Experimental study on the influence of gas pressure on CH4 adsorption-desorption-seepage and deformation characteristics of coal in the whole process under triaxial stress

Abstract

CH4 adsorption-desorption-seepage characteristics of coal is an important indicator to measure the gas extraction capacity of coal seam. The deformation of coal caused by CH4 adsorption-desorption-seepage is an important factor affecting the extraction ability of CH4. Synchronous test experiments of CH4 adsorption-desorption-seepage and whole-process coal deformation under different gas pressures were carried out by using the self-developed comprehensive test experimental system of gas-solid coupling for coal measure gas. The timeliness characteristics of CH4 adsorption-desorption-seepage and coal deformation were analyzed. The influence of gas pressure on CH4 adsorption-desorption-seepage and coal deformation was clarified. The quantitative relationship between CH4 adsorption-desorption-seepage and coal deformation was discussed. A whole-process strain model of coal for CH4 adsorption-desorption-seepage was established. The results showed that increasing gas pressure could promote CH4 adsorption-desorption-seepage of coal. The higher the gas pressure was, the greater the capacity of CH4 adsorption-desorption-seepage in coal was. Increasing gas pressure could promote the expansion/contraction deformation caused by CH4 adsorption-desorption-seepage in coal. The higher the gas pressure was, the stronger the deformation ability of CH4 adsorption-desorption-seepage in coal was. The expansion or contraction deformation caused by CH4 adsorption-desorption-seepage in coal was simultaneously affected by coal bedding structure and external stress. The axial/circumferential strain caused by CH4 adsorption-desorption-seepage in coal showed exponential function, quadratic function, and exponential function growth relationship with adsorption capacity, desorption capacity, and seepage capacity of CH4, respectively. The stress in the process of CH4 adsorption-desorption-seepage in coal covered the expansion stress of adsorbed CH4 to coal matrix, the compression stress of free CH4 to coal matrix, and the expansion stress of free CH4 to coal microporous fracture. Considering three kinds of deformation effects by CH4 in two states, the whole process strain model of CH4 adsorption-desorption-seepage in coal was established, which could effectively reflect the deformation timeliness characteristics of the whole process of CH4 adsorption-desorption-seepage in coal. This research provides some theoretical support for the efficient extraction of coal seam gas.