Cryogenic Fracture Dynamics in Coal: Crack Initiation and Propagation Mechanisms Induced by Liquid CO2 Phase Transition.

Liquid CO2 fracturing is an anhydrous fracturing technology with broad application prospects, but the low-temperature impact effect on the injection process has attracted insufficient attention to the strengthening mechanism of fracture network formation. In order to explore the low-temperature impact effect of liquid CO2 fracturing the coal seam, according to the tensile stress criterion and the effective stress principle, the coal fracture propagation criterion considering the low-temperature impact of liquid CO2 is constructed. The self-designed true triaxial liquid CO2 fracturing low-temperature impact initiation and propagation platform is used to explore the energy release characteristics and fracture network evolution mechanism in the process of coal fracture propagation combined with acoustic emission monitoring. The results show that when the injection temperature of liquid CO2 is decreased from 10 to -10 °C, there is a significant increase in the temperature difference between the coal body and the liquid CO2 interface. This induced a stronger temperature gradient field, generating a substantial temperature-induced tensile stress concentration at the crack tip region. Consequently, the peak pressure inducing crack propagation was effectively reduced, with the propagation pressure decreasing by 10.5%. The proportion of temperature stress increased from 10.46 to 23.13%. Both the peak-induced fracture pressure and the temperature stress exhibited a variation rate of 0.054 MPa per unit increase in temperature difference. The increase in the number of branching cracks ranged from 33.3 to 56.25%, with an average increase of 45% in the number of branching cracks for every 5 °C decrease in temperature, and the crack network density and distribution range increased significantly with the decrease in temperature. This study reveals the mechanism by which the low-temperature impact of liquid CO2 intensifies the evolution of fracture networks in coal bodies, providing theoretical support for optimizing liquid CO2 fracturing technology in coalbed reservoir modification applications.