Experimental Study on Evolution of Fracture Network and Permeability Characteristics of Bituminous Coal Under Repeated Mining Effect

In a group of deep coal seams, mining of upper multilayers generates cyclic loading–unloading stresses in bottom layers, improving their permeability and enhancing coalbed methane gas drainage. In this study, the respective repeated mining stress path was introduced into the seepage test. A high-resolution 3D X-ray microscopy imaging system was used for computed tomography scanning of coal samples. The crack characteristics and stress–permeability relationships were analyzed based on the experimental results. It was found that the permeability increased approximately by 5.8 times after the loading–unloading cyclic tests. The fracture volume and rate grew obviously, and the complexity degree of the fracture network increased. The permeability evolution during the test was closely related to stress–strain state of the coal sample with variable response characteristics of axial deviatoric stress, confining pressure, and axial pressure. The realization of different numbers of loading–unloading cycles in the tests revealed similar fracturing effects of the initial and later cycles, whereas the largest fracturing effect was obtained within the effective stress range from 4 to 11 MPa. The mechanical deformation was found to play an important role in the stress–permeability relationship. The crack closure and elastic deformation at low stress-state caused the permeability loss, while crack growth and plastic deformation at high-stress state increased the permeability. The plastic strain significantly grew with the number of loading–unloading cycles, but the fracturing effect of different numbers of cycles in the critical fissure point was nearly the same. The results obtained are considered instrumental in studying the pressure relief and extraction of coalbed methane during upper protective layer mining in coal seam groups.