Abstract
Exploring the damage differences between different coal rank coal reservoirs subjected to liquid nitrogen (LN2) cooling is of great significance to the rational development and efficient utilization of coalbed methane. For this purpose, the mechanical properties, acoustic emission (AE) characteristics and energy evolution law of lignite and bituminous coal subjected to LN2 cooling were investigated based on the Brazilian splitting tests. Then, pore structure changes were analyzed to reveal the difference in the microscopic damage between lignite and bituminous coal after LN2 cooling. The results showed that compared with bituminous coal, the pore structure of lignite coal changed more obviously, which was manifested as follows: significant increases in porosity, pore diameters, and pore area; a larger transformation from micropores and transition pores to mesopores and macropores. After LN2 cooling, the thermal damage inside lignite and bituminous coal was 0.412 and 0.069, respectively. The thermal damage reduced the cohesive force between mineral particles, leading to the deterioration of the macroscopic physical and mechanical properties. Simultaneously, denser AE ringing counts and larger accumulated ringing counts were observed after LN2 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 LN2 cooling, leading to more complicated crack formation paths and a higher fragmentation degree, such as that of lignite coal.
Gas seepage laws based on dual porosity and dual permeability: Numerical simulation and coalbed methane extraction practice
