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 (Swi) and in residual oil saturation (Sor). 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, ζ 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. ζ potential of the carbonate is found to become more negative with the dilution of the brine. After the sample is aged with oil, the ζ potential changed, indicating an alteration in wettability.