Multi-scale characterization of pore evolution reveals how thermal shock unlocks coal permeability

The inherently low permeability of coal seams remains a major obstacle to the economically viable development of coalbed methane resources. Liquid nitrogen fracturing has recently emerged as a promising waterless stimulation technology capable of inducing strong thermal shock and enhancing coal permeability. However, the role of multi-scale pore evolution in controlling permeability enhancement remains poorly understood. In this study, a series of low-field nuclear magnetic resonance experiments were conducted to characterize the pore system of coal before and after thermal shock. Saturation and centrifugation techniques were further employed to control free water and bound water within the pore system. The results indicate that permeability enhancement is not solely attributed to the formation of new pores that increase total porosity. Besides, thermal shock induces a synergistic evolution of pore characteristics, including the development of flow channels, the improvement of pore connectivity, and a reduction in structural heterogeneity. Particularly, the decrease in fractal dimension reflects a more homogeneous and better-connected pore system, while the increase in maximum pore diameter indicates the expansion of dominant flow pathways. These findings demonstrate that permeability improvement results from the combined effects of pore generation, structural optimization, and channel enlargement across multiple pore scales. This study offers valuable guidance for optimizing cryogenic stimulation strategies to improve gas migration and production efficiency in low-permeability coal reservoirs.