3D fracture network and anisotropic permeability in Cenozoic red-bed mudstone under freeze-thaw cycles: A case of the northeastern Tibetan plateau

Cenozoic red-bed mudstone is widely distributed globally as a typical geological disaster-prone lithology. In cold, high-altitude regions, freeze-thaw (F-T) cycles significantly affect the mechanical properties and permeability of this mudstone by increasing internal fracturing. However, systematic studies on its fracturing patterns and permeability under F-T conditions remain limited. This study employed integrated three-dimensional (3D) X-ray computed tomography (CT) and the lattice Boltzmann method (LBM) to characterize the 3D fracture networks and unsaturated permeability of Cenozoic red-bed mudstone from the northeastern Tibetan Plateau during progressive unidirectional F-T cycles. Fracture network analysis showed that increasing F-T cycles caused fractures to expand and intersect, whereas the lengths of intersection lines and individual fracture areas decreased. Conversely, the number of corner points, total fracture count, cumulative fracture area, and fractal dimension all increased, with fractures exhibiting a clear vertical directional preference. Based on experimental conditions and frozen front characteristics, fractures propagated horizontally in a Y-shaped junction pattern, forming an irregular hexagonal grid, while extending vertically perpendicular to isothermal surfaces. Permeability analysis revealed that both saturated and unsaturated permeability increased significantly with more F-T cycles. However, anisotropic fracture distribution resulted in distinct differences between horizontal and vertical permeability. Notably, the anisotropy of unsaturated permeability diminished with decreasing saturation and increasing F-T cycles. These findings highlight the significant anisotropic characteristics of fracture development and permeability in Cenozoic red-bed mudstone under F-T cycles. This integrated methodology is essential for characterizing 3D fracture propagation and provides novel insights into the hydro-mechanical behavior of red-bed lithology in similar regions worldwide.