Coal pores and fracture development during CBM drainage: Their promoting effects on the propensity for coal and gas outbursts

This study explored the occurrence of coal-rock dynamic disasters during the low-temperature oxidation of coalbed methane (CBM) reservoirs for different coal ranks by investigating the dynamic development of pores in coal during low-temperature oxidation. This research monitored dynamic changes in the diameters and numbers of pores in different rank coals during low-temperature oxidation using nuclear magnetic resonance (NMR) and the P-wave rock measurement system. Microscopically, by utilizing industrial component analysis technology and gas chromatography, this study determined the dynamic changes occurring in the composition of coal and the concentrations of representative gases in the low-temperature oxidation of different rank coals. By adopting a new comprehensive index K, which is a combination of the coal-rock hardness (f) and the initial velocity (△p) of a gas emission, to predict a gas outburst, the authors predicted coal-rock dynamic disaster hazards in the low-temperature oxidation of different rank coals. The experiment showed that there are similarities and differences in pore development during the low-temperature oxidation of different rank coals. The similarities are illustrated in the consistencies underlying fracture development. Specifically, in the early stage of low-temperature oxidation of coal (30-130 °C), due to the evaporation of water, the dehydration of compounds containing crystalline water and the decomposition and volatilization of volatiles, micro-pores in coal expand and connect to form meso-pores. In the late stage of oxidation (130-230 °C), the macromolecular compounds and volatiles in coal oxidize and decompose such that meso-pores expand and connect to form macro-pores and micro-fractures. However, as the metamorphic degree of the coal increases, the oxidation resistance and thermal stability of the coal improves, and the initial temperature for the development of pores with the same diameter increases. Based on the concept of the comprehensive prediction index K, the probability of a coal-rock dynamic disaster occurring in a CBM reservoir gradually increases as the oxidation temperature increases. The lower the metamorphic degree of the coal, the faster the growth rate. Therefore, the oxidation temperature at which the probability of a coal-rock dynamic disaster increases by 50% is used as the critical temperature for taking fire prevention measures, i.e., 90 °C and 130 °C for lignite and bituminous coals, respectively.