Free convective controls on sequestration of salts into low-permeability strata: insights from sand tank laboratory experiments and numerical modelling

Using sand tank experiments and numerical models, local-scale solute-transport processes associated with free convection in both the region surrounding as well as within discrete low-permeability strata are explored. Different permeability geometries and contrasts between high- and low-permeability regions are tested. Results show that two free convective processes occur at different spatial and temporal scales. In the high-permeability region, salinisation was rapid and occurred predominantly by free convective flow around the low-permeability blocks (interlayer convection). A free convection flow field also became concurrently established within the low-permeability lenses (intralayer convection). It was found that upward vertical flow created by the large-scale interlayer free-convective flow field in the high-permeability region retards salinisation of the lenses as buoyant freshwater displacements oppose the downward penetration of dissolved salts. Salinisation of the low-permeability structures eventually takes place from below as saltwater is dragged upwards. This bottom up convective salinisation process of low permeability strata has not been reported in previous literature. These results demonstrate that variable-density sequestration of solutes driven by a source resident above the low-permeability layer is a complicated function of the geometry of the permeability distribution and the permeability contrast between low- and high-permeability regions.