Enhanced dispersion in intermittent multiphase flow
Résumé
Transport of solutes during flow through porous media plays a central role in many geological, biological, and engineered systems. Whereas solute dispersion has been extensively studied for single phase flow, less is known about dispersion in the presence of multiple immiscible fluid phases. The presence of several immiscible fluid phases in the pore space can significantly modify transport. This scenario occurs in many natural and engineered systems, including unsaturated soils, hydrocarbon reservoirs or ice systems. Several studies have investigated how the presence of multiple fluid phases influences dispersion of solute species. Most of them have considered steady flows and showed that flow heterogeneity resulting from the presence of multiple phases increases effective dispersion. However, in transient multiphase flows the interplay between phases is expected to trigger highly intermittent velocity fields. However, it is not known how such intermittency affects dispersion properties. Here, we use numerical simulations of transport in two-phase flow to explore longitudinal and transverse dispersion. While longitudinal dispersion evolves linearly with flow velocity, as expected, we demonstrate that transverse dispersion exhibits a sublinear dependency with flow rate. We explain this behavior by a continuous activation and de-activation of different flow pathways, leading to intermittent flow patterns. We derive a theoretical framework that relates longitudinal and transverse dispersion to the flow statistics. We demonstrate a mechanism where transverse dispersion increases at a given position when flow rate decreases. This result provides a new scaling of lateral transport in porous media with velocity, beyond mechanical dispersion and diffusion