Permeability characteristics of CH4, CO2, and N2 during the whole process of adsorption in coal with accumulated pressure under triaxial stresses

This study aimed to explore the seepage characteristics of N2, CH4, and CO2 in coal during the whole process of adsorption with cumulative gas pressure. A self-developed device was utilized to conduct the adsorption isotherm experiment, and the theoretical method based on the gas state equation was utilized to calculate the real-time permeability of gases during adsorption. The gas flow and velocity at the inlet and outlet were calculated on the basis of the pressure and time change law. Results showed that, during adsorption with accumulated pressure, the average pore pressure of N2 increased, whereas that of CO2 decreased. As cumulative pressure increased, the gas flow at the inlet gradually decreased, whereas that at the outlet increased. The flow and velocity of CO2 at the inlet were the largest, followed by CH4. On the contrary, the flow and velocity of N2 at the outlet were the smallest, followed by CH4. On the basis of the gas flow and velocity at the inlet and outlet, the adsorption amount and permeability of gases in coal were calculated. Gas adsorption isotherm conformed to Langmuir characteristics. Under certain gas pressure, the real-time permeability showed a quadratic form with adsorption amount, and the real-time gas permeability decreased with the increase of cumulative pressure. The real-time permeability of N2 was greater than that of CH4 and CO2, followed by CH4. The research can provide theoretical basis for deep coalbed methane development and carbon sequestration in coal seam, even gas flow in porous media.