Functionalization-Driven Wettability Control by Ionic Liquids at CO ₂ –Brine–Carbonate Interfaces: Implications for CO ₂ Geo-storage in Saline and Depleted Carbonate Reservoirs

The depleted oil reservoirs in Saudi Arabian oilfields often represent a viable option for effective CO2 geo-storage. However, understanding and controlling the rock–CO2–brine wettability are very essential for the effective and secure CO2 geo-storage under harsh reservoir conditions. Carbonate reservoirs are sensitive to their wettability changes, since even the presence of traces of organic residues will alter their surface charges, which will shift their overall wettability more toward oil-wet or CO2-wet, and thus, it influences their relative capillarity as well. In this study, we investigated the role of molecular functionalizations in imidazolium-based ionic liquids (ILs) in order to understand their efficacy on altering the wettability of rock–CO2–brine wettability systems in the harsh carbonate reservoirs. Systematically, we conducted the contact angle measurements of the various rock–CO2–SW systems with and without IL treatments at various temperatures (25–80 °C) and pressures (14.7–3000 psi). In this, we studied clean calcite, clean carbonate, and oil-wet carbonate samples in order to cover both saline aquifers and depleted oil reservoirs. The clean calcite and carbonate samples were observed to remain water-wet with the measured contact angles below 43° at all the studied temperatures and pressures, but they slightly increased upon increasing their CO2 pressure due to the increase in their CO2 density. However, treating these samples with ILs significantly shifted the wettability more toward water-wet with contact angles in the range of 9–24°. The untreated oil-wet carbonate reservoirs were initially identified to be strongly CO2-wet with the measured contact angle range of 102–134°, and treating these samples with ILs dropped the contact angles drastically to the range of 23–71° and restored the water-wet characteristics. The −COOH-functionalized ILs outperformed with the lowest contact angle of 23–45°. The moderate reduction of the CO2–SW interfacial tension with the addition of these ILs further supports the effective CO2 capillary trapping. The observed results suggest that proper functionalizations of chemicals, such as −COOH-functionalized ILs, can provide a highly surface-active, efficient, and brine-compatible approach to reverse oil-wet or CO2-wet reservoirs back to water-wetness, and thus, it can enable better CO2 geo-storage potential under harsh reservoir conditions, especially at high CO2 pressure conditions.