Gravitational Instability and Convection in a Granular Porous Medium: Pore Scale Experimental Study and Implications for Solubility Trapping Of CO2
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Abstract:
Geological sequestration of CO2 is considered as one of the few efficient ways to mitigate the increased global warming scenario. Solubility trapping stands as one of the key trapping mechanisms in CO2 sequestration. It dictates the typical timescale for safely storing CO2 in deep aquifers. A less dense layer of supercritical CO2 collects over a denser brine solution on CO2 injection under the cap rock. As CO2 mixes with brine, it incidentally forms a mixture heavier than pure brine, thus triggering a gravitational instability. Solubility trapping efficiency can be estimated by how fast this heavy layer is removed by the convective instability, thereby fuelling subsequent dissolution of CO2 into brine. Solubility trapping results in irreversible storage of CO2 at the bottom of the aquifer where dissolved CO2 is eventually converted into carbonate minerals through chemical trapping. Although most experimental studies of this phenomenon have been carried out in Hele-Shaw setups, as analogues to two dimensional (2D) porous media, few existing 3D numerical simulations indicate that its convection structure may be different from its 2D counterpart. Further, the effect of pore scale detail on the instability and subsequent convection is still unknown. To unravel these aspects, we have developed a laboratory scale experiment based on refractive index matching (RIM) of the liquid to the granular solid matrix. The setup does not currently involve continuous dissolution of CO2 at the top boundary but uses an analogue solute instead. It thus aims to unravel the miscible Rayleigh-Taylor instability dynamics in presence of a granular medium, and how it is differs from the dynamics predicted by Darcy-scale models. We focus on the onset time, non-linear time, mixing time, plume amplitude and plume speeds for the instability. The granular medium, with grains of diameter 3 mm, has dimensions 45x45x1 cm3. The 2D visualization in based on a solute dye that differentiates between the heavier (top) and lighter (bottom) fluid. The 3D visualization relies on a horizontal laser sheet that scans across the setup and triggers fluorescence in the lighter fluid whereas the heavier fluid is RIM to the solid grains. Upscaling the results from these experimental measurements provides insights into the trapping efficiency in geological porous media.