No abstract

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 stressstate 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.

In coalbed methane (CBM) exploitation, liquid nitrogen (LN2) fracturing has attracted much attention as a promising and effective means to enhance coal seam permeability. Considering the bituminous coal in the Xutuan coal mine of Anhui Province, China, the permeability characteristics of coal samples under different pressure and stress conditions, and a single LN2 treatment and cyclic fracturing conditions were studied. Additionally, an evaluation of the macroscopic cracking behavior of coal was carried out. LN2 treatment can be very effective in promoting the generation of macroscopic fissures in the coal, and the degree of damage increases with the number of fracturing cycles. The results of a single LN2 treatment showed that under a constant confining pressure, the permeability increased exponentially with the increase in gas pressure. The cyclic LN2 treatment showed that permeability first decreased and then increased with increasing gas pressure, which is consistent with the quadratic function distribution. The permeability of the coal sample increases exponentially with the number of cycles, but the growth rate gradually slows down. For the same absolute fracturing time, cyclic fracturing had a better fracturing effect than a single cycle, and the ratio of the permeability increment is 126.3%‐213.8%. The research results provide some guidance for the development of the LN2 freeze fracturing technology.