A new approach modeling permeability of mining-disturbed coal based on a conceptual model of equivalent fractured coal

Although many permeability models have been proposed in last few decades to characterize the permeability evolution in poroelastic coalbed methane (CBM) reservoirs, none of the existing model is capable of defining the failure-involved coal permeability behavior which is the common situation for deep coal in-seam boreholes and over matured CBM reservoirs. In this study, a conceptual model named equivalent fractured coal (EFC) was initially proposed to characterize the structure of plastic deformation with new fracture generation. Based on the EFC model, the fracture dilation with deviatoric stress evolution within coal bulk is quantified by an exponential function, and a plastic deformation-based coal permeability (PDP) model was then proposed by incorporating the quantified fracture generation and dilation into a poroelasticity-based permeability model we built previously. Then the PDP model was validated by matching it with the tested permeability data in the unloading-flow experiments. The experiment results show that at the initial stage, the permeability increases slowly with a decrease of the confining pressure, which is mainly caused by the increase of fracture aperture. Later, with the further decrease of the confining pressure, the permeability increases by orders of magnitude, this is dominantly resulted from the generation of large amounts of new cracks. In addition, the results show that the PDP modeled permeability evolution well agreed with the experimental measured results for the whole process of unloading. This suggests that the PDP model cannot only predict the permeability variation under fracture generation and dilation process during plastic deformation, but also can describe that in the case of elastic deformation. The primary implications for this study is to provide a new mechanism-based approach to predict the permeability behavior in deep CBM reservoir during primary depletion or coal seams disturbed by underground coal mining.