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We study the fractionation of two components between a well-mixed gas and asaturated convecting porous layer. Motivated by geological carbon dioxide(CO$_2$) storage we assume that convection is driven only by the dissolvedconcentration of the first component, while the second acts as a tracer withincreased diffusivity. Direct numerical simulations for convection at highRayleigh numbers reveal that the partitioning of the components, in general,does not follow a Rayleigh fractionation trend, as commonly assumed. Initially,increases in tracer diffusivity also increase its flux, because the diffusiveboundary layer penetrates deeper into the flow. However, for $D_2\geq 10\,D_1$, where $D_1$ and $D_2$ are, respectively, the diffusion coefficients ofCO$_2$ and the tracer in water, the transverse leakage of tracer between up-and down-welling plumes reduces the tracer flux. Rayleigh fractionation betweencomponents is only realized in the limit of two gases with very largedifferences in solubility and initial concentration in the gas.

Rayleigh fractionation in high-Rayleigh-number solutal convection in porous media