Fluid flow in discrete joint sets: Field observations and numerical simulations

The distribution of flow within conductive joint sets is influenced by the geometric arrangement of joints and the hydraulic properties of both joints and matrix. We use finite element simulations with an equivalent porous me dia joint representation to understand the distribution of flow through joi nts and porous matrix. Isolated joints in a porous media create characteris tic flow perturbations in the matrix with reduced fluid potentials near the upstream joint tip, elevated potentials near the downstream tip, and flow shadows adjacent to the joint. In more complex joint systems, flow in any g iven joint is influenced by its proximity to other joints, resulting in cha racteristic enhancement or reduction of flow velocities. The permeability r atio (equivalent joint permeability divided by matrix permeability) plays a major role in determining the distribution of flow within complex joint sy stems. When the permeability ratio is <3.0 orders of magnitude, all joints are indirectly connected to the flow system through the matrix. As joint co nductivity increases, flow becomes increasingly localized into directly con nected joints. When the permeability ratio exceeds 6.5 orders of magnitude, significant flow occurs only in the directly connected joints. We compare these numerical results with field observations from an ancient reactive fl ow system now exposed at the Earth's surface. In the field, 32% of joints a re associated with chemically altered halos. By explicitly representing map ped joint distributions in numerical simulations, we estimate that 32% of t he joints would conduct significant volumes of fluid if joint permeability is 5 orders of magnitude greater than the matrix permeability. This corresp onds to an insitu joint aperture of 2.3 mm, closely resembling the 1.8-mm a verage joint aperture measured on the present-day outcrop.