Convective dissolution of CO 2 in reactive alkaline solutions: Active role of spectator ions

Abstract

Upon dissolution of carbon dioxide (CO 2 ) in deep saline aquifers, various chemical reactions are likely to take place between dissolved CO 2 and reactants dissolved in the brine, which may drastically impact the mixing of stored CO 2 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 CO 2 into the host phase. To do so, we study experimentally in a Hele-Shaw cell the reactive and convective dissolution of gaseous CO 2 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 set-up, 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 CO 2 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 CO 2 .

Density profiles around A+B→C reaction-diffusion fronts in partially miscible systems: A general classification

“…However, their spatial dependence, i.e. the type and number of extrema, typically remains the same over time as shown previously for specific zones [15,16,46]. Therefore, we can analyze the shape of the density profiles to predict possible scenarios for the development of buoyancy-driven instabilities.…”

“…This zone has been illustrated by the experiments of CO 2 dissolving into aqueous solutions of alkalis (δ B /δ C ≈ 2, R B /R C ≈ 0.4). In that case, the instability develops faster than without reaction [15,46]. The concentration of C above and at the reaction front is larger than in the equal diffusivities case (Figs.…”

“…This is the case e.g. of alkyl formate [17] or gaseous CO 2 [44][45][46] dissolving into an aqueous solution. Even when a reaction takes place in the host phase, it is not possible to observe a stratification statically stable everywhere: at least above the reaction front, the density of the solution decreases downwards due to the contribution of the dissolving species to density (Fig.…”

“…The concentration of C above and at the reaction front is larger than in the equal diffusivities case (Figs. 2d and 4), which increases the density jump at the origin of convection and thus opposes the stabilizing effect of the minimum [46].…”

Chemically-driven convective dissolution

“…Recently, chemical reactions have been shown to be able to affect such convective dynamics. [9][10][11][12][13] In particular, it has been demonstrated theoretically [14][15][16][17][18][19][20][21][22][23] and experimentally 14,15,20,21,[24][25][26] that A + B -C type of chemical reactions can amplify or slow down convective fingering when all three species are in solution depending on their relative contribution to density. Reactions can therefore not only allow for the storage of larger amounts of CO 2 during convective dissolution because they consume CO 2 (reactive effect) but can also accelerate the development of convection (reaction-induced convective effects).…”