Modeling of dynamic permeability evolution in CH4-containing coal based on elastic-plastic deformation

The study on the evolution characteristics of coal permeability is of great significance for rationally determining gas extraction parameters and increasing gas extraction efficiency. In order to study the effects of different coal stresses and gas pressures on coal permeability, the experiment on the deformation of stress-loaded coal and gas adsorption-diffusion was conducted, the segmented dynamic model of coal permeability was established, and the rationality of the established model was verified by the experimental results. The results shown that the gas adsorption amount and coal deformation both shown a Langmuir-type with the increasing gas pressure, and the dynamic diffusion coefficient of gas decreased exponentially with time. As the gas pressure decreased, the expansion deformation of the stress-loaded coal decreased, and the permeability increased gradually. The permeability and expansion deformation of stress-loaded coal gradually decreased with the increasing stress. The coal permeability shown a “V” shape with continuous stress loading, and it reached the smallest at the stress peak. The coupling between matrix and fracture deformation caused by gas adsorption, the dynamic diffusion of gas in matrix, and the mass exchange between matrix and fracture were all considered in the established permeability model of coal. The rationality of established segmented model of coal permeability was verified by the experimental results. The permeability model of coal based on elastic deformation can reflect the permeability evolution at the stage of elastic deformation. Within the experimental range, the absolute error between the experimental test and numerical simulation results of coal permeability was −0.135×10−15～0.296×10−15 m2, and the absolute error of volumetric strain of the coal due to gas seepage was −0.327×10−5～2.026×10−5. The permeability model considering plastic deformation can also reflect the permeability after stress peak. The error between experimental and numerical results was −0.435×10−15～0.997×10−15 m2.