Cryogenic stimulation using liquid nitrogen (LN2) has emerged as a promising alternative to conventional hydraulic fracturing for enhanced geothermal systems offering reduced environmental impact and improved stimulation efficiency. This study investigates the evolution of macro-mechanical properties and fracture structures in granite subjected to repeated high-temperature heating and LN2 cryogenic impact cycles—conditions simulating artificial thermal reservoir stimulation in hot dry rock environments. Standardized granite specimens were treated with varying thermal–cryogenic cycles followed by comprehensive characterization at multiple scales. Ultrasonic velocity measurements and spectral analyses were employed to assess internal damage and crack development induced by thermal–mechanical fatigue. Uniaxial compression tests were conducted to evaluate the degradation of mechanical parameters such as strength, stiffness, and failure mode. Furthermore, micro-computed tomography and three-dimensional laser profilometry techniques were integrated to quantify pore-fracture network evolution and surface morphology variations. The results demonstrate a progressive decline in mechanical integrity with increasing treatment cycles accompanied by enhanced connectivity and complexity of fracture networks. This study elucidates the coupled damage mechanisms induced by thermal shock and cryogenic contraction and provides experimental insights for optimizing LN2-based reservoir stimulation strategies in deep geothermal applications.
Li et al. (Fri,) studied this question.