Introduction

Convective mixing of fluids in porous media is a physical process that manifests in complex flow patterns. In solutal convective mixing, the presence of a fluid component in the background fluid results in a density contrast between fluid containing the solute and fluid without. This density difference gives rise to a pressure gradient which drives motion.

Recently, a significant body of scientific literature has been concerned with density-driven convective mixing in porous media due to its applicability to geological storage of carbon dioxide, see Emami-Meybodi et al. (2015) for an extensive literature review. In the case where CO is injected into a saline aquifer with a bounding cap rock, buoyancy drives vertical migration of the mobile CO (CO in the supercritical gas phase), which then spreads beneath the cap rock to form a thin, laterally extensive plume. In time, the gaseous CO begins to dissolve into the local formation water, leading to a small increase in density of the saturated brine at the top of the aquifer of approximately . Diffusion of the dissolved CO allows further dissolution, a process that leads to a gravitational instability whereby a denser fluid lies atop a less dense one. After a sufficient period of time, vertical fluid motion is induced as vertical acceleration overcomes diffusion, and CO-rich water descends to the lower part of the reservoir.

The process of convective mixing can significantly increase the rate of dissolution of CO in the formation water and hence reduce the amount of mobile CO. This can significantly reduce the risk of leakage into overlying aquifers, increasing the security of storage.