Exploration of pore structure evolution and damage mechanism of coal under liquid nitrogen freeze-thaw cycles

Liquid nitrogen (LN2) freeze-thaw cycle, a new waterless fracturing method, has a significant potential in coalbed methane (CBM) production. The ultra-low temperature of LN2 causes continuous damage to coal's pore structure and mechanical properties during the freeze-thaw cycles. A series of related experiments were conducted to investigate the damage mechanism of freeze-thaw cycles. The same coal samples were tested by ultrasonic tester, a nuclear magnetic resonance (NMR) instrument, and a comprehensive mechanics test system after freeze-thaw cycles. The experimental results show that the fracture gradually expanded with an increased width under the action of freeze-thaw cycles. Some secondary fractures were formed along the original fracture and connected to form a fracture network. As the number of freeze-thaw cycles increased, the micropores transformed to mesopores and macropores. The proportion of micropores gradually decreased, while mesopores and macropores continuously increased. The porosity of coal samples was positively correlated, while the wave velocity was negatively correlated with the number of freeze-thaw cycles. The uniaxial compressive strength and elastic modulus of coal samples showed a slight and then rapid decrease with increasing freeze-thaw cycles. Based on rock damage mechanics, wave velocity, elastic modulus, and twoparameter coupling models were chosen to define the damage of coal samples by freeze-thaw cycles. It was found that the predicted values of the coupled model were closest to the experimental values. The two-parameter coupling model has a good prediction of the uniaxial compressive strength of coal samples in freeze-thaw cycles without destroying the coal samples. The results would guide LN2 fracturing technology.