Gas-Flow Microchannels in Coal Due to Microwave-LN2 Cycles: Adsorption/Desorption Behavior and Nanoscale Surface Morphology

LN2 fracturing improves the permeation structure of the reservoir, but the lengthy thawing time restricts the efficiency of CBM development. In the study, efficient pore structure modification of bituminous coal is achieved through rapid cycles of LN2 cold soaking and microwave thawing. The research shows that the microstructure and surface morphology of the coal change significantly during the cycles, including the increase of the external specific surface area and pore volume, and accelerate the iteration of micromeso-macropores. The volume increase peak of mesopores and macropores reaches 90.78% and 101.42% at the 15th and 20th cycles, which further confirms the development of the pore structure, with more gas adsorption/desorption points and secondary pores beginning to appear. In addition, the heat–cooling coupling affects the damage mechanism of the coal structure, transitioning from crack propagation to matrix fracture and developing into the trend of main cracks expanding followed by convergence. The average fractal dimension increases by 26.23%–75.10%, with rapid freeze–thaw cycles increasing the multiselectivity of gas-flow channels. The findings provide a reference for the integration and optimization of anhydrous fracturing technology for coal reservoirs.