Abstract: Shale gas development is faced with challenges such as difficulties in reservoir stimulation and low permeability. Liquid nitrogen fracturing, as a waterless reservoir stimulation technology, can circumvent the water sensitivity damage and environmental pollution issues associated with traditional hydraulic fracturing. However, the mechanism by which its thermal shock effect influences the mechanical degradation of shale remains unclear. To investigate the mechanical strength variation and fracture morphology characteristics of deep shale gas reservoirs under liquid nitrogen freezing, uniaxial compression experiments were conducted to measure the mechanical strength of shale at different temperatures and liquid nitrogen cycling times. The failure modes of shale at various temperatures were compared, and microstructural changes before and after liquid nitrogen freezing were analyzed. The results show that the deterioration and damage characteristics of shale are governed by cooling time, preheating temperature, and thermal gradient effects, manifested as microcrack propagation, increased fracture network complexity, and reduced mechanical strength. A higher thermal gradient enhances the deterioration effect of liquid nitrogen on the mechanical strength of the rock. Increased cycling times further exacerbate matrix damage and strength weakening, although the effect diminishes and stabilizes over time. The failure mode of shale is significantly influenced by bedding angles, with a minimum compressive strength observed at 60°, and a 90° bedding angle exhibits higher mechanical strength. In the case of strong non-homogeneity of shale, liquid nitrogen can effectively form a complex network of seams and reduce the mechanical strength. The synergistic damage model of “temperature-circulation-structure” established in this study provides a key theoretical support for the time regulation, optimization of circulation parameters, orientation of layers and non-homogeneous effect of liquid nitrogen fracturing in deep shale reservoirs.

Key words: liquid nitrogen fracturing, deep shale, temperature effect, mechanical strength, damage evolution