Interfacial tension (IFT) reduction and wettability alteration (WA) are the two most important mechanisms for chemical EOR. When two mechanisms are combined, however, the relative contribution of each mechanism is hard to determine. Researchers have obtained different, sometimes conflicting, conclusions. Numerical simulation studies usually suggest that, when WA happens, IFT should not be reduced. In this work, we investigated experimentally how IFT reduction impacts oil recovery when wettability alteration happens. To answer this question, this study tests the oil recovery potential of novel zwitterionic surfactants (ZS) and Gemini surfactants (GS) for oil-wet carbonate rock. A chelating agent was added to alter the properties of the surfactants to get comparable WA performance but different IFT.
The surfactants used in this study were a zwitterionic surfactant Erucamidopropyl Sulfobetaine, and a locally synthesized amino-amine-based cationic Gemini surfactant. Diethylenetriamene pentaacetate (DTPA) chelating agent was used. Indiana limestone was selected to represent carbonate rocks. The spinning drop method was applied in IFT measurements. Static contact angle under room conditions and reservoir conditions was conducted to evaluate the WA performance of studied materials. Spontaneous imbibition tests were conducted using oil-aged limestone core plugs. By comparing the IFT reduction, WA, and oil recovery, the relative importance of the mechanism was reflected.
Results show that investigated zwitterionic surfactant reduces IFT more than Gemini surfactant. However, Gemini surfactant showed much better WA performance than zwitterionic surfactant. When DTPA was added to zwitterionic surfactant, the wettability alteration performance of zwitterionic surfactant was improved. The mixture of DTPA and zwitterionic surfactant (DTPA/ZS) showed comparable wettability performance compared to DTPA and Gemini surfactant (DTPA/GS). However, the IFT value of DTPA/ZS is much lower compared to DTPA/GS. In spontaneous imbibition tests, the mixture of DTPA/ZS recovered oil much slower than the mixture of DTPA/GS. This shows that when wettability alteration happens, reducing IFT to the ultralow condition has a negative effect on oil recovery. This is because when wettability is altered to water-wet conditions, the capillary force becomes a driving force that helps oil production. Reducing IFT will reduce the magnitude of capillary force, thus negatively impacting the oil recovery rate.
This study shows that for the spontaneous imbibition process in low permeability carbonate rock, IFT reduction negatively impacts the oil recovery when the rock wettability is altered from oil-wet to water-wet.