低炭素社会構築のためのCCS

[3206] A Modelling of Mass Transfer due to Natural Convection in Porous Medium

^○王蕾¹, 兵藤陽光¹, 坂井成基¹, 末包哲也¹ (1.東京工業大学)

司会:佐々木久郎(九州大学)

Keywords:Mass transfer, Rayleigh number, Natural convection, Rayleigh-Bénard fingers, Visualization

Carbon dioxide (CO₂) injected into the saline aquifers dissolves into the brine, leads to a convective process at the reservoir scale that governs the strength of CO₂ mass transfer into the brine, promoting stable long-term storage. The quantification of mass transfer of CO₂ into the brine is of great importance to evaluate a long-term fate of CO₂.
In this study, we conducted laboratory experiment by applying an analogous fluids system (NaCl solution, sodium iodide and a mixture of methanol, ethylene glycol) that simple models are able to obtain the characteristic of the natural convection in reality saline formation. A three-dimensional visualization scheme employing micro-focused X-ray computer tomography (CT) technology to observe natural convection has been developed in this work by using a plastic resin particles packed bed as a porous medium. Based on the reconstruction software, the distribution of local concentration can be evaluated from CT images as the X-ray attenuation depends on the concentration of sodium iodide.
Under the reservoir, the CO₂ saturated heavy brine sinking downward and light virgin brine floating upward. The process of formation and development of Rayleigh-Bénard fingers induced by density difference can be visualized. It is able to mimic the convective mixing in deep saline formation. During the process, downwelling of high density fingers resulting in a decrease in finger number and an increase in finger diameter by the coalescence with neighbouring fingers. The finger diameter grows with time, but remains the same, because of the mixing between the upward and downward flowing regions. As the Rayleigh number increases, the finger extending occurs more quickly. The mass transfer increases with the Rayleigh number also. The dimensional flux Sherwood number is associated with Rayleigh number following the power law relationship.