Permeability Evolution of Damaged Coal Near a Wellbore

The evolution of gas permeability in coal a wellbore is not only induced by the damage of coal matrix but also by the dilation deformation of fractures. This study focuses on two fundamental issues: evolutions of the coal matrix damage and the dilation deformation of fractures, and their impacts on the evolution of gas permeability. As the volumetric strain increases with the number of fractures, the equation of coal damage can be considered as a function of the volumetric strain. Then a stress-depended permeability model with the damage incorporated is proposed for the coal matrix. The permeability evolution model for the fractures under the influence of shearing dilation is obtained based on the Hurst coefficient which presents the roughness of fracture surface. These two permeability models for the coal matrix and the fractures, respectively, are incorporated into a fully coupled model of gas flow field and coal deformation field. A 2D case of the wellbore cross-section is presented to show the permeability evolution. The result shows that the gas permeability increases drastically in the wellbore due to the influence of the effective stress and the damage factor. The result also indicates that the shear displacement plays a dominant role on the evolution of fracture permeability around the wellbore.