In partially miscible two-layer systems within a gravity field, buoyancy-driven convective motions can appear when one phase dissolves with a finite solubility into the other one. We investigate the influence of chemical reactions on such convective dissolution by a linear stability analysis of a reaction-diffusion-convection model. We show theoretically that a chemical reaction can either enhance or decrease the onset time of the convection, depending on the type of density profile building up in time in the reactive solution. We classify the stabilizing and destabilizing scenarios in a parameter space spanned by the solutal Rayleigh numbers. As an example, we experimentally demonstrate the possibility to enhance the convective dissolution of gaseous CO_{2} in aqueous solutions by a classical acid-base reaction.
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 .
Chemical reactions can impact mixing in partially miscible stratifications by affecting buoyancy-driven convection developing when one phase dissolves into the other one in the gravity field. By means of combined nonlinear simulations and experiments, we explore the power of an A + B → C type of reaction to either enhance or refrain convective dissolution with respect to the nonreactive system depending on the relative contribution to density of the dissolving species A, of the reactant B initially dissolved in the host phase and of the product C. Nonlinear simulations are performed by solving reaction-diffusion-convection equations describing the dissolution and reactive dynamics when a less dense phase of A is layered on top of a reactive denser solution of B, in which A is partially miscible with a given solubility. The spatio-temporal dynamics and convective patterns observed in the numerical study compare favorably with experiments carried out with (i) a liquid alkyl-formate stratified on top of an aqueous solution in which the ester dissolves and undergoes a hydrolysis reaction and (ii) gaseous CO dissolving into an aqueous solution of NaOH. We show that the same reaction type can induce a different effect on the convective dynamics depending on the reactant in the host phase. The efficiency of convective dissolution in partially miscible systems can hence be controlled by the chemicals present in the host fluid and their concentration. The direct comparison between the convective dynamics observed during CO dissolution in an aqueous phase and in the ester/water stratification validates the latter as a convenient liquid-liquid model system for the interpretation of the impact of chemical reactivity in geological CO sequestration.
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