Convective CO2 dissolution: Analog experiments and direct numerical simulations

Geological carbon capture and storage, in which CO2 is injected into deep saline aquifers for permanent storage, forms an integral part of the CO2 mitigation strategies. At representative conditions CO2 is buoyant and therefore may leak into surface waters or the atmosphere. A possible route to stable CO2 storage is through convective dissolution of CO2 into the ambient brine; the density of brine increases with CO2 saturation. We consider this mechanism using a novel analogue fluid system; a methanol ethylene-glycol (MEG) solution which is buoyant compared to fresh water, but whose density upon mixing with water exceeds that of either end member. Experiments were conducted in a deep, two-dimensional porous medium so that effects of the bottom boundary could be neglected. We observe that the buoyant layer of MEG solution convectively dissolves into the underlying fresh water at a constant rate proportional to the Darcy buoyancy flux, γ = KΔρg/μ, where K is the permeability of the porous medium, Δρ, is the maximum density difference between fresh water and the water-MEG mixture, g is the gravitational acceleration, and μ is the viscosity of the water. Direct numerical simulations of the nonlinear dynamics show the same dependence of the dissolution rate on the buoyancy flux, and allow us to extend the experimental results over a larger range in the governing parameter. We conclude that the convective dissolution rate of CO2 into the ambient brine is likely to be fast in high permeability aquifers like the Sleipner injection site on the continental shelf of Norway, but may be significantly slower in low permeability aquifers or aquifers where high amounts of dissolved solids reduce the CO2 solubility and hence the density difference driving the convection.