Pore-Scale Supercritical CO2- Brine Drainage Fingering in Mixed-Wet Micromodels

Displacement of resident brine by supercritical CO2 (scCO2) during geological carbon sequestration involves unstable drainage due to the unfavorable viscosity ratio between invading scCO2 and the resident brine. The unstable drainage process can be complicated by non-uniform wetting of the solid surface, resulting from scCO2 induced wettability changes. These changes occur in local patches where water films are thin, and not where the water film is thicker resulting in a mixed wet system. In this study, pore-scale drainage fingering of scCO2-brine in mixed-wet micromodels was investigated in a heterogeneous micromodel under a wide range of displacement rates from log Ca of -8.1 to -4.1 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 fingering of the OTS solution and the location of the wettability alteration in micromodels are well-controlled. Two mixed-wet systems, namely capillary mix-wet (fingering controlled by capillary force) and viscous mixed-wet (fingering controlled by viscous force) were established for subsequent scCO2-birne displacement. Results show in the mixed-wet and untreated water-wet micromodels, capillary fingering dominated the displacements at small displacement rates (logCa<-6.4), while at large displacement rates (log Ca > -6.1), viscous fingering developed with increased CO2 saturation. A crossover zone with decreased CO2 saturation was observed at the intermediate rates. Quasi-steady state scCO2 saturations are highest in the viscous mixed-wet and lowest in the capillary mixed-wet micromodels for the three fingering regimes investigated. Locally, tortuosity and connection of invading scCO2 pathways vary in the different wetting micromodels. Both scCO2 distribution and saturation suggest the importance of pore-scale heterogeneities in grain surface wettability. This study adds to the understanding of the complex pore-level drainage pattern and scCO2 saturation in various porous media. For field-scale GCS, the non-uniformity in solid surface wettability will inevitability play a critical role in scCO2 migration and overall storage capacity.