Impact of saturation on dispersion and mixing in porous media
Résumé
In partially saturated porous media, the spatial distribution of wetting (e.g., water) and non-wetting (e.g., air) phases causes the liquid flow to be focused onto narrow and complex flow paths, leaving large volumes of wetting fluid trapped in between non-wetting phase clusters. The impact of the resulting highly heterogeneous wetting fluid velocity distributions on the dispersion and mixing of a solute in this wetting phase is critical for predicting reactive transport processes that take place in partially saturated porous media. We study the dependence of dispersion and mixing on the saturation degree using a 2D experimental setup consisting of cylindrical grains built using soft lithography. The joint injection of the two phases (wetting and non-wetting) provides a controlled homogeneous saturation in the medium. The simultaneous measurement of the flow velocity field, the spatial distribution of the wetting and non-wetting phases, and the tracer concentration field are used to investigate the relationship between the flow field complexity induced by desaturation and dispersion/mixing properties.We analyze the temporal behavior of the mean concentration gradient and the scalar dissipation rate, which quantify the temporal variation of the concentration variability and the potential for mixing-controlled chemical reactivity. The formation of preferential flowpaths in unsaturated flows is found to have an important impact on the mixing behavior. While the mean concentration gradient decays in time for saturated flow following the classical diffusive smoothing of concentration gradients, the creation of highly channelized finger structures in unsaturated flows induces persistently large concentration gradients which decay slowly in time. The highly resolved concentration field images show that this effect is due to i) a drastic increase of the surface available for creating concentration gradients across the finger boundaries, ii) the existence of dead-ends with a wide range of sizes, and iii) the recursive coalescence of tracer fingers at critical bottle necks formed between air clusters. These results suggest that large concentration gradients may persist over a broad range of times in unsaturated media thus affecting the reactivity of hydrological systems.