Investigation of the Effects of Density-Driven Convection on Solute Mixing in Porous Media with Application for CO2 Sequestration

Abstract:

Convective mixing is a process that greatly enhances the transport of mass between two miscible fluids and that plays an important role in both engineered and natural processes. In the context of geologic CO₂ sequestration, density-driven convection of CO₂-rich reservoir fluid is one of the fundamental mass transport mechanisms for mixing and storing significant quantities of CO₂ in the subsurface. Due to a slightly higher density of CO₂-rich water as compared to fresh water, gravitational instabilities occur and characteristic, perpendicular elongated finger-like patterns form that are enhancing the mixing between CO₂ and water compared to a purely diffusive process.

We investigate the effects of density-driven convection on the mixing dynamics of CO₂ in saline aquifers over a range of Rayleigh numbers by a CFD-FEM model that simulates the convective mixing in a two-dimensional porous media. While previous studies have focused almost exclusively on evaluating the dissolution flux, we extend the analysis to include various statistical measures of mixing that make use of the local spatial structure of the concentration field in the medium.
Owing to the difficulty of imaging the time-dependent convective process, experiments so far have largely focused on two-dimensional systems (e.g. Hele-Shaw cells). However, the convective fingers are propagating into all three spatial directions and neglecting the third spatial dimension imposes a strong restriction on the lateral spreading of the plumes. To explore the flow pattern within a three-dimensional medium, we developed an experimental procedure by applying X-ray CT imaging and 3D reconstructions that allow visualisation of the spatial and temporal evolution of the plumes non-invasively. To imitate the dissolution process of CO₂ in brine under laboratory conditions, we use salt with a high X-ray attenuation coefficient that dissolves in water and creates a heavier solution than pure water. The method has been successfully tested and results are expected to provide more representative information towards the investigation of convective mixing in the context of CCS. Insights into the complex three-dimensional mixing structures will additionally support the elucidation if two-dimensional scaling laws can successfully predict three-dimensional behaviour.