Experimental study of miscible Rayleigh-Taylor convection in a granular porous medium

More than 60% of greenhouse gas emissions are due to CO₂ released from fossil fuels and industrial processes [1]. It is expected that by 2035, the expected increase in CO₂ emissions will be 37.2 Gt/yr [2]. To reduce the resulting further adverse effects in climate changes, geological sequestration of CO₂ provides an effective solution for carbon capture and storage (CCS) [2-4]. Dissolution trapping of CO₂ in deep saline aquifers is a trapping mechanism that allows for long term storage. When CO₂ is injected into the subsurface geological layers, the supercritical CO₂ (sCO₂) dissolves into the aquifer’s aqueous solution positioned below. The formation of a layer of CO₂-enriched brine at the upper interface of the liquid domain results in unstable stratification which evolves into gravitational convection [2-5].

To evaluate the storage capacity and the efficiency of the trapping, it is necessary to understand the dynamics of the instabilities and convection, and the affect of granular media properties on them. To do so, we perform a 2D experimental study in a 3D granular medium and use Darcy scale simulations to complement our experimental findings [6]. Analog experiments are performed by using two miscible fluids with a density contrast between them. In doing so we decouple the gravitational instability process from the dissolution process; the latter is not modeled in our experiment. We match the refractive index of the fluids to that of the granular medium to allow for optical measurement of the concentration field. We observe that there is substantial difference in convection development time scales between the experimental results and the Darcy scale simulations performed with the experimental macroscopic parameters (porosity, permeability, dispersivity lengths, density contrast). We attribute this to the role played by pore scale heterogeneity of the velocity field, which cannot be predicted by Darcy scale models. This would suggest that Darcy scale simulations [2, 4,6] significantly overestimate the typical time scale of the convection.

[1] Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC 2014.

[2] Emami-Meybodi, H., Hassanzadeh, H., Green, C. P., & Ennis-King, J. (2015). Convective dissolution of CO2 in saline aquifers: Progress in modeling and experiments. International Journal of Greenhouse Gas Control, 40, 238-266.

[3] Bachu, S. (2008). CO2 storage in geological media: Role, means, status and barriers to deployment. Progress in energy and combustion science, 34(2), 254-273.

[4] Pau, G. S., Bell, J. B., Pruess, K., Almgren, A. S., Lijewski, M. J., & Zhang, K. (2010). High-resolution simulation and characterization of density-driven flow in CO2 storage in saline aquifers. Advances in Water Resources, 33(4), 443-455.

[5] Nadal, F., Meunier, P., Pouligny, B., & Laurichesse, E. (2013). Stationary plume inducedby carbon dioxide dissolution. Journal of Fluid Mechanics, 719, 203-229.

[6] Dhar, J., Meunier, P., Nadal, F. & Méheust, Y. (2021). Convection dissolution of CO2 in  2D and 3D porous media: the impact of hydrodynamic dispersion. Submitted to Physics of Fluids.