Mechanism of Thermal Stress-Induced Fracture in Rock Masses under Liquid Nitrogen Fracturing Conditions

Under liquid nitrogen (LN2) fracturing conditions, rock is rapidly cooled to 77 K, generating intense temperature gradients and thermal stress. Studying the coupled thermal-stress fracture behavior is key to revealing the mechanisms of crack initiation, propagation, and coalescence at low temperatures. This study combines numerical simulation with visualized experiments using poly(methyl methacrylate) (PMMA) specimens, through a self-designed LN2 fracturing system that enables comparison among LN2, vaporized nitrogen, and water-based fracturing. The investigation focuses particularly on thermal-induced damage and its correlation with crack propagation behavior. Key findings include: (1) LN2’s low viscosity reduces PMMA initiation pressure by 30%, with an additional 20% reduction from thermal stress, totaling a 50% decrease compared to hydraulic fracturing, while forming a highly tortuous radial thermal micro-crack network; (2) LN2 damage occurs in two stages: pre-cooling, where wellbore wall temperature drops from 293 to 77 K in 100 s and thermal stress reaches 30 MPa within 75 s, leading to stress concentration and initial cracking; and fracturing, where strong thermo–mechanical coupling alters crack propagation paths and increases the damage rate to 24%, far exceeding the 6% from pure mechanical loading; (3) A Mode I stress intensity factor model incorporating wellbore pressure, thermal stress, and crack internal pressure is established, supported by a crack–length–based dynamic pressure control method for optimized fracture guidance. These results provide a theoretical foundation for the controllable design of complex thermally induced fracture networks.