Permeability evolution and displacement efficiency of CO2 displacing CH4 in coal seam under different pressures

CO2-enhanced coalbed methane recovery (CO2-ECBM) can significantly improve gas extraction efficiency while simultaneously inducing permeability reduction. However, the evolution on permeability during displacement is still indistinct. To examine the evolution of permeability and gas displacement efficiency, a suite of experiments is performed to measure the strain characteristics and gas flow rates of coal samples. A mixed gas permeability model is established, and the advantages of injecting pressure at different phases of displacement are discussed. The finding reveals that the coal exhibits higher deformability in the direction perpendicular to bedding planes. CO2 adsorption significantly reduces permeability. While the displacement process is taking place, the permeability is expressed in the increase and decrease phases, and finally, it displays a stable tendency. Under different pressures, the permeability decreases to 95.9%-98.2% of the initial permeability, respectively. The CH4 flow rate experiences decreases, increases, and sustained decline phases during displacement. A stepwise increase in gas injection pressure during different displacement stages enhances the gas displacement efficiency.