Effect of CO2 injection on CH4 desorption rate in poor permeability coal seams: An experimental study

CO2 injection into coal seams is an effective way for reducing greenhouse gas emissions and increasing gas recovery in coal seams. To study the influence rule of CO2 injection on the CH4 desorption rate, an experimental system of CO2 displacing CH4 was used to analyse the changes in gas composition, outlet flow rate, coal seam temperature, and displacement efficiency parameters during CO2 injection. From the perspective of enhancing CH4 recovery, the CO2 critical injection amount (Vinc) is 297.4 L and critical injection time is 188.75 min. When the CO2 injection amount V < Vinc, the amount of CH4 desorption, CO2 storage, and CH4/CO2 gas content all increased. When V > Vinc, except CH4 desorption amount remained almost unchanged, the other parameters increased. In the process of CO2 displacing CH4, the coal seam temperature rises (exothermic process), showing a law of “hierarchical displacement”. With the injection of CO2, the pore pressure first decreased and then increased, the CH4 partial pressure decreased, and the effective diffusion coefficient first increased and then decreased. After the injection of CO2, the desorption time of CH4 was 269.45min earlier and the desorption rate of CH4 was increased by 12.43 %.