Modeling Directional Water Flux in Fractured Rock

Accounting for directional water flow is important in evaluating recoverable water resources in fractured rocks. Fracture orientation, aperture, frequency, and continuity affect water flow and could cause flux to vary with direction. Thus, the potential yield of a well in a fractured rock depends on the location of the well relative to groundwater flow pathways determined by the fracture system. Although advances in characterization of fractured rock provide basic data needed for numerical simulation of flow, current technology typically does not incorporate fracture characteristics or their uncertainties adequately. This paper described a two-tier technology for modeling water flow in a fractured rock, based on using two independent analysis codes sequentially. The first code models discrete fractures explicitly, using a statistical fracture distribution model as input, to calculate the spatial distributions of directional permeability and porosity representing the fractured-rock system. Directional permeability is based on the orthotropic model. The second code uses the porosity and directional permeability distributions in continuum analysis to model groundwater flow. The paper evaluated the approach through numerical simulation of a water-flow experiment in a fractured-rock block 20 × 20 × 20 m³. The simulated specimen features a central recharge well and four observation wells in different directions and equidistant from the recharge well. Results from four fracture distribution cases and one recharge rate show the steady-state flow rate in an observation well varies with the direction of the well location relative to the recharge well. The directional effects on flow rates were evaluated relative to the flow rates in an isotropic system (that has no directional dependence).