Investigation of geometric morphology and fluid flow characteristics of natural fracture in tight sandstone under non-loading and true triaxial cyclic loading

Understanding the flow characteristics of rock fractures under stress is critical for many geological engineering applications. In this study, flow experiments are conducted on tight sandstone samples with a single natural fracture under true triaxial cyclic loading using the geotechnical consulting and testing systems. The geometric morphology of the fracture is scanned before and after loading using a profilometer. An improved cubic law is developed by including correction factors for stationary roughness, surface tortuosity, and hydraulic tortuosity. The evolution of fracture permeability during cyclic loading of each principal stress is measured using the steady-state method. The results show that (1) the surface tortuosity of the natural fracture correlates as a binary quadratic function with its fractal dimension and joint roughness coefficient. (2) The improved cubic law model has higher accuracy in predicting the permeability of the opening natural fracture than other commonly used modified cubic law models. (3) The principal stresses exhibit an anisotropic influence on fracture permeability. During cyclic loading of principal stress parallel to the fracture, the changes in fracture permeability are neglectable. (4) During cyclic loading of principal stress perpendicular to the fracture, the fracture permeability decreases significantly in the first loading cycle, exhibiting a hysteresis effect. In subsequent cycles, the changes in fracture permeability are nearly reversible, indicating the stress-memory effect of the natural fracture. This study provides direct evidence for the hysteresis and stress-memory effects in the permeability evolution of fractured rock during true triaxial cyclic loading.