Investigation of the permeability anisotropy of 2D fractured rock masses

• A HAF factor is derived to estimate the permeability anisotropy of fractured rocks.
• The effectiveness of the HAF factor is verified by numerical simulations.
• The effects of fracture patterns on the permeability anisotropy are studied.
• Define an ACI to evaluate the directional hydraulic connectivity of fracture system

A hydro-geometric anisotropy factor is derived for better correlation with the permeability anisotropy of field-scale fractured rock masses. The hydro-geometric anisotropy factor considers the orientation, length, spacing and hydraulic aperture properties of a discrete fracture network. Numerical simulations are carried out by using a pipe network method which simulates the fractures as connected and directed pipes obeying Darcy's law to validate the correlation between the hydro-geometric anisotropy factor and the permeability anisotropy of fracture networks. Fracture patterns from simple to complex with a scale of 10 m by 10 m are generated to study the effects of different fracture distributions on the hydro-geometric anisotropy of fracture networks and thus on the permeability anisotropy. It is found that the aperture size and distribution have a significant effect on the permeability anisotropy that affects the main permeable direction of the fracture networks. The permeability anisotropy varies following a power relation with the change of the intersection angle between two fracture sets. Varying the number and length ratio of the fracture sets also alters the permeability anisotropy. In all fracture patterns, the hydro-geometric anisotropy factor gives an accurate correlation with the permeability anisotropy. An anisotropic conductivity index is further defined to evaluate the directional hydraulic connectivity of the fracture system. Based on this approach, the permeability anisotropy of a fracture network system can be quickly assessed through the correlation with the hydro-geometric anisotropy factor and the anisotropic conductivity index.