Mechanistic Evaluation of Al₂O₃ and TiO₂ Nanofluids on Wettability and Interfacial Properties for Geological CO₂ Storage in Sandstone Reservoir

Geological CO₂ storage in quartz-rich sandstone is often limited by unfavorable wettability and high capillary entry pressures. This study examines how Al₂O₃ and TiO₂ nanofluids modify wettability and CO₂–water interfacial tension under reservoir conditions (8.27 MPa, 323 K). To isolate the intrinsic effect of nanoparticles, salinity was intentionally excluded and all nanofluids were prepared in deionized water (DIW), thereby enabling direct evaluation of nanoparticle-rock and nanoparticle–interface interactions without ionic interference. Nanofluids (0.05 and 0.1 wt %) were preinjected into a sandstone core at a volume of 0.5 pore volumes (PV). Subsequent measurements included contact angle, CO₂–water interfacial tension, and core-flooding responses. Contact angle and interfacial tension were quantified in situ using a high-pressure drop shape analyzer, while CO₂ storage efficiency and water displacement were determined from dynamic injection experiments. Al₂O₃ nanofluids increased the contact angle from 64.6° (DIW) to nearly 80°, thereby reducing displacement pressure from 3.214 to 1.264 kPa with negligible changes in interfacial tension. In contrast, TiO₂ nanofluids reinforced hydrophilicity while reducing interfacial tension by 14–16%, resulting in displacement pressures as low as 1.120 kPa. These interfacial modifications enhanced CO₂ displacement efficiency to 63.58–70.21% and increased water displacement up to 45.5%. Porosity changes remained below 2%, indicating minimal pore plugging. By distinguishing the wettability-dominant mechanism of Al₂O₃ from the interfacial tension-dominant mechanism of TiO₂, this study provides mechanistic insight into nanoparticle selection for CO₂ storage applications. Although the DIW-based system represents a simplified environment, the results clarify fundamental interfacial responses induced by nanoparticles and establish a foundation for future validation under realistic brine salinities.