Micro-damage model of gas-bearing coal under load and instability identification criteria

The distribution of pores and skeletons within coal reservoirs significantly affects the migration of gases and the occurrence of gas dynamic disasters. To further explore the micro-damage mechanisms in gas-containing coal, a detailed study of the micro-damage process in gas-containing coal was conducted. Atomic force microscopy was employed to conduct in-situ tests on the surfaces of protruding and non-protruding coal samples before and after loading. The results indicate that the surface structure of the coal samples changes after loading, with a reduction in closed pore diameter, damage to some pores, and a tendency for connectivity between adjacent closed pores. Before loading, the pores in coal samples exhibit irregular distribution, while after loading, pore connectivity increases, and the number of open pore throats slightly increases. Loading leads to a reduction in the modulus of coal skeleton in protruding coal samples due to pore connectivity, while non-protruding coal samples experience internal structure compaction, resulting in a slight increase in elastic modulus due to their higher strength. Micro-damage types and concepts in coal were defined, and the stress distribution characteristics around coal pores and the coal skeleton were analyzed, revealing the micro-damage mechanisms in gas-containing coal under different conditions. Simultaneously, the factors influencing the closed-cell micro-gas explosion were discussed. The stress at the end of a slender elliptical hole is greater along the hole wall, making it more susceptible to closed-cell micro-gas explosions. Two forms of occurrence of open-pore micro-damage were described, revealing the constraining effect of the "bottleneck effect" on micro-damage. Inherent fractures were identified as the weak link in the coal skeleton, and the evolution of their rupture was analyzed. Utilizing theories such as linear elastic fracture mechanics, elastic-plastic mechanics, and permeation mechanics, criteria for detecting pore damage and coal instability under stress disturbances were established. The micro-damage characteristics of gas-containing coal and the mechanisms inducing coal and gas outbursts were summarized, and the research direction of coal and gas outburst was prospected.