Mixing-induced reactive transport experiments in heterogeneous and variably saturated porous media

Mixing-induced reactions are an essential feature of environmental flow and transport processes. They control many reactive transport processes, including mineral precipitation rates and contaminant remediation processes. Natural porous media are characterized by a strong structural heterogeneity, which impacts solute mixing and, therefore, the resulting chemical reaction rates. Establishing a quantitative link between pore-scale heterogeneity and mixing/reaction rates in saturated and unsaturated conditions remains an open question. Here, we study pore-scale solute mixing using high-resolution experimental measurements to quantify the overall reaction rates and product concentrations. Our goals are to study the impact of structural heterogeneity on 1) reaction rates and products during saturated flow and 2) the spatial arrangement of fluid phases during unsaturated flow and its impact on reaction rates and products.

We use two-dimensional porous media consisting of circular posts in a Hele-Shaw-type flow cell. We control heterogeneity by varying the posts’ diameters disorder and correlation length; increasing this length introduces more structure in the porous medium. We utilize an irreversible oxidation reaction to produce fluorescein from its non-fluorescent form. The Damköhler number is sufficiently larger than unity, so the reaction rate is mixing-controlled. We inject a non-fluorescent tracer pulse into the porous medium sample filled with the oxidating reactant under saturated and unsaturated flow conditions. We analyze periodic fluorescence intensity images to track the evolving solute concentration field. The reaction rates and the total reaction product mass are calculated directly from the concentration images.

Solute concentration images show that increasing the spatial correlation length under saturated flow conditions leads to enhanced reaction front stretching and elongation as the solute travels along preferential pathways. Due to this overall stretching, the reaction front is locally more compressed perpendicular to the elongation direction. In a non-correlated, randomly disordered porous medium, overall stretching is reduced, and the front is less compressed locally. Under unsaturated flow conditions, a main preferential flow path characterizes the correlated porous medium. In contrast, the non-correlated medium is characterized by a higher degree of branching and splitting in the velocity field. Solute pulse focusing in the correlated porous medium sample reduces reaction front stretching compared to the non-correlated porous medium, under unsaturated conditions. Under these conditions, the reaction rate increases more than the saturated case due to the unsaturated flow pattern's enhanced reaction front stretching. This effect is more pronounced for the non-correlated sample, where flow path splitting and reaction front stretching are more significant. This work shows that structural heterogeneity has a considerable effect on reactive solute transport and that this effect depends on the system’s saturation.