Experimental study on the evolution of three-dimensional fracture structure and enhanced permeability of bituminous coal under high and low temperature conditions

Abstract A systematic study of the three-dimensional reconstructed coal microstructure under low-temperature liquid nitrogen shock and cryogenic-heat shock combined with high-temperature convective heating is of great significance for the improvement of permeability of coal seams by low-temperature media. By combining CT scanning experiments with three-dimensional reconstruction techniques, the pore and fracture structures, their spatial distribution, and connectivity within the coal matrix under single-cryogenic (SC) and cryogenic-heat (CH) treatment conditions were investigated. A pore network model of connection fractures with statistical pore radius, throat radius, and coordination number distribution was constructed, based on the "Volume Fraction module" in Avizo. Finally, the effect was verified by a permeability analyzer. Experimental results indicate that SC treated coal surfaces exhibit partially connected, axially developed fractures. And Coal sample that was subjected to CH treatment showed greater fracture apertures. Significant changes in the fracture volume distribution after different treatments were found by statistical analysis. Specifically, the fraction of fracture volume in CH treated samples between 1010-1011 µm3 decreased from 51.5–5.6%, while the segment from 1011-1012 µm3 jumped from 0–85.47%. Additionally, under different treatment conditions, the overall trend of the number of pores and throats increased and then decreased as the pore radius and throat radius increased, and this trend is more pronounced in coal samples treated with CH. The permeability of gas is largely constrained by the degree of expansion of internal fissures within rocks, the permeability of coal samples decreases with the increase of confining pressure.