Improving coal seam permeability and displacing methane by injecting liquid CO2: An experimental study

•The change rule of coal pore fracture and mechanical properties before and after liquid CO2 leaching is analyzed.•The stress–strain curves of coal samples before and after liquid CO2 leaching were obtained.•The characteristics of CO2 displacement of CH4 in coal under different gas injection pressure are compared. To study coal permeability after the coal has been treated with liquid CO2, coal samples were first treated in a purpose-built liquid CO2 soaking system. After samples were treated in this system, cracks on the samples’ surfaces, the coal’s pores structure, and its mechanical properties were studied using scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and uniaxial compression tests. The results show that large and complex fracture networks are generated on the surface of the treated samples. During liquid CO2 soaking, some of the micropores are enlarged and transformed to mesopores and this leads to adsorption and seepage pores becoming connected. Compared with untreated coal, the uniaxial compressive strength and elastic modulus of the treated coal samples decreased by 20.3%–44.6% and by 8.7%–43.5%, respectively. An experiment on methane displacement by CO2 showed that the CH4 concentration in the displaced gas for the same displaced gas flow rate increased in tandem with the gas injection pressure. At the same injection pressure, the CH4 concentration decline increased in tandem with the displacement flow rate.