Effect of layered heterogeneity of a porous medium on density-driven natural convection

Abstract

In carbon dioxide capture and storage (CCS), the shift to the dissolution trapping of CO₂ in brine from the capillary trapping and physical trapping by caprock depends on the mass transfer associated with density-driven natural convection at a reservoir scale. In this study, natural convection between miscible fluids in a porous medium with layered structure is visualized three-dimensionally by means of an X-ray microtomography. When a finger associated with Rayleigh-Taylor convection passes through the interface of the layered heterogeneous structure with an increasing permeability, because of an increase in the finger extension velocity, the finger diameter and the concentration in the finger decrease. On the other hand, when a finger passes through the interface with a decreasing permeability, the finger diameter increases and the concentration in the finger remains the same, which suggests that the change in the concentration in the finger shows nonlinearity against the permeability. In present study, because the thickness of the initial interface is lower than the thickness of the layer structure, the onset time of convection depends on the property of the porous medium just around the initial interface. In addition to that, the finger extension velocity depends on the local property of the porous medium and the empirical equation derived for the uniform porous medium agrees well with the local finger extension velocity estimated for each layer.