Influences of softening behaviour of coal on evolution of its permeability by considering matrix-fracture interactions

The evolution of dynamic permeability is an important basis for improving the production of coal bed methane. In order to obtain the influences of the softening behavior of coal from matrix-fracture pressure interactions on the evolution of its permeability, a dual-pore permeability model with modulus reduction ratio from differential pressure is obtained through theoretical analysis and is validated based on the permeability transient method tests and the finite element numerical simulation software COMSOL. The experimental results show that the strain is divided into the initial, rapid growth and equilibrium phases based on the characteristics of the curve change. During the rapid growth phase, the slope of the strain curve increases from 1 to 3 MPa with slopes of 83.77, 270.54, 440.92 m/s respectively. The modulus-softening coefficient is a function of the strain and its value increases. Furthermore, a dual-pore permeability model with modulus-softening coefficient is obtained by proposing a conceptual model for open and closed fractures. The experimental data are consistent with the results of the improved permeability model, demonstrating that the modulus-softening coefficient dominates the dynamic evolution of the permeability. Finally, the numerical simulation method can be used to monitor the pressure in the coal matrix compared to the experimental method. Thus, the pressure difference between the matrix and the fracture reveals the mechanism of permeability evolution in coal samples.