The two most important EOR mechanisms for oil-wet carbonate rock are IFT reduction and wettability alteration. By altering rock surface wettability, the resulting positive capillary pressure can help imbibe water and displace oil from the rock matrix. Researchers have studied the wettability alteration of materials covering surfactants, nanofluids, alkalis, salts, and so on. Chelating agents are found to alter carbonate rock wettability recently. This work aims at the wettability alteration and EOR production by three chelating agents in different salinity condition when used alone or combined with surfactants.

Three commonly used chelating agents were studied. Indiana limestone and Guelph dolomite were selected to represent carbonate rocks. Rock samples were dipped in chelating agent solutions at different concentrations and salinity, in ambient and reservoir conditions. Differences in contact angle values due to the treatment reflect the wettability alteration performance. Mixtures of chelating agents and surfactants (VES AGA-97, gemini surfactant) were prepared and tested in the spontaneous imbibition study.

Results show that all tested chelating agents could strongly alter the wettability of carbonate samples from oil-wet to water-wet at relatively low concentrations (~0.4 wt%). When salinity increased, however, higher concentrations were required. Salts showed different effects on the wettability alteration by DTPA. The negative effect lowered in the sequence: NaHCO3 > Na2SO4 ≈ NaCl ≈ MgCl2. CaCl2 showed a positive effect, indicating enhanced performance in carbonate formations due to the abundant Ca2+ ions. A real-time contact angle study under reservoir conditions revealed that the oil drop decreased in diameter while increasing in height, indicating a higher tendency to be removed from the rock surface. The gemini surfactant + DTPA mixture has stronger wettability alteration potential. The VES + DTPA mixture has a stronger IFT reduction potential. The spontaneous imbibition results showed that for relatively tight carbonate rock, wettability alteration has a higher significance than IFT reduction.

This study shows the outstanding wettability alteration performance of chelating agents, suggesting possible applications in oil-wet carbonate formations with high salinity. Besides, this study suggests that in low permeability formations, wettability alteration contributes more to oil recovery than IFT reduction.

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