Investigating Rayleigh–Bénard convection, its onset, and its effect on mass transfer are crucial for understanding the effective dissolution processes between carbon-dioxide (CO2)-saturated and CO2-free brines in CO2 geological storage. However, most previous investigations on the onset of convection and mass transfer properties were conducted by 2D and 3D numerical simulations. Only a few 3D experimental studies of Rayleigh–Bénard convection were reported. In this study, we proposed a non-destructive experimental approach to visualize the three-dimensional (3D) fingering structure of Rayleigh–Bénard convection using a micro-focused X-ray computed tomography (X-ray CT) scanner. The experimental conditions covered the Rayleigh number (Ra) range between 3400 and 23000, which also includes the Ra value of the actual condition of gas reservoir field. Results show that fingers and coalescence dynamics significantly developed upon increasing Ra. At higher Ra, the finger concentration experienced greater dilution due to the combined effects of unsteady finger motion and enhanced dispersion strength. The maximum finger extension velocity is linearly proportional to Ra. The convective mass flux (Sh) correlated with Ra by a power-law relationship. The findings in this study provide a better understanding of the influence of Ra on mass transport properties and can be used as a reference for developing the convective mass transfer model of Rayleigh–Bénard convection in 3D porous media.
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