Variation in permeability during CO2-CH4 displacement in coal seams: Part 1-Experimental insights

The injection of CO2 into deep coal seams can improve the recovery of CH4 and contribute to the geological sequestration of CO2. To reveal the controlling factors associated with CO2 displacing CH4, experiments were conducted to simulate the process under different confining pressures (8, 12, 16, and 20 MPa) and CO2 injection pressures (1-6 MPa). The results show that coal can adsorb more CO2 than CH4. However, the permeability of CH4 is greater than that of CO2 under inlet gas pressures of 1-5 MPa. For CH4 saturated coals, the CH4 desorption volume is higher under CO2 displacing conditions compared to pure CH4 desorption processes under the same pressures. The coal adsorbed CO2 decreases under high pore and confining pressures, indicating the displacement results may be better in shallow buried coals. The axial, radial, and bulk strains of coal show steady, sharp, and then slow declining trends with decreasing pore pressures, and the inflection points generally occur at 5.25 and 3.25 MPa. Meanwhile, the strain in three directions decreases exponentially with the increase of effective stress, and the radial strain is higher than the axial strain under each confining pressure. With the combined influences of effective stress, matrix swelling/shrinkage, and gas slippage, the permeability decreases first and then increases with the decrease of pore pressure. With the decrease of pore pressure and the increase of confining pressure, the combined influence of the three factors on coal permeability gradually weakens. The results would be beneficial for controlling the injection process and permeability variation predicting during CO2 sequestration and the replacement of CH4 in coals at different burial depths.