New Insight into the Synergism Between Engineered Water and Nonionic Surfactants for Improving Spontaneous Imbibition in Oil-Wet Carbonates

Altering the wettability of rock matrix to water-wet has been recognized as an effective approach to enhance oil recovery from fractured carbonate reservoirs. This can be achieved using various techniques, including but not limited to, water salinity/composition modification (engineered water) and surfactants injection. This study features an experimental evaluation of engineered-water-surfactant (EWS) performance in wettability alteration of oil-wet carbonate having high salinity formation water (196 g/L) and moderate temperature (75 °C), representative of an actual Omani carbonate reservoir. Two commercially available polyethoxylated nonionic surfactants were used in this study. The surfactants were mixed to obtain an average ethylene oxide (EO) units equals to 12 (C₁₃EO₁₂). The surfactant solutions were prepared at concentration of 0.3 wt. % using diluted formation water (100 fold) and spiked with either 50 or 100 mmole/L of Mg²⁺ and Ca²⁺. The crude oil was acquired from the targeted reservoir. The experimental workflow included the following measurements: cloud point, interfacial tension (IFT) against crude oil, contact angle (CA) on calcite surfaces, and, spontaneous imbibition in Indiana limestone outcrops. Results revealed that increasing the concentration of either Mg²⁺ or Ca²⁺ had a minor effect on the cloud point temperature (≤2 °C), despite the increase in salinity. On the other hand, increasing divalent ions concentrations further reduced the IFT by 1 order of magnitude and reduced the IFT equilibrium time. However, the minimum IFT value obtained was 0.06 mN/m, which still falls within the range of Winsor Type-I. From CA measurements it was found that low salinity nonionic surfactant was able to alter the wettability from oil-wet to weak water-wet state. Spiking the surfactant solution with 50 mmole/L of divalent ions proved to be more effective than 100 mmole/L as the latter concentration showed a negligible impact on the CA. A strong water-wet state (27°) was achieved after treatment with surfactant solution spiked with 50 mmole/L of Mg²⁺. Unexpectedly, the system that yielded strong water-wet state resulted in the lowest oil recovery by spontaneous imbibition (37 %). While, the remaining EWS systems were able to substantially improve oil recovery (69-74 %) compared with the reference low salinity surfactant system (51 %). One interesting finding; the additional oil recovery is inversely related to the difference between the cloud point of the EWS system and the testing temperature. The findings of this study clearly indicate that strong water-wet state does not always result in high oil recovery from carbonates. The role of divalent ions in boosting the solubility along with the interfacial activity of nonionic surfactants is promising; it provides a new insight and a new approach for EWS implementation to obtain optimum synergism that leads to higher oil recovery.