Electrical Resistivity Monitoring of Saline Tracer Fingering at Pore Scale under Partially Saturated Conditions

Time-lapse electrical resistivity tomography is a widely used geophysical method to remotely monitor water saturation and the migration of contaminant plumes. The effects of heterogeneous solute concentrations below the resolution limits of the tomogram are commonly ignored. We have adapted an experimental set-up to study the effects of sub-resolution solute heterogeneities on the effective bulk electrical resistivity. We used a 2D analogous porous medium consisting of a Hele-Shaw cell. We monitored simultaneously the bulk electrical resistivity and the spatial distribution of the water/air phases and the saline solute concentration field in the water phase using a fluorescent tracer injected together with the saline solute and a high-resolution camera. We performed saline tracer tests under full and partial water saturations. The bulk electrical conductivity measured at the scale of the medium were confronted to electrical conductivity computed numerically from the measured spatial distributions of the fluid phases and the salinity field. We find that the air distribution, saline tracer fingering, and mixing phenomena all result in large changes in the measured and simulated bulk resistivities by creating preferential flow paths or barriers for electric current at the pore scale.