The role of capillary forces during buoyant migrati
on of CO2 is critical towards plume immobilization within the post-injection phase of a geological carbon sequestration operation.
However, the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in-situ plume
evolution. To overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic CO2 plumes in the presence of a background
hydraulic gradient, we conducted two unique long-te
rm experiments in a 2.44 m × 0.5 m tank. X-ray attenuation allowed measuring the evolution of a CO2-surrogate fluid saturation, thus providing direct insight into capillarity- and buoyancy-domin
ated flow processes occurring under successive
drainage and imbibition conditions. The comparison
of saturation distributions between two
experimental campaigns suggests that layered-type h
eterogeneity plays an important role on non-
wetting phase (NWP) migration and trapping, because
it leads to (i) longer displacement times
(3.6 months vs. 24 days) to reach stable trapping c
onditions, (ii) limited vertical migration of the
plume (with center of mass at 39% vs. 55% of aquife
r thickness), and (iii) immobilization of a
larger fraction of injected NWP mass (67.2% vs. 51.
5% of injected volume) as compared to the
homogenous scenario. While these observations confirm once more the role of geological
heterogeneity in controlling buoyant flows in the s
ubsurface, they also highlight the importance of
characterizing it at scales that are below seismic
resolution (1-10 m).