Assessment of Factors Controlling Low Salinity Water Flooding in Carbonates: An X-Ray Photoelectron Spectroscopy Insight

Low salinity water (LSW) flooding, as a wettability modifying agent, is observed as one of the viable technologies for improving oil recovery in carbonates, owing to its low cost, low carbon footprint and effectivity. However, the optimum salinity (i.e. dilution factor) and wettability state to trigger the low salinity effect remain subjects of conflicting views. Thus, the main objective of this study was to investigate the factors controlling the low salinity effect (LSE) quantitatively and qualitatively. Series of interfacial tension (IFT) measurements, contact angle (CA) analysis on calcite surfaces, and, spontaneous imbibition tests in Indiana limestone outcrops were carried out at 75°C. The experiments were performed using synthetic formation water (FW) having high salinity (196 g/L), which was diluted by various factors: 2 times (97.9 g/L), 4 times (49 g/L), 10 times (19.6 g/L), 20 times (9.8 g/L) and 100 times (1.96 g/L), and crude oil sample obtained from the targeted carbonate reservoir. Furthermore, surface composition of calcite surfaces before and after wettability modification were analyzed using X-ray photoelectron spectroscopy (XPS). The measured IFT between formation water and crude oil was equal to 9 mN/m and it reduced to ∼6 mN/m upon decreasing the salinity. Such IFT reduction could contribute to the recovery mechanism. CA results showed that the wettability of the calcite surfaces was altered from an initial oil-wet state towards an intermediate-wet state, upon reducing the salinity from 196 to 19.6 g/L. Further reduction of salinity resulted in no wettability alteration and the calcite surfaces remained oil-wet. It was found that the positive effect of increasing rock dissolution, by decreasing salinity, in reducing oil-wetness is opposed with the negative role of sulfate in shifting the wettability towards oil-wet due to the resultant increase of the pH of LSW solution. The oil recoveries by spontaneous imbibition were in perfect agreement with CA results, as maximum oil recovery (∼50 %) was obtained for the LSW system (19.6 g/L) that shifted the wettability from oil-wet towards intermediate-wet. From XPS analysis, it was observed that the atomic percentage of C-C/C-H peaks reduced after treatment with LSW suggesting the calcite was slightly dissociated during treatment. In addition, calcium dissociation was more dominant than carbon after treating with 1.96 g/L, while for the case of 19.6 g/L, carbon dissociation was more dominant. The findings of this study provide a deeper understanding of the factors influencing the LSE in carbonates by coupling the change in surface composition with wettability alteration. It is interesting to state that although none of the LSW solutions used in this study could alter the surface wettability towards a water-wet state, some of the intermediate-wet systems were able to efficiently increase oil recovery by spontaneous imbibition.