  The density-driven convection coupled with chemical reaction is the preferred
mechanism for permanently storing CO2 in saline aquifers. This study uses a 2D
visual Hele-Shaw cell to evaluate and visualize the density-driven convection
formed due to gravitational instabilities, also known as Rayleigh-Taylor
instability. The primary goal of the experiments is to understand the various
mechanisms for the mass transfer of gaseous CO2 into brine with different
initial ionic concentrations and flow permeability. Moreover, the impact of CO2
flow rates, injection locations, reservoir dipping angle, and permeability
heterogeneity is also investigated. We observed that the presence of salts
resulted in earlier onset of convection and a larger convective finger
wavelength than the case with no dissolved salts. In addition, experimental
data showed a higher lateral mixing between CO2 fingers when dipping is
involved. The visual investigation also revealed that the CO2 dissolution rate,
measured by the rate of the convective fingers advance, depends on the type and
concentration of the ions present in the brine. The CO2 dissolution for
solutions with varying salt dissolved, indicated by the area of the
pH-depressed region, is observed to be 0.38-0.77 times compared to when no salt
is present. Although convective flow is slowed down in the presence of salts,
the diffusive flux is enhanced, as observed from both qualitative and
quantitative results. Moreover, the reduced formation permeability, introduced
by using a flow barrier, resulted in numerous regions not being swept by the
dissolved CO2, indicating an inefficient dissolution. We also investigated the
effect of discrete high conductivity fractures within the flow barriers, which
showed an uneven vertical sweep and enhanced flow channeling. Lastly, the
parameters regarding CO2 leakage risk during storage are identified and
discussed.
