Cracking Patterns and Damage Evolution Characteristics of Coal with Bedding Structures Under Liquid Nitrogen Cooling

Abstract

Liquid nitrogen (LN₂) fracturing has various advantages, such as low reservoir damage, minimal environmental impact, and excellent permeability. In this study, the cracking pattern and damage evolution characteristics of bedded coal subjected to LN₂ fracturing were investigated. The deterioration features of the mechanical parameters and failure mechanisms were examined in a comparable manner using Brazilian splitting tests. Additionally, the damage characteristics of bedded coal during LN₂ fracturing were explored. The results indicated that LN₂ cooling promoted the development of thermal cracks, consequently reducing the effective bearing capacity of the coal. Randomly distributed thermal cracks actively contributed to macroscopic crack propagation, increasing the proportion of shear cracks and the complexity of the fracture surface. Different bedding angles led to distinct failure modes, significantly impacting the proportion of shear cracks and the fracture surface complexity. Moreover, the bedding planes constantly influenced the propagation direction of the fracturing cracks, resulting in a macroscopic damage zone that expanded preferentially at the weak bedding planes with the borehole at the center. With increasing bedding angles, both the degree and rate of damage of coal decreased sequentially. Consequently, it was feasible to employ LN₂ fracturing in low-permeability reservoirs along the bedding planes, facilitating swift and efficient reservoir fracturing.