Methane and helium adsorption of coal and its related deformation under different temperature and pressures

Abstract

ABSTRACT In general, the adsorption/desorption of free and adsorbed gas in coal will cause the coal matrix swelling/shrinkage. However, there is no constant conclusion on how the coal deforms under the two types of gas under different temperature and pressure conditions. Herein, a self-developed constant-temperature adsorption/desorption and deformation test system is used to monitor the axial and radial deformations of coal caused by the adsorption of different gases. The effects of effective stress and gas-self adsorption on the deformation in the process of expansion of gas adsorbed–coal are studied. The results showed that at the initial stage of gas adsorption by coal, the axial deformation of coal exceeds the radial deformation. Under the same injection pressure, the volumetric strain caused by injecting He and CH4 into coal first increases and then decreases with increasing temperature. During the CH4adsorption of coal, the volumetric strain of coal caused by effective stress exceeds that caused by adsorption stress. With increasing injection pressure, coal deformations caused by adsorption stress varied insignificantly. At a constant temperature, the CH4 adsorption stress increases slightly with increasing injection pressure. Under a constant injection pressure, the decrease in the CH4 adsorption stress is large when the temperature rises from 40°C to 60°C and is small when the temperature rises from 60°C to 80°C. The effect of temperature on the CH4 adsorption stress is more significant than that of the injection pressure. Finally, the ratio of volumetric strain caused by adsorption stress to that caused by effective stress decreases with increasing temperature, and it is approximately constant at temperatures between 60°C and 80°C. In general, the volume deformation caused by adsorption stress is mainly affected when the temperature is <60°C; thus, 60°C can be regarded as the threshold temperature at which the gas adsorption of coal is significantly changed.