Experimental study of microstructure and mechanical properties-acoustic emission characterization of high-rank coal under supercritical CO2 action

The supercritical CO2 fracturing technology has significant potential for developing coal fracture and increasing gas permeability. Understanding its influence mechanism on coal is crucial for advancing research and practical engineering applications of this technology. In order to accurately characterize the changes of pore-fracture structures and mechanical properties of the high-rank coal under the action of supercritical CO2, taking high-rank coal from Zhongmacun Mine of Coking Coal Group as the experimental object, the changes of pore and fracture structure of the high-rank coal before and after supercritical CO2 treatment were analyzed by Brunauer-Emmet-Teller (BET) and Mercury intrusion porosimetry (MIP), and the mechanical and acoustic emission characteristics of the high-rank coal before and after supercritical CO2 treatment were tested by combining with uniaxial compression and acoustic emission experimental system. The results show that supercritical CO2 had a good effect on pore expansion and permeability enhancement of the high-rank coal. After supercritical CO2 treatment, the percentage of micro-small pore volume of the high-rank coal decreased, the percentage of medium and large pore volume increased, and the total pore volume of the high-rank coal increased after supercritical CO2 treatment. Supercritical CO2 had an obvious deteriorating effect on the mechanical properties of the high-rank coal. The uniaxial compressive strength and modulus of elasticity of the high-rank coal after supercritical CO2 treatment were significantly decreased by 70.06% and 55.56%, respectively. The acoustic emission signal activity of the high-rank coal after supercritical CO2 treatment decreased significantly, and the uniaxial compressive time, cumulative ring counts and cumulative energy of the high-rank coal were reduced by 98.68 s, 95.14×103, and 200.30 V·ms, with the decreases of 46.65%, 37.65%, and 50.03%, respectively. The proportion of accumulated ringing count and accumulated energy in the quiet period of high rank coal increased significantly, while the proportion in the slow period decreased. The proportion in the surge period of accumulated ringing count decreased, while the proportion of accumulated energy increased slightly. The research results will be conducive to promoting the mechanism research, and have certain guiding significance for the coalbed methane extraction of deep high-rank coal and CO2 underground storage.