NON-UNIFORM DISPLACEMENT AND RESIDUAL TRAPPING OF SUPERCRITICAL CO2 IN MIXED-WET MICROMODELS

Supercritical CO₂ (scCO₂)-induced wettability changes have been observed for thin water films, where the contact between the CO₂ and mineral surface is close. In the natural system, one might expect wettability changes to occur in patches where there are thin water films, and not where the water film is thicker – a mixed wet system. In this study, we investigated the pore-scale displacement and residual trapping of scCO₂ in a mixed-wet heterogeneous micromodel at 85 bars and 45^◦C. We created mixed-wet systems by applying octadecyltrichlorosilane (OTS, 0.2% by vol. in hexane) to modify wettability in a micromodel, while it is undergoing drainage with ethylene glycol (EG) as the wetting phase. With varying injection rates, the distribution of OTS solution and EG, as well as the wettability alteration in micromodel, is well-controlled, and two mixed-wet systems, namely capillary mixed-wet (at slow displacement rate) and viscous mixed-wet (at high displacement rate) were established for subsequent scCO₂-brine displacement tests. Results show very different drainage fingering patterns and imbibition bypass as a function of displacement rates from log Ca of -8.1 to -4.1 in the capillary and viscous mixed-wet systems, as well as in the untreated water-wet micromodel. Quasi-steady state scCO₂ saturations after drainage show highest values in viscous mixed-wet micromodel, and are lowest in the capillary mixed-wet micromodel, indicating the effects of both pore-scale heterogeneities in pore geometry and wettability. During imbibition, brine bypass dominated the displacement and determined the residual CO₂ saturation distribution. Pore filling and snap-off mechanisms, though suppressed by the brine bypass, show varying contributions to the flow pathway and residual saturations within the three wetting types of micromodels. The non-uniform displacement and residual trapping of scCO₂ observed in the mixed-wet micromodels differ from that in the water-wet and uniform intermediate-wet systems, showing the need to consider the role of mixed-wet grain surfaces on the scCO₂ plume migration and safe and permanent storage.