Onset of convection in a porous medium in the presence of chemical reaction

Using scaling, we show that the stability of a buoyant boundary layer in a porous medium in the presence of a first-order chemical reaction is fully determined by the nondimensional number Da/Ra-2 = k(r)aD phi mu(2)/(k Lambda rho(0)g)(2), where Da = k(r)aL(Z)(2)/(D phi) is the Damkohler number and Ra = k Delta rho(0)gL(Z)/(mu D phi) is the solutal Rayleigh number. The time for onset of convection is shown to increase with rising Da/Ra-2. Above a critical Da/Ra-2 approximate to 2 x 10(-3), no convection occurs as reaction stabilizes the diffusive layer at a finite thickness. This thickness decreases with increasing Da/Ra-2, becoming zero at Da/Ra-2 approximate to O(1). As applied to CO2 geostorage, our results suggest distinct regimes for CO2 transport in saline aquifers.