Wettability Alteration during Low-Salinity Water Flooding

Low-salinity water flooding (LSWF) for hydrocarbon recovery has attracted industrial attention, owing to its simplicity and economic feasibility. Although this topic has received numerous studies, mechanisms driving the low-salinity effect remain poorly understood. This study is aimed at investigating the direct effects of injecting low-salinity brine (0.6 and 0.2 M NaCl) as the non-wetting fluid and Soltrol 130 as a synthetic wetting fluid on outcrop "Austin Chalk" rock samples. The petrophysical properties of rock samples were estimated by saturating the core samples with high- and low-salinity brine at laboratory conditions. Experiments were conducted for unsteady-state and steady-state flow for both imbibition and drainage processes. A shift to the right has been observed for the relative permeability curve of 0.2 M NaCl along with a drop in irreducible water saturation (S-wi) and in residual oil saturation (S-or). Furthermore, the results have shown a reduction in irreducible water saturation from 22.2 to 18.7% when using 0.2 M NaCl compared to 0.6 M NaCl. The current research demonstrates that ionic interactions among rock, oil, and brine compositions would alter the in situ wettability of the carbonate samples from oil-wet/mixed-wet to more water-wet conditions. A correlation is found among the double-layer expansion, zeta potential, and wettability alteration during LSWF. Moreover, improved oil recovery takes place during LSWF only when a repulsive electrostatic force between oil-brine and mineral-brine interfaces is induced by the change in brine composition. zeta potential of the carbonate is found to become more negative with the dilution of the brine. After the sample is aged with oil, the zeta potential changed, indicating an alteration in wettability.