Study on Coal Seam Damage Caused by Liquid Nitrogen Under Different Ground Temperature Conditions

The thermal stress caused by the ultra-low temperature of liquid nitrogen (LN2) can seriously affect the porosity of the coalbed. In this paper, the effects of various temperature differences on the LN2 damage were studied by changing the initial temperature, so as to explore the effect of LN2 on coal seam with different buried depth. The X-ray diffraction (XRD), scanning electron microscope (SEM), wave velocity, acoustic emission (AE), and uniaxial compression experiments were used in the experiments. The experimental results show that LN2 causes a lot of damage to coal and the LN2 effect increase at first and then decrease with the increase of the initial temperature. When the initial temperature is 293 K, before and after liquid nitrogen treatment, the wave velocity damage of the coal sample reaches 0.2207 and the compressive strength decreases by 27.92%. These two values are 0.3697 and 47.37% at the initial temperature of 323 K, and 0.2727 and 28.27% at the initial temperature of 353 K. This is because if the temperature exceeds 353 K, it will cause a 3.17% drop in water content, thus reducing the damage caused by LN2, resulting in the overall effect slightly lower than that at 323 K.