Reactive-convective dissolution in a porous medium: the storage of carbon dioxide in saline aquifers

We quantify the destabilising effect of a first-order chemical reaction on the fingering instability of a diffusive boundary layer in a porous medium. Using scaling, we show that the dynamics of such a reactive boundary layer is fully determined by two dimensionless groups: Da/Ra², which measures the timescale for convection compared to those for reaction and diffusion; and β_C/β_A, which reflects the density change induced by the product relative to that of the diffusing solute. Linear stability and numerical results for β_C/β_A in the range 0-10 and Da/Ra² in the range 0-0.01 are presented. It is shown that the chemical reaction increases the growth rate of a transverse perturbation and favours large wavenumbers compared to the inert system. Higher β_C/β_A and Da/Ra² not only accelerate the onset of convection, but crucially also double the transport of the solute compared to the inert system. Application of our findings to the storage of carbon dioxide in carbonate saline aquifers reveals that chemical equilibrium curtails this increase of CO₂ flux to 50%.