been established to numerically simulate the penetration and replacement performance of gas injection replacement, and the breakthrough curve characteristics of gas injection replacement and mass transfer predicted with different influencing factors, which revealed the dynamic response mechanism of coal seam gas injection. The experimental and simulation results show that the penetration curves and gas temperature change curves at the outlet end agree with the experimental results both qualitatively and quantitatively. Gas adsorption capacity determines gas diffusion resistance in coal particles. Compared with N₂ injection to drive the CH₄ penetration curve, CO₂ injection to drive the same curve is steeper, and the penetration time is longer. As the effective pore diffusivity is smaller, the coal particle size is larger, the gas injection pressure is higher, and the gas injection penetration time is shorter. Moreover, the effective pore diffusivity is larger, the coal particle size is smaller, the gas injection pressure is lower, and the average CH₄ replacement rate is larger during gas injection replacement.</p>