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 pro-posed 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.

Automated summary

A thermal-mechanical-damage (TMD) model was proposed to investigate the effects of liquid nitrogen on the temperature, stress, and damage of coal samples. The results showed that the coal sample experienced a rapid drop in surface temperature upon contact with liquid nitrogen, leading to the formation of low-temperature and high thermal stress regions. These regions gradually expanded into the interior of the specimen as the freezing time increased. The temperature and stress changes in coal samples exhibited three main stages: rapid cooling, slow cooling, and stable temperature. Higher initial coal sample temperature resulted in more significant instantaneous thermal stress, more severe damage, and weaker mechanical properties. These findings provide a theoretical basis for the application of liquid nitrogen fracturing in coalbed methane extraction.