Development of permeability anisotropy during coalbed methane production

Although coal-gas interactions have been comprehensively investigated, prior studies have focused on one or more component processes of effective stress or sorption-induced deformation and for resulting isotropic changes in coal permeability. In our previous work, a general porosity and permeability model was developed to represent both the primary medium (coal matrix) and the secondary medium (fractures) under variable stress conditions. In this study the permeability model is extended to define the evolution of gas sorption-induced permeability anisotropy under the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement, and implemented into a fully coupled model for coal deformation, gas flow and transport in the matrix system, and gas flow and transport in the fracture system. Furthermore, the model incorporates a heterogeneous distribution of fractures in coal. The results demonstrate that under the condition of constant reservoir volume the interaction between fracture and matrix is controlled by the sorption-induced strain only, while under stress controlled conditions, the differential stress may elevate the gas sorption-induced coal permeability anisotropy.