Estimation of REV size and three dimensional hydraulic conductivity tensor for a rock mass through discrete fracture fluid flow modeling

A new methodology is presented to determine 3-D hydraulic conductivity tensor for a fractured rock mass through 3-D discrete fracture fluid flow modeling. First, a 3-D stochastic fracture network model was built and validated for a gneissic rock mass based on the fracture data mapped from scanline surveys at the site. This validated fracture network model was combined with the fracture data observed on a borehole to generate a stochastic-deterministic fracture network system in a cubic block around each packer test conducted in the same borehole. Each packer test was simulated numerically applying a developed discrete fracture fluid flow model to estimate the influenced region or effective range for the packer test. A block size of 60 feet with the packer interval located at the center of this block was estimated for the influenced region. Using this block size, the average flow rate per unit hydraulic gradient (defined as the transmissivity multiplied by mean width of flow paths) field for fractures was calibrated at different depth regions around the borehole by numerically simulating the packer tests conducted at different depth regions. The average flow rate per unit hydraulic gradient of the fractures in the immediate vicinity of the borehole was considered to be quite different to the average flow rate per unit hydraulic gradient of the fractures at a significant distance away from the borehole. A relation was developed to quantify the ratio between these two parameters. Representative Elementary Volume (REV) for the hydraulic behavior of the rock mass was then estimated to be a block size of 50 feet. Finally, the hydraulic conductivity tensor in 3-D was obtained. The principal directions of hydraulic conductivity were found to be consistent with the existed fracture system. Further, the geometric hydraulic conductivity calculated was found to be comparable to the hydraulic conductivity estimated through the radial flow assumption in continuum porous media.