Structure–Performance Relationships of Metal Oxide Nanofluids for Enhanced Oil Recovery: Balancing Colloidal Stability, Interfacial Regulation, and Pore Transport

The effect of nanofluid assistance in improving the recovery rate depends on the balance between colloidal stability, interfacial regulation, and the transport capacity of nanoparticles in porous media. Under the same preparation and experimental conditions, this study systematically compared four nanofluids of SiO2, TiO2, Al2O3, and Fe3O4 to establish their structure-performance relationship. Through a variety of complementary characterization methods and core flooding experiments, the colloidal properties, rheological behavior, interfacial characteristics, adsorption characteristics, transport ability, and oil displacement effect of each system were evaluated. The results show that the comprehensive performance of SiO2 nanofluids is the best, which is closely related to its good dispersion stability, interfacial regulation ability, and effective propagation in porous media. At a concentration of 0.30 wt%, the final recovery rates of SiO2, TiO2, Al2O3, and Fe3O4 were 52.28%, 41.01%, 31.54%, and 21.49%, respectively, while the water flooding recovery rate was only 18.35%. Although Fe3O4 shows strong adsorption and retention ability, particle aggregation and pore-throat retention limit its effective transport; in contrast, the moderate adsorption of SiO2 not only maintains continuous interfacial regulation, but also avoids excessive retention. After 30 days of aging, the dispersion stability of SiO2 remains above 90%. The above results show that the nanoparticle screening used for EOR should comprehensively consider the synergistic effect among colloidal stability, interfacial activity, adsorption-retention behavior, and deep pore transport, and should not rely on a single performance parameter.