Study on strain characteristics of coal during CH₄ adsorption–desorption process

In order to clarify the deformation law of the coal body in the adsorption-desorption process and reveal its intrinsic dominant mechanism, a study on the strain characteristics of the coal body in the CH₄ adsorption-desorption process was carried out. The isothermal CH₄ adsorption-desorption experimental test of the coal body was carried out at room temperature on an independently constructed experimental platform. Using He as the reference gas, the transient and residual strains during CH₄ adsorption-desorption process were systematically analyzed in the pore pressure range of 1.0-5.0 MPa, and the deformation law of coal body during the interaction between coal and gas was studied in depth. The results show that coal has complex pore structure and strong adsorption capacity, CH₄ adsorption causes swelling and deformation of the coal body, and desorption leads to contraction of the coal body, which triggers a series of coupled effects of stress change and seepage behavior. In the CH₄ adsorption-desorption process, the pore pressure and coal body strain showed a positive correlation trend. With the increase of pore pressure, the volumetric strain induced by CH₄ adsorption on the coal body increases significantly, and the residual strain after desorption also increases. Compared with He, the adsorption swelling strain produced by CH₄ under the same pressure condition is 1.77-3.52 times, and the desorption residual strain is 2.02-3.36 times, indicating that CH₄ adsorption-desorption has an obvious irreversible deformation characteristic on the coal body. The swelling deformation caused by CH₄ adsorption is mainly dominated by the filling and swelling elastic deformation of the pore-crack structure, the adsorption swelling deformation of the coal matrix, and the pore damage deformation induced by the adsorption stress, while the residual deformation after desorption is mainly controlled by the micro-damage effect caused by the release of pore pressure, the reorganization behavior of the coal molecular structure during desorption, and the irreversible deformation of the pore structure, among other factors.