Preferential Solute Transport in Low Permeability Zones During Spontaneous Imbibition in Heterogeneous Porous Media

Abstract

Multiphase flows in porous media, and the associated solute transport processes, are controlled by a combination of gravity, capillary, and viscous forces. Geologic heterogeneity influences flow and transport processes, yet gaps exist in our understanding of how heterogeneity impacts capillary-driven transport, such as during spontaneous imbibition. Here we use positron emission tomography, combined with a newly developed method for conducting spontaneous imbibition experiments, to observe solute transport into low permeability regions during imbibition. Using an experimentally parameterized 2D numerical model, we demonstrate how capillary-driven flow near the imbibition front can carry solutes into low permeability regions. This process displaces solutes from high permeability zones, while the cumulative amount of solute in low permeability zones increases, opposite of what is observed under fully saturated solute transport conditions. These results highlight the complex flow and solute transport behavior that arise during multiphase displacements in heterogeneous geologic porous media.

Plain Language Summary

Solutes and contaminants dissolved in a fluid, such as water, are carried along with the fluid as it migrates in the subsurface. In some subsurface settings, there are multiple fluids present. For example, above the water table a zone of soil and rock has pores filled with different fractions of water and air; in geologic carbon storage reservoirs the rock has pores filled with different fractions of supercritical CO₂ and brine. The nature of the geologic setting controls how these fluids displace one another in the subsurface. In this study, we perform experiments that are quantified with medical imaging techniques to study how solutes are carried with water during a specific type of displacement process known as spontaneous imbibition. These imaging techniques allow us to see inside geologic rock cores and monitor solute migration and immobilization processes. Results of the experiments and associated mathematical modeling highlight a mechanism of solute immobilization that is unique to the process of spontaneous imbibition. These results may help explain why some contaminants can be immobilized for extended periods of time before reaching groundwater supplies.