Anisotropic flow characteristics in single-fractured tensile and shear fractures in granite subjected to various confining stress

Anisotropic flow characteristics in single-fractured tensile and shear fractures in granite subjected to various confining stresses were investigated by experiments and experimental results-based flow simulations. Anisotropic characteristics in aspects such as the permeability reduction rate, stress dependency of permeability, and flow paths in fracture and fracture aperture distribution were analysed and compared between tensile and shear fractures. The main conclusions are as follows: (1) for tensile fractures, the permeability in the X direction, k(X), is slightly lower than the permeability in the Y direction, k(Y), and the average permeability anisotropic coefficient, alpha(k) (alpha(k) = k(X)/k(Y)), is in the range 0.236-0.779. For shear fractures, k(X) is obviously lower than k(Y), and alpha(k) is in the range 0.038-0.163; therefore, the permeability anisotropy in shear fractures is stronger than in tensile fractures. (2) Strong channelling effects exist in both shear and tensile fractures. The stress-dependence coefficient of permeability is 0.731 MPa-1 in the X direction and 0.365 MPa-1 in the Y direction for tensile fractures, and 0.034 and 0.010 MPa-1, respectively, for shear fractures, indicating that the dependency of permeability on stress also shows anisotropy. (3) An empirical model for alpha(k) based on stress and fracture aperture variograms is proposed. Based on alpha(k), permeability along both the X and Y directions can be well predicted, especially under relatively high confining stress.