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.