Onset of convection in anisotropic porous media subject to a rapid change in boundary conditions

Previous studies of fluid convection in porous media have considered the onset of convection in isotropic systems and the steady convection in anisotropic systems. This paper bridges between these and develops new results for the onset of convection in anisotropic porous media subject to a rapid change in boundary conditions. These results are relevant to sedimentary formations where the average vertical permeability is some fraction gamma of the average horizontal permeability. Linear and global stability analyses are used to define the critical time t(c) at which the instability occurs as a function of gamma and the dimensionless Rayleigh-Darcy number Ra-* for both thermal and solute-driven convection in an infinite horizontal slab. Numerical results and approximate analytical solutions are obtained for both a slab of finite thickness and the limit of large slab thickness. For a thick slab, the increase in t(c) as gamma decreases is approximately given by (1+root gamma)(4)/(16 gamma(2)). One important application is to the geological storage of carbon dioxide where it is shown that the use of an effective vertical permeability in estimating the critical time is only valid for low permeabilities. The time scale for the onset of convection in geological storage can range from less than a year (for high-permeability formations) to decades or centuries (for low-permeability ones). (c) 2005 American Institute of Physics.