Convective dissolution of CO2 in reactive alkaline solutions: Active role of spectator ions

Upon dissolution of carbon dioxide (CO2) into deep saline aquifers, various chemical reactions are likely to take place between dissolved CO2 and reactants dissolved in the brine, which may drastically impact the mixing of stored CO2 in the reservoir. Our objective is to understand how the nature of the dissolved chemical reactants affects the convective dynamics generated by the dissolution of CO2 into the host phase. To do so, we study experimentally in a Hele-Shaw cell the reactive and convective dissolution of gaseous CO2 into aqueous solutions of bases MOH where M+ is an alkali metal cation. We quantify the effect of the counter-ion M+ on the convective dynamics. Using a schlieren optical setup, we compare the convective patterns in pure water to those in different alkaline solutions of various concentrations. For any reactant MOH studied, the fingering instability develops faster in the reactive case than in pure water, and convection is enhanced if the concentration of the reactant is increased. Furthermore, changing the counter-ion M+ modifies the onset time and the non-linear development of the fingering instability. We explain these experimental results by theoretically analyzing the reaction-diffusion density profiles developing in the solution. We find that changing the counter-ion M+ of the base modifies the density profile, not only through solutal effects but also through differential diffusivity effects. This highlights that the spectator ion M+, despite not participating actively in the acid-base reaction, impacts the development of the hydrodynamic instability. Our results suggest that, in the context of CO2 sequestration, the details of the chemical composition of the storage site should be taken into account for more accurate modeling of the reactive transport of dissolved CO2.