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 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 dropped rapidly when the coal sample was first contacted with LN2 . The 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 evident 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 4000s) was a stable temperature stage. As the initial temperature of coal sample was higher, the instant thermal stress of the coal sample was greater, the damage was more serious, and the mechanical performance reduction was more severe. The findings would provide a theoretical foundation for the LN2 fracturing in CBM extraction.