Mechanical degradation characteristics and permeability evolution law of coal under liquid nitrogen freeze–thaw cycles

Considering that liquid nitrogen (LN2) refracturing may become a potential coalbed methane extraction technology, this study systematically studied the changes in physical mechanics, permeability, and fracture behavior of coal treated with LN2 freeze–thaw using experimental methods such as uniaxial compression, acoustic emission monitoring, scanning electron microscope (SEM) and permeability testing, and three dimension (3D) cross-sectional scanning. The crack phase field model was used to reveal the thermal cracking mechanism of coal during the freeze–thaw process. Research has found that LN2 freeze–thaw can lead to the deterioration of the physical and mechanical properties of coal, and the deterioration effect increases with the increase in freeze–thaw cycles. In addition, this method effectively improves the degree of fragmentation and cross-sectional roughness of coal after damage, thereby enhancing the permeability of coal. Based on the numerical calculation results of the thermo-mechanical coupling phase-field model, it was found that the initiation and propagation of microcracks are mainly caused by temperature gradients and stress mismatches between mineral particles. In addition, compared to the freezing process, the size and area of microcracks that appear during the thawing process are larger. The overall research results have important guiding significance for the utilization of LN2 fracturing in coalbed methane mining.