A Two‐Phase, Pore‐Scale Reactive Transport Model for the Kinetic Interface‐Sensitive Tracer

Abstract

AbstractPrevious laboratory experiments with the kinetic interface sensitive (KIS) tracers have shown promising results with respect to the quantification of the fluid‐fluid interfacial area (IFA) under dynamic, two‐phase flow conditions. However, pore‐scale effects relevant to two‐phase flow (e.g., the formation of hydrodynamically stagnant/immobile zones) are not yet fully understood, and quantitative information about how far these effects influence the transport of the tracer reaction products is not yet available. Therefore, a pore‐scale numerical model that includes two‐phase, reactive flow, and transport of the KIS tracer at the fluid‐fluid interface is developed. We propose a new method to quantitatively analyze how the mass of the KIS‐tracer reaction product in the flowing water is affected by the presence of the immobile zones. The model employs the phase field method (PFM) and a new continuous mass transfer formulation, consistent with the PFM. We verify the model with the analytical solutions of transport involving advection, reaction and diffusion processes. The model is tested for two‐phase flow conditions in a conceptual 2D slit. The applicability of the model is demonstrated in NAPL/water drainage scenarios in a conceptual porous domain, comparing the results in terms of the spatial distribution of the phases and solute concentration. Furthermore, we distinguish the mobile and immobile zones based on the local Péclet number, and the corresponding IFA, and solute mass in these two zones is quantified. Finally, we show that the solute mass in flowing water can be employed to selectively determine the mobile part of the IFA.