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We numerically examine solutal convection in porous media, driven by thedissolution of carbon dioxide (CO2) into water---an effective mechanism for CO2storage in saline aquifers. Dissolution is associated with slow diffusion offree-phase CO2 into the underlying aqueous phase followed by density-drivenconvective mixing of CO2 throughout the water-saturated layer. We study thefluid dynamics of CO2 convection in the single aqueous-phase region. Acomparison is made between two different boundary conditions in the top of theformation: (i) a constant, maximum aqueous-phase concentration of CO2, and (ii)a constant, low injection-rate of CO2, such that all CO2 dissolves instantlyand the system remains in single phase. The latter model is found to involve anonlinear evolution of CO2 composition and associated aqueous-phase density,which depend on the formation permeability. We model the full nonlinear phasebehavior of water-CO2 mixtures in a confined domain, consider dissolution andfluid compressibility, and relax the common Boussinesq approximation. Wediscover new flow regimes and present quantitative scaling relations for globalcharacteristics of spreading, mixing, and a dissolution flux in two- andthree-dimensional media for both boundary conditions. We also revisit thescaling behavior of Sherwood number (Sh) with Rayleigh number (Ra), which hasbeen under debate for porous-media convection. Our measurements from thesolutal convection in the range 1, 500<Ra<135, 000 show that the classicallinear scaling Sh ~ Ra is attained asymptotically for theconstant-concentration case. Similarly linear scaling is recovered for theconstant-flux model problem.

Solutal convection in porous media: Comparison between boundary conditions of constant concentration and constant flux