We study water flow through observation boreholes using coupled numerical simulations of Navier-Stokes (in open spaces) and Darcy flow (in porous media) using COMSOL Multiphysics. The objective of this work is to analyze different measuring techniques used in observation boreholes such as dilution tests, Colloidal Borescope (CB) and In-Well-Point-Velocity-Probe (IWPVP). We also incorporate the Point Velocity Probe (PVP), which is directly buried in the aquifer instead of inserted in an observation borehole. In our numerical simulations and interpretation, the water velocity field along the whole borehole volume and through the borehole slots is solved explicitly. Screen geometry (slot opening, distance between slots etc.) as well as shape of different devices inserted in the borehole (such as the CB or the IWPVP) can be explicitly considered to analyze how they disturb the water velocity within the borehole.
Interpretation of measurements taken in boreholes requires the conversion of observed borehole velocity u_0 to aquifer Darcy flux q_inf. This conversion is typically done through a proportionality factor alpha=u_0/q_inf. In this contribution we present numerical simulations of IWPVP measurements. For the case of IWPVP, experiments conducted in a controlled sandbox are used to compare alpha values obtained numerically with those measured in the lab and test how screen geometry (number of slots, length and opening of slots, screen width and diameter etc.) as well as orientation may influence observed velocities in the borehole. The results of IWPVP deployment at ADYCHATS pilot scale site are also discussed to analyze the implementation of IWPVP in the field scale.
Acknowledgements: The authors thank TOTAL for funding this R&D project and giving permission to publish this paper.