Effects of matrix-fracture interaction and creep deformation on permeability evolution of deep coal

A new model considering effects of matrix-fracture interaction and creep deformation on permeability of deep coal is established under the condition of triaxial stress state. An internal swelling coefficient is defined to characterize the degree of matrix-fracture interaction. The internal swelling coefficient, reflecting contribution of matrix swelling deformation by gas adsorption to the change of bulk volume and fracture volume, is introduced to a classical stress-strain constitutive equation. Moreover, the proposed permeability model is validated by experimental data-based permeability estimated by the fractional derivative transient pulse method and the nonlinear least square method is used to determine the model parameters. It is indicated that the permeability decreases gradually in the primary and secondary creep stage, consistent with the physical process in which the initial fracture and pores are gradually compacted during the creep process. In addition, it is found that with the decrease of pore pressure, the proposed permeability model appears to be better than the existed various classical models in term of description of the evolution trend of permeability. The effects of creep deformation and internal swelling coefficient on the permeability model are discussed, indicating that under the condition of constant effective volumetric strain, a small volume of the internal swelling coefficient makes a leading role of the effective volumetric strain in permeability evolution, and a large volume of the internal swelling coefficient leads to a domination role of the adsorption swelling strain increment in permeability evolution of deep coal.