Study on damage characteristics and influence scope of coal-rock saturated with freeze–thaw water

The cryogenic cycling freeze–thaw technique demonstrates significant applicability in extracting coalbed methane (CBM) from low-permeability saturated coal seams. However, due to the inability to effectively predict the degree of structural damage to coal-rock at different times and impact radii in practical engineering, it severely affects the efficient extraction of CBM. Therefore, by conducting stepwise low-temperature single-cycle freeze–thaw tests on water-saturated coal-rock specimens at − 196 ℃ (the low temperature of liquid nitrogen), − 45 ℃, − 30 ℃, and − 15 ℃, the damage characteristics of the coal-rock specimens after freeze–thaw are determined and analyzed. Additionally, a physical model is established to calculate the damage range of liquid nitrogen freeze–thaw in coal-rock. Finally, through comparison with existing research results and numerical simulation of liquid nitrogen low-temperature freezing heat transfer, the accuracy of the experiments and physical models is ensured. The research results indicate that with the increase in stepwise low temperatures, the temperature stress caused by temperature changes gradually decreases, leading to a gradual reduction in the freeze–thaw damage to the coal-rock specimen structure. It is calculated that under the − 15 ℃ low-temperature environment, the temperature stress is only 0.72 MPa, which represents the effective temperature for enhancing transparency in freeze–thaw damage to coal-rock structures within a single cycle. Based on the established physical model and combined with experimental results, it is possible to rapidly and accurately predict the degree of damage to coal-rock at different locations, the impact range of liquid nitrogen injection wells, and the time required for heat transfer. This aids researchers in scientifically locating liquid nitrogen injection wells and CBM extraction wells, thereby enhancing work efficiency, reducing costs, and promoting efficient, green CBM extraction.