Wettability Alteration in Low-Permeability Sandstone Reservoirs by “SiO₂–Rhamnolipid” Nanofluid

The wettability condition of a rock’s surface is an enormous parameter for enhanced oil recovery (EOR) in low-permeability reservoirs. It determines the spreading of injected water and the adhesion of residual oil on rock surfaces. In this study, a new nanofluid composed of silica nanoparticles and rhamnolipid in brine was utilized as an effective agent to alter the wettability of a rock’s surface. In order to obtain rock surfaces with different initial wettabilities, hexamethyldisilazane (HMDS) was used for the sample preparation. The measurement of three-phase contact angles for water/air/rock and brine/oil/rock were performed to explore the contact situation between a rock surface and movable fluids, and the spontaneous imbibition test researched the process of oil recovery with nanofluid. By evaluating the experimental measurements, nanofluid showed a significant potential in wettability alteration from oil-wet toward water-wet state, which is an important mechanism for “scraping” oil drops from a rock surface during nano-EOR process. Moreover, a total of nine coreflooding tests were performed to investigate the effects of rock wettability on waterflooding and nanofluid flooding, respectively. The results revealed that an intermediate-wet state of a rock’s surface is in favor of the oil recovery especially for waterflooding, and nanofluid, resulting in an additional 5.3–6.8% increments in oil recovery compared to conventional waterflooding. In conclusion, nanofluids show a significant potential for EOR from low-permeability cores, implying a wide application in oil fields.