Sorption-Induced Permeability Change of Coal During Gas-Injection Processes

Our study has two features. First, laboratory experiments measured the change of the permeability of coal samples 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 (CH{sub 4}), nitrogen (N{sub 2}), and carbon dioxide (CO{sub 2}), as well as binary mixture of N{sub 2} and CO{sub 2} of different compositions. The coal pack was initially dry and free of gas, then saturated by each test gas at a series of increasing pore pressures at a constant effective stress until steady state was reached, 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 CO{sub 2} in the injection gas increases, the permeability of the coal decreases. Pure CO{sub 2} leads to the greatest permeability reduction among all the test gases. However, 10 to 20% by mole of N{sub 2} helps to preserve permeability significantly. According to the mixed-gas adsorption isotherms, adsorption and the selectivity of a particularmore » gas species on coal surfaces is a function of pressure and the gas composition. Therefore, we conclude that loading 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.« less