Permeability-enhanced rate model for coal permeability evolution and its application under various triaxial stress conditions

Knowledge on permeability enhancement is of great significance for the exploitation of coalbed methane, shale gas, and tight gas. In this study, the volumetric strain of rocks is considered to be one of the most important permeability change factors. A new model based on the concept of the permeability-enhanced rate (PER), which is referred to as the permeability ratio variation of per volumetric strain, is derived based on Darcy's law, the tablet flow model, and the theories of flow through porous media and capillary flow. To validate the PER model using coal samples under triaxial stress conditions, loading tests at different gas pressures and loading-unloading tests at different loading-unloading rates are conducted. In the loading tests, the PER evolution is quite different for the different gas pressures. The PER value at a gas pressure of 1 MPa reached 108.40, which was 1.69, 1.41, and 1.57 times as high as the PER value at 0.5 MPa, 3 MPa, and 5 MPa, respectively. In the loading-unloading tests, the PER value was calculated based on the initial porosity and the relative increment in the volumetric strain during the whole loading-unloading process. A lower loading-unloading rate implied a more connected failure surface, and the PER value was greatly enhanced in the post-peak phase. The results show that a good relation between the deformation and seepage characteristics is developed in the PER model. The PER can not only capture the failure process and describe the pore-fracture connectivity improvement induced under loading conditions but can also quantitatively evaluate the permeability evolution, which provides an easy and fundamental method to describe the relative degree of permeability change in the recovery of unconventional gas.