The seepage driving mechanism and effect of CO2 displacing CH4 in coal seam under different pressures

The technology of CO2-ECBM holds the potential to enhance the efficiency of coalbed CH4 extraction by boosting the driving force of CH4 seepage and the desorption of adsorbed CH4 in coal. To investigate the seepage characteristics and displacement effects of gas during CO2-ECBM, the experiment of CO2 displacing CH4 in coal was conducted using a triaxial experimental system within a displacement pressure range of 1.5–4.0 MPa. The results showed that increased displacement pressure led to large coal swelling. This swelling caused a reduction in permeability, leading to high resistance to gas flow and the generation of elevated pore pressure. The distribution of the pore pressure in coal displayed a nonlinear feature, predominantly influenced by coal swelling during CO2 adsorption process. With the injection of CO2, the pressure field in the coal transitioned from an unstable to a stable state. The coal strain induced by CO2/CH4 adsorption-desorption varied, leading to the variation in pore pressure gradient and seepage flow rate, which ultimately impacted the displacement efficiency. The evolution of pressure field during CO2-ECBM provides valuable guidance for displacement effects. It contributes to a parametric optimization for CO2-ECBM application in CH4 extraction. •The displacement effect during CO2-ECBM was analyzed.•The permeability evolution of coal was elucidated.•The nonlinear feature of pressure field was revealed.•Optimization of displacement pressure in CO2-ECBM application was discussed.