Investigate on mechanical properties and fracture characteristics of sandstone under different liquid nitrogen treatment states

Tight sandstone gas reservoirs can potentially be developed more efficiently with the use of liquid nitrogen (LN₂) fracturing. In order to evaluate the effects of LN₂ freeze-thaw on the physical and mechanical properties, as well as the failure behavior of sandstone, the research involved conducting a range of experiments and numerical simulations on sandstone samples subjected to various LN₂ treatment conditions. The results indicated that the freeze-thaw process had a detrimental effect on the physical and mechanical properties of the sandstone, leading to varied degrees of weakening. The reduction percentages of tensile strength and fracture toughness, which are two key indicators, were found to be 14 % and 21.4 % respectively. Furthermore, there were the bigger degree of volume fracture, multiple branching and tortuous macroscopic cracks and macro fracture surface with larger roughness (increased by15 %) in the freeze-thaw sandstone after destruction. The physical mechanical properties of the freezing sandstone were significantly enhanced, but the complexity of the tensile failure path increased. Numerical simulations indicate that a large temperature gradient leads to the formation of damage zones within the sandstone matrix, and microscopic observations show a significant increase in the length and width of microcracks within the matrix. This indicates that the low temperature of LN₂ fully releases the damage potential of initial defects in sandstone, the damage area participates in the formation of crack propagation paths and increases the randomness of path direction selection, which increases the degree of sandstone volume fracture. The research results are anticipated to offer a scientific foundation and data references for the efficient development of tight sandstone gas resources.