CO₂ Adsorption/Desorption, Induced Deformation Behavior, and Permeability Characteristics of Different Rank Coals: Application for CO₂-Enhanced Coalbed Methane Recovery

To understand and evaluate the CO2 injectivity in different coal seams, low-, middle-, and high-rank coals from Shanxi Province of China were collected to conduct CO2 adsorption/desorption, induced swelling/shrinkage, and permeability experiments. Results show that the adsorption/desorption amount, swelling/shrinkage deformation, and permeability depend on the coal rank. The CO2 adsorption/desorption amount of high-rank coal is the largest, followed by middle-rank coal, and that of low-rank coal is the smallest. The swelling/shrinkage strain and initial permeability of coals follow the sequence middle-rank coal > low-rank coal > high-rank coal. The percentage reductions of permeability of low-rank coal, middle-rank coal, and high-rank coal are 57.46, 48.50, and 71.17% when CO2 adsorption reaches the equilibrium state, indicating that the permeability of high-rank coal is more sensitive for the CO2 adsorption swelling. The swelling and shrinkage deformation presents obvious three-dimensional anisotropic characteristics; the deformation in the vertical bedding plane direction (VBD) is the maximum, the second is that in the parallel face cleat direction (PFD), and the deformation in the parallel butt cleat direction (PBD) is the maximum. The developmental characteristics of cleats and the distribution of macerals in coal contribute largely to the anisotropic deformation of coal induced by CO2 adsorption–desorption. The permeability of coal shows a U-shaped change trend of first decreasing and then increasing after CO2 adsorption because the permeability of coal is first dominated by the CO2 adsorption swelling and then is dominated by the reduction of effective stress. The swelling behavior and permeability attenuation of coal seams after CO2 injection are unavoidable; adopting the reservoir stimulation methods to produce more complex fracture networks is the key to improving CO2 injectivity. Combining the reservoir stimulation methods with CO2-ECBM technology may be an important development direction of the CCUS in coal seams.