Experimental Determination of Hydraulic Anisotropy in Fractured Formations

Abstract

Discontinuities in a rock mass are defined as the planes of weakness of structural origin and commonly include fractures, faults, and joints. Many numerical solutions have been proposed which are based on stochastic generation of fractures to represent the actual field conditions; however, this approach neglects the role of the actual discontinuities, and results in misleading conclusions. A model was constructed based on fracture trace mapping. Fracture traces are the surficial expressions of the planes of weakness present in the underlying rock mass and can be seen on aerial photographs and Landsat images. Two fractured formations present in the Black Hills region of South Dakota were selected as prototypes. The Madison Formation composed of limestone and dolomite, and the Minnelusa Formation consisting of sandstone and shale with interbeds of limestone and dolomite. Fracture trace patterns served as overlays for the laboratory flow experiments. A two-dimensional flow cell was constructed. To simulate the field conditions, cuts were made of the fracture trace patterns in impervious glazed vinyl sheets. This resulted in smooth walled polygonal shaped pieces which were glued firmly onto the flow cell. The flow was thus restricted within the fracture walls, conforming to the parallel plate analogy. By scalingmore » down the hydrodynamic field conditions, outflow was measured for each run. The plot of the measured data resulted in the best fit ellipse. The major and minor principal axes of this ellipse indicate the maximum and minimum hydraulic conductivities. A close match of the experimental hydraulic conductivity values with that of the field data validated the use of fracture trace analysis for determining the hydraulic anisotropy in fractured formations.« less