Triaxial experiments on adsorption deformation and permeability of different sorbing gases in anthracite coal

The gas ad-/de-sorption induced deformation of coal is one of the key parameters that influences the coal permeability and the long-term coalbed methane (CBM) production. Three coal samples were collected from No.3 coal seam of Permian Shanxi Formation in the southern Qinshui Basin at which is the most productive CBM field in China. Triaxial experiments of CO2, CH4 and N2 adsorption induced deformations in coal were carried out by a laboratory desorption-seepage testing system. The law of adsorption induced deformations of different gases and their influences on the coal permeability were comprehensively analyzed. For anthracite, the experimental results indicate that axial, radial and volumetric sorption induced strains increase as increasing of adsorption equilibrium pressure if the core was kept under a constant effective stress defined as the difference between the average external stress and pore pressure. And the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane; the axial strain, radial strain and volumetric strain induced by CO2 adsorption is higher than that by CH4 and N2. At a constant effective stress, the permeability of N2 in coal is higher than that of CH4 and CO2, with the increase of adsorption equilibrium pressure, the adsorption swelling strain of coal matrix increases. With the increase of volumetric strain, the permeability declines by a negative exponential function during the adsorption process; the behaviors of adsorption deformation and adsorption capacity of different gases in coals follow similar trends.