Differences in Petrophysical and Mechanical Properties Between Low- and Middle-Rank Coal Subjected to Liquid Nitrogen Cooling in Coalbed Methane Mining

Exploring the damage differences between different coal rank coal reservoirs subjected to liquid nitrogen cooling is of great significance to the rational development and efficient utilization of coalbed methane (CBM). For this purpose, the mechanical properties, acoustic emission (AE) characteristics, and energy evolution law of lignite and bituminous coal subjected to cooling treatment were investigated based on Brazilian splitting tests. Then, pore structure changes were analyzed to reveal the differences in microscopic damage between lignite and bituminous coal after a cooling treatment. The results showed that compared with bituminous coal, the pore structure of lignite coal changed more obviously, which manifested as follows: significant increases in porosity, pore diameters, and pore area and a larger transformation from micropores and transition pores to mesopores and macropores. After the cooling treatment, the thermal damage inside lignite and bituminous coal was 0.412 and 0.069, respectively. Thermal damage reduced the cohesive force between mineral particles, leading to the deterioration of the macroscopic physical and mechanical properties. Simultaneously, denser acoustic emission ringing counts and larger accumulated ringing counts were observed after cooling. Moreover, the random distribution of thermal damage enhanced the randomness of the macrocrack propagation direction, resulting in an increase in the crack path tortuosity. With more initial defects inside coal, a more obvious thermal damage degree and wider damage distribution will be induced by cooling treatment, leading to more complicated crack formation paths and a higher fragmentation degree, such as that of lignite coal.