Our study has two features. First, laboratory experiments measured the change of the absolute permeability of a coal pack as a function of pore pressure and injected gas composition at constant effective stress. Second, adsorption solution theory described adsorption equilibria and aided interpretation. The gases tested include pure methane (CH4), nitrogen (N2), and carbon dioxide (CO2), as well as N2 and CO2 binary mixtures. The coal pack was initially dry and free of gas, then saturated by each test gas at a series of increasing pore pressures and a constant effective stress until steady state. Thus, the amount of adsorption varied while the effective stress was held constant. Results show that (i) permeability decreases with an increase of pore pressure at fixed injection gas composition and (ii) permeability change is a function of the injected gas composition. As the concentration of CO2 in the injection gas increases, the permeability of the coal decreases. Pure CO2 leads to the greatest permeability reduction among all the gases tested. However, a small fraction of N2, 10% to 20% by mole, helps to preserve permeability significantly. According to the mixed-gas adsorption isotherms, adsorption and the selectivity of a particular gas species for a coal surfaces is a function of pressure and the gas composition. Therefore, we conclude that loading of coal surfaces with adsorbed gas at constant effective stress causes permeability reduction. Finally, gas adsorption and permeability of coal are correlated simply to extend the usefulness of study results.
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