Experimental comparisons of gas adsorption, sorption induced strain, diffusivity and permeability for low and high rank coals

Abstract

Because of coal pore structure, the adsorption and diffusion behaviors are quite different for low and high rank coals. Differences of gas adsorption-diffusion and adsorption deformation of low and high rank coal and its permeability evolution were carried out in isothermal adsorption experiment and desorption-seepage testing system. The law of adsorption and adsorption induced strain, diffusion of low and high rank coal and its influence on the coal permeability were revealed. It turns out that, the methane adsorption-diffusion and adsorption induced-strain of low and high rank coal increase with the increase of the adsorption equilibrium pressure. Because of the control of pore structure, the diffusion property of low rank coal sample is higher than that of high rank coal sample. And the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane. Sorption-induced strain of high rank coal is higher than that of low rank coal, which is related to the amount of gas adsorption in coal. The Langmuir volumetric strain is about twice as high as that of the low rank coal. The law of gas adsorption deformation of coal can be described by a Langmuir isotherm adsorption equation. The influence of methane adsorption-induced swelling strain on the permeability of high rank coal is higher than that of low rank coal. At a constant effective stress, the permeability of low rank coal is higher than that of high rank coal, and with the increases of adsorption equilibrium pressure, the permeability decline by a negative exponential function of different rank coals; the rate of permeability reduction of high rank coal is higher than that of low rank coal.