Laboratory testing of real-time flux measurements in fractured media

Abstract

The In-Well Point Velocity Probe (IWPVP) was originally developed for use in screened porous media wells. The tool can characterize small-scale aquifer heterogeneity from wells that have been carefully developed to ensure an open screen and good hydraulic connection with the surrounding aquifer. This capability led to the hypothesis that the IWPVP could be adapted to characterize flow in fractured media, which features small-scale openings (i.e. fractures) that resemble screen slots. To test this hypothesis, a lab-scale fracture flow apparatus (FFA) was fabricated. The FFA consisted of two parallel acrylic plates that could be separated by a fixed distance, simulating a fracture with a known aperture. Two lengths of PVC well casing served as a cylindrical access port and were centrally placed in the plates. This mock-well left the interface with the fracture open – analogous to a fracture intersecting an uncased borehole. The probe was calibrated experimentally by relating known fluxes through the FFA with those measured inside the IWPVP. The ratio of these fluxes was found to be predictable for two aperture sizes (500 μm and 1290 μm), based on a flux balance and the Cubic Law. Additionally, the ability of the probe to discern flow direction in a fracture was tested and found to be accurate within ±20° under the conditions of the laboratory testing. The results suggest that an IWPVP could be reliably used in fractured media to directly measure horizontal flux and flow direction in a single fracture without reference to Darcy’s Law.