Multi-dimensional Imaging and Characterization of Convective Mixing in a Porous Media

The dissolution of CO2 into reservoir brines is one of the key trapping mechanisms during CO2 sequestration in deep saline aquifers. The dissolution at the CO2-brine interface induces a buoyant instability in the aqueous phase following a local brine density increase in the range of 0.1-1% depending on pressure, temperature, and salinity. As a result the CO2 -saturated brine mixes with fresh brine to form characteristic finger-like patterns. This downward flow pushes fresh brine to the CO2-brine interface and further enhances dissolution. This phenomenon is referred to as convective mixing. A study has been undertaken to investigate convective mixing in a 3D opaque porous medium. A novel protocol is presented using X-ray Computed Tomography (X-ray CT) to image the evolution of convective mixing over time. Results are presented for experiments carried out at ambient conditions using a spherical bowl (diameter of 20 cm) packed with glass beads (diameter, 0.5 mm). Surrogate fluids are used that provide good x-ray contrast whilst maintaining a maximum density differential comparable to the one observed in a supercritical CO2-brine system (about 10 kg/m3). We use a mixture of methanol and ethylene glycol (MEG) at three different ratios (and doped with KI) and brine. We perform two repeats for each fluid pair and during a typical experiment scans are taken at regular time intervals for up to 10 hours. 3D images of the bowl are reconstructed (fig. 1) with (2x2x2) mm3 voxels. The experiments are classified by Rayleigh number covering the range Ra = 5,000-25,000. As expected, higher Ra leads to early development of instability, with the plume moving faster towards the bottom of the bowl. The computed dissolution flux supports these visual observations and confirms that dissolutions enhanced mixing produces fluxes that are significantly larger than the corresponding purely diffusive scenario. While quantitative agreement is observed from repeated experiments, we note that the characteristic microscopic features differs (e.g. finger distribution), even when a homogenous packing is considered. We observe that mixing is not necessarily more efficient with a higher Ra, as in a closed domain the faster movement of the plume towards the bottom leads to an early convective shutdown and a situation that is solely controlled by diffusion.