Quantitative study of density-driven convection mass transfer in porous media by MRI

Abstract

CO2 geologic storage in deep saline aquifers is a promising technology to reduce greenhouse gas emissions and maintain the pace of economic development. The dissolution and convection of CO2 injected into brines are the primary trapping mechanisms to enhance storage efficiency and provide long-term storage security. The spatial mass transfer characteristics of convection in porous media remain poorly understood. In this study, two kinds of analogous fluid pairs were used as the equivalents of CO2-saturated brine and in situ brine. Density-driven convection experiments using magnetic resonance imaging (MRI) technology in heterogeneous porous media were performed at various stages, including the diffusion stage, the unstable trigger stage and the convection development stage. Combined with the spatial evolution of the fingers and the change in MRI mean intensity, the local fluid concentration and overall mass transfer behavior could be quantified. Physical quantities associated with mass transfer by convection, such as the mixing layer thickness and scalar dissipation rate at the diffusion stages, the onset time at the unstable trigger stage, the average velocity of the front finger, the finger number, the transverse concentration gradient of the finger, and the sweep area of the finger at the convection development stage, were also analyzed systematically. We found that convective mixing substantially promotes mass transfer over pure diffusion, and the dissolution rate can be further enhanced by improving the injection method at each stage. The range of Ra in this study covers most CO2 capture and storage (CCS) sites and can provide basic data for engineering applications.