Phases saturation control on mixing driven reactions in 3D porous media

TL;DR

This study uses 3D MRI imaging to investigate how phase saturation influences mixing and reaction rates in unsaturated porous media, revealing that saturation levels significantly affect reaction dynamics and the limitations of current models.

Contribution

It introduces a 3D experimental approach and a new model to better predict reactive transport in unsaturated porous media, addressing limitations of existing continuum models.

Findings

Reaction rate increases with saturation due to interface deformation.

Faster mixing and reaction front stretching occur at lower saturation.

Reaction extinction duration is longer at lower saturation levels.

Abstract

Transported chemical reactions in unsaturated porous media are relevant across a range of environmental and industrial applications. Continuum scale dispersive models are often based on equivalent parameters derived from analogy with saturated conditions, and cannot appropriately account for processes such as incomplete mixing. It is also unclear how the third dimension controls mixing and reactions in unsaturated conditions. We obtain 3$D$ experimental images of the phases distribution and of transported chemical reaction by Magnetic Resonance Imaging (MRI) using an immiscible non-wetting liquid as a second phase and a fast irreversible bimolecular reaction. Keeping the P\'eclet number (Pe) constant, we study the impact of phases saturation on the dynamics of mixing and the reaction front. By measuring the local concentration of the reaction product, we quantify temporally resolved effective reaction rate ($R$). We describe the temporal evolution of $R$ using the lamellar theory of mixing, which explains faster than Fickian ($t^{0.5}$) rate of product formation by accounting for the deformation of mixing interface between the two reacting fluids. For a given Pe, although stretching and folding of the reactive front are enhanced as saturation decreases, enhancing the product formation, this is larger as saturation increases, i.e., volume controlled. After breakthrough, the extinction of the reaction takes longer as saturation decreases because of the larger non-mixed volume behind the front. These results are the basis for a general model to better predict reactive transport in unsaturated porous media not achievable by the current continuum paradigm.