Evaluation of coal damage and cracking characteristics due to liquid nitrogen cooling on the basis of the energy evolution laws

• Change in energy evolution laws of coal due to liquid nitrogen cooling was evaluated by uniaxial compression tests.
• The effect of liquid nitrogen cooling on coal damage and cracking characteristics was analyzed using energy evolution laws.
• The cryogenic cracking mechanism of liquid nitrogen on coal was discussed.

Liquid nitrogen used as fracturing fluid can significantly reduce reservoir temperature and cause damage and cracking effect on rock because of its extremely cryogenic characteristic. To investigate the cryogenic damage and cracking characteristics of liquid nitrogen on coal, coal samples were cool-treated with liquid nitrogen. Then, uniaxial compression tests were conducted on the cool-treated coal samples and intact samples (not cool-treated). The energy evaluation laws of intact and cool-treated samples during deformation and failure were compared. Results showed that liquid nitrogen cooling was able to improve the initial damage degree of coal, thereby leading to a significant effect on the energy evolution laws of coal. The elastic energy, dissipated energy, and absorbed energy of cool-treated samples were less than those of intact samples when the samples ruptured. During the initial loading stage, the ratio of dissipated energy of cool-treated samples was greater than that of intact samples. However, as the coal samples were about to rupture, the ratio of dissipated energy of intact samples was greater than that of cool-treated samples. The change in energy evolution laws of coal was mainly caused by the growth of micro-cracks inside coal, which can reflect the damage characteristics due to liquid nitrogen cooling. After liquid nitrogen cooling, the amount of micro-cracks inside coal increased, resulting in the deterioration of mechanical properties and the improvement of the fracturing performance in coalbed methane.