Determining the Directional Hydraulic Conductivity of a Fractured Rock Mass

Abstract

The directional hydraulic conductivity of a fractured rock mass is dependent upon several factors including fracture network density, geometry, connectivity, mineralization and the effects of the contemporary in-situ stress field. Its estimation is critical to the numerical modelling, understanding and predictability of fluid flow within fracture networks. Directional hydraulic conductivity is typically measured directly from standard bulk hydraulic tests via the use of multiple observation wells, however, in the absence of these standard tests its estimation is problematic. This study demonstrates an alternative approach to its preliminary estimation through the use of coupled hydromechanical, discrete fracture network models constructed from the geological, hydrogeological and geomechanical characteristics of a field test site. This approach explicitly represents a deformable fracture network and the effects of the contemporary in-situ stress field, which allows for a more detailed evaluation of stress-dependent fracture permeability, anisotropic fluid flow and directional hydraulic conductivity trends within a fractured rock mass. These trends are depicted in terms of estimates of fracture deformation, fracture flow rates and hydraulic conductivity ellipses throughout the entire fracture network. Despite its limitations, the results of this method were found to be consistent with field observations and can provide valuable inputs for large-scale, continuum-based aquifer modelling problems and at poorly instrumented sites where sufficient number and quality of standard hydraulic tests are not available to calibrate the model.