Echo tracer dispersion in flows of polymer solutions through porous media: A tool for detecting weak permeability heterogeneities?

Tracer dispersion in Newtonian and shear-thinning fluids (scleroglucan-water polymer solutions) flowing through single and double porosity grain packings has been studied experimentally using both classical transmission dispersion and echo dispersion (in the latter, the concentration variation front is pumped back through a detector at the inlet after penetrating for a chosen distance into the sample). Transmission dispersion increases markedly in both types of samples with the shear thinning index of the fluid at all Péclet numbers (except when molecular diffusion is dominant). Echo and transmission experiments give nearly identical dispersivity values for Newtonian fluids while echo dispersivity is lower than transmission for shear thinning ones. The normalized dispersivity difference has same order of magnitude for single and double porosity samples and increases with the shear thinning exponent α (by a factor of 2 between α = 0.35 and α = 0.60). This difference may be due to heterogeneities inducing permeability variations of small amplitude over distances of the order of the sample section : their influence on tracer dispersion is partly reversible with respect to a change of the flow direction and is only detectable if it is amplified by the shear-thinning properties of the fluid. Received 19 September 2001