The performance of surfactant-enhanced oil recovery (EOR) in fractured carbonates relies on spontaneous imbibition or low IFT-aided gravity drainage. This work investigated the synergism between wettability effects and IFT reduction mediated by a variety of surfactants through experiments and numerical simulation studies. Experiments have shown that oil can be recovered from oil-wet Silurian dolomite fracture blocks either by capillarity driven imbibition, gravity-driven imbibition or low tension-aided gravity drainage. The mixture of wettability alteration (WA) surfacant with IFT reduction surfactant exhibits the synergistic effect on the imbibition oil recovery from oil-wet carbonate rocks. It was found that divalent ion scavengers help the wettability altertion capability of some sulfonate surfactants in hard brine, which leads to the high oil recovery up to 70% OOIP (IFT+WA), compared with oil recovery of 30-50% OOIP by sulfonate surfactant only (only IFT reduction). We proposed a mechanism that the presence of a sufficient amount of divalent ion scavengers in the anionic surfactant formulation reduces the free divalent cations in hard brine, which then promotes the release of surfactant monomers from the micelles and enhances wettability alteration by surfactant adsorption. The UTCHEM simulation results confirmed the existence of synergism between IFT reduction and WA in spontaneous imbibition processes. According to the capillary desaturation curve (CDC), that residual oil saturation after gravity drainage is approximately 10% to 20% higher than gravity-driven spontaneous imbibition when two processes have the similar trapping numbers, confirming that the wettability alteration contributes to the ultimate oil recovery.

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