Chemically-driven convective dissolution.

When a solute A dissolves in a host phase with a given solubility, the resulting density stratification is stable towards convection if the density profile increases monotonically along the gravity field. We theoretically and numerically study the convective destabilization by reaction of this dissolution when A reacts with a solute B present in the host phase to produce C via an A + B→C type of reaction. In this reactive case, composition changes can give rise to non-monotonic density profiles with a local maximum. A convective instability can then be triggered locally in the zone where the denser product overlies the less dense bulk solution. First, we perform a linear stability analysis to identify the critical conditions for this reaction-driven convective instability. Second, we perform nonlinear simulations and compare the critical values of the control parameters for the onset of convection in these simulations with those predicted by linear stability analysis. We further show that the asymptotic dissolution flux of A can be increased in the convective regime by increasing the difference ΔRCB = RC-RB between the Rayleigh numbers of the product C and reactant B above a critical value and by increasing the ratio ß = B0/A0 between the initial concentration B0 of reactant B and the solubility A0 of A. Our results indicate that chemical reactions can not only initiate convective mixing but can also give rise to large dissolution fluxes, which is advantageous for various geological applications.

Enhanced convective dissolution due to an A + B → C reaction: control of the non-linear dynamics via solutal density contributions.

Chemical reactions can have a significant impact on convective dissolution in partially miscible stratifications in porous media and are able to enhance the asymptotic flux with respect to the non-reactive case. We numerically study such reactive convective dissolution when the dissolving species A increases the density of the host phase upon dissolution and reacts with a reactant B present in the host phase to produce C by an A + B → C reaction. Upon varying the difference ΔRCB = RC-RB between the Rayleigh numbers of the product C and the reactant B, we identify four regimes with distinct dynamics when the diffusion coefficients are the same. When ΔRCB < 0, the density profiles are non-monotonic and the non-linear dynamics are seen to depend on the relative values of the density at the interface and the initial density of the host phase. For ΔRCB > 0, the monotonic density profiles are destabilizing with respect to the non-reactive case above a certain critical value ΔRcr. We analyze quantitatively the influence of varying ΔRCB and the ratio ß = B0/A0 of the initial concentration of B and the solubility of A on the asymptotic steady flux, the wavelength of the fingers and the position of the reaction front. In the context of CO2 geological sequestration, understanding how such reactions can enhance the dissolution flux is crucial for improving the efficiency and safety of the process.