Fine characterization of the pore and fracture structure and strength degradation mechanism of gas bearing coal

To explore the strength degradation mechanism of gas bearing coal, the uniaxial compression tests were performed on coal under different initial gas pressures by using self-developed gas-solid coupling test system, and the pore and fracture structures of gas bearing coal were characterized by SEM, high-pressure mercury injection, low-temperature liquid nitrogen adsorption and micro-CT scanning system. The mechanical and non-mechanical effects of different gas occurrence states on pores and fractures were described respectively, and the internal relationship between pore and fracture failure and the loss of macroscopic strength of gas bearing coal was revealed. The results show that the degradation degree of average uniaxial compressive strength of coal increases with the increase of initial gas pressure. The pore structure and pore size distribution of coal were characterized by multi-means, and it is found that the development of fracture structure of gas bearing coal is not obvious. The proportion of isolated pores is relatively large, and the connectivity between them is poor, which is not conducive to gas seepage. The method of jointly characterizing the pore structure and pore size distribution of coals can correct the errors caused by "shielding effect" of micropores and transition pores and the errors caused by the "compression effect" of coal matrix or pore and fracture failure caused by the sample size. Adsorbed gas leads to the fracture and failure of micro-elements through non-mechanical action, mechanical action of expansion stress and gas wedge effect of free gas on coal body. The mathematical model of mechanical property deterioration and macro strength loss of gas bearing coal based on the micro view angle was established. From the model, it is found that the action of gas leads to the shift of the center of the Mohr circle to the left, the envelope of Mohr-Coulomb strength to the right, and the cohesion becomes smaller, and finally it leads to the loss of macro strength of coal.