Characteristics of pore fractal and energy dissipation evolution in sandstone uranium reservoirs under liquid nitrogen cyclic cold impact

Liquid nitrogen cyclic cold impact (LNCCI) is an emerging technology for low-temperature fracturing and permeability enhancement in sandstone uranium reservoirs. However, the evolution mechanisms of sandstone pore structure under LNCCI remain to be clarified. To address this, sandstone samples were first treated with LNCCI. Then, multiscale pore structure characterization was conducted using scanning electron microscopy (SEM), low-pressure nitrogen gas adsorption (LP-N 2 GA), and mercury intrusion porosimetry (MIP). Additionally, fractal theory was applied to quantitatively describe the evolution of the pore structure. Furthermore, uniaxial compressive strength (UCS) tests were performed to evaluate the mechanical properties of LNCCI-treated sandstone, and energy evolution during sample failure was analyzed based on energy dissipation theory. The results show: (1) LNCCI weakens grain cementation, induces fractures, and increases intergranular pores, which further leads to treated samples exhibiting a bimodal pore size distribution with elevated counts of micropores, macropores, and ink-bottle-shaped pores. (2) Fractal analysis reveals that smoother micropore surfaces, increased small-pore complexity, reduced large-pore complexity, and improved pore connectivity. (3) As LNCCI cycles increases, sandstone strength decreases, while plastic deformation and dissipation energy during failure rise linearly. The research results provides theoretical and technical support for LNCCI-based fracturing and permeability enhancement in low-permeability sandstone uranium deposits.