Three-dimensional structure of natural convection in a porous medium: Effect of dispersion on finger structure

Density-driven natural convection in a porous medium is of great interest in the geoscience and geoengineering fields, especially for carbon dioxide capture and storage (CCS). In this study, we applied a novel experimental approach to three-dimensional (3D) imaging of density-driven natural convection in a porous medium. Miscible fluid pairs with nonlinear density properties (a sodium chloride solution and a mixture of methanol and ethylene glycol doped with sodium iodide) were used to model density-driven natural convection from 3D imaging captured by X-ray computed tomography (CT). Slight fluctuations that appeared at the interface grew into large “fingers,” which interacted and merged with the neighboring fingers. The fingers extended vertically downward without changing their locations, forming a columnar structure. The finger-extension velocity increased in line with the Rayleigh number and was correlated with the characteristic velocity. The 3D images captured the local concentration of fingers in the course of convective mixing. We found that the decrease in finger-number density was not only related to the Rayleigh number but was also affected by transverse dispersion between the downward- and upward-flow regions. There was an exponential relationship between the finger-number density and the transverse dispersion coefficient. Transverse dispersion caused broadening of the fingers and reduction in the finger-number density because of the interaction and merging of fingers. In carbon dioxide (CO2) geological storage applications, this transverse dispersion influences the long-term-dissolution process of CO2 injected into aquifers.