Roles of coal heterogeneity on evolution of coal permeability under unconstrained boundary conditions

• The performance of coal permeability models is evaluated against analytical solutions.
• Permeability models are incapable of explaining permeability net reductions.
• Final equilibrium permeability for homogeneous cases is higher than the initial one.
• Coal permeability net reduction is obtained for the heterogeneous cases.
• Successful match with experimental data verifies the role of heterogeneity on coal permeability evolution.

Coal permeability models based on constrained conditions such as constant volume theory can successfully match unconstrained experimental data and field observations. However, these models have a boundary mismatch because the boundary of permeability models is constrained while experiment boundary is free displacement or unconstrained. What the mechanism is to require such a boundary mismatch has not been well understood. In this study, a full coupled approach was developed to explicitly simulate the interactions of coal matrixes and fractures. In this model, a matrix-fracture model is numerically investigated after incorporating heterogeneous distributions of Young's modulus, Langmuir strain constant in the vicinity of the fracture. The impact of these local heterogeneities of coal mechanical and swelling properties on the permeability evolution is explored. The transient permeability evolution during gas swelling process is investigated and the difference between the final equilibrium permeability and transient permeability is compared. With the heterogeneity assumption, a net reduction of coal permeability is achieved from the initial no-swelling state to the final equilibrium state. This net reduction of coal permeability increases with the fracture (injection) pressure and is in good agreement with laboratorial data under the unconstrained swelling conditions. Coal local heterogeneity in vicinity of fracture can therefore be the mechanism of the above mismatch.