Numerical investigation of temperature distribution and thermal damage of heterogeneous coal under liquid nitrogen freezing

As a new water-free fracturing fluid, liquid nitrogen (LN2) has been widely studied and applied in coalbed methane (CBM) production. The heterogeneity of the reservoir has a significant effect on the mechanical strength, heat transfer and damage form of coal. Herein, a thermal-mechanical-damage (TMD) model was proposed based on the damage mechanics and micro-element theories. The numerical results indicated that the surface temperature of the coal sample experienced a rapid drop when the coal sample first came into contact with LN2. Low-temperature and high thermal stress regions were generated on the surface of the specimen. As the LN2 freezing time increased, the low-temperature and high thermal stress regions gradually expanded into the specimen's interior. The temperature and stress changes in coal samples showed three main stages: stage I (the first 200 s) was a rapid cooling stage, stage II (200–4000 s) was a slow cooling stage, and stage III (after 4000 s) was a stable temperature stage. A higher initial coal sample temperature led to more significant instantaneous thermal stress, more serious damage and weaker mechanical properties. The findings would provide a theoretical basis for the LN2 fracturing in CBM extraction.