Effect of gas impurity on the convective dissolution of CO₂ in porous media

Growing needs for energy and the essential role of fossil fuels in energy market require attempts such as carbon dioxide (CO₂) sequestration in saline aquifers to stabilize and mitigate atmospheric carbon concentrations. The possibility of co-injection of impurities along with CO₂ allows for the direct disposal of flue gas and hence a significant reduction in the cost of CO₂ sequestration projects by eliminating the separation process. In this study, the results of series of novel experiments in a high-pressure visual porous cell are reported, which allow for visually and quantitatively examining the dynamics of convective dissolution in brine-saturated porous media in the presence of an overlying impure free gas phase with conditions that reflect more closely the subsurface conditions. We find cases with 10% (respectively, 20%) N₂ impurity show higher (respectively, lower) dissolution flux and greater (respectively, smaller) pressure drop at early time as compared to those with pure CO₂. To shed light on this non-trivial behavior, image analysis is performed and hydrodynamic dispersion is revealed as the root to this behavior. Robust scaling relations for the obtained dispersion coefficients and transition time to the shut-down dissolution regime are reported based on dimensionless numbers. Additionally, multiphysics numerical simulations are performed and coupled with the captured dispersion quantities. Our findings elucidate how the presence of impurity in gas stream affects the overall efficiency of CO₂ convective mixing, and how its essence can be leveraged in practice while improving the numerical models.