Rock mechanical properties of coal in cryogenic condition

Liquid nitrogen (LN₂) fracturing is a kind of non-aqueous fracturing technology, which is expected to provide a new and efficient way for coalbed methane (CBM) development. The mechanical properties of coal under LN₂ freezing are very important for studying the mechanism of LN₂ fracturing. However, most of the current research is limited to studying mechanical properties of rocks after being frozen by LN₂ and returned to room temperature. In this paper, the effect of LN₂ freezing on the mechanical properties of coal was studied. Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions. In addition, standard electron microscopy (standard SEM) and cryo-electron microscopy (Cryo-SEM) were used to compare the fracture morphology of coal samples at room temperature and LN₂ temperature. The results showed that LN₂ freezing can damage and improve the mechanical properties of coal simultaneously. The strength of saturated coal under freezing is higher than that of dry coal, and the filling of ice can enhance the mechanical strength of coal. In addition, the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN₂ freezing. The main findings of this study are the keys to the research of LN₂ fracturing mechanisms in CBM reservoirs.