Laboratory-scale electrical resistivity and fluorimetric monitoring of saline tracer tests under partially saturated conditions

Abstract : Time-lapse electrical resistivity tomography is widely used to remotely monitor water saturation and contaminant plumes. Petrophysical relationships are needed to link electrical measurements to subsurface properties of primary interest. Most petrophysical relationships are based on mixing laws or upscaling procedures (e.g., differential effective medium, volume averaging) that are well understood in saturated media at homogeneous pore water composition. The effects of heterogeneous solute concentrations (i.e., fingering of a saline tracer) and fluid distributions (i.e., air and water) below the resolution of geophysical tomograms are currently ignored in most hydrogeophysical studies. We adapted an existing experimental set-up to study the effects of sub-resolution heterogeneities on the effective bulk electrical resistivity. We used a 2D analogous porous medium consisting of a Hele-Shaw cell containing a single layer of 4500 cylindrical solid grains built by soft lithography. We monitored the bulk electrical resistivity between two electrodes at a temporal resolution of 2 s. At the same time, we monitored the phase distribution and the local concentration field using a fluorescent tracer and a high-resolution camera (27 pixels per mm, 12 bit images). Our experiments included drainage and imbibition, as well as high salinity tracer tests under full and partial saturations. The measured bulk electrical resistivities are currently compared to those computed (1) numerically at the pore scale based on the local salinity and saturation fields obtained by image processing and (2) by applying petrophysical relationships defined at the scale of the porous media.We expect that this work will give rise to more robust upscaled petrophysical models and better understanding of sub-resolution effects in hydrogeophysics. The experimental approach will be extended to include spectral induced polarization and self-potential measurements under different water saturations.