(55e) Influence of Permeability on the Relative Contribution of Wettability Alteration and IFT Reduction in Oil Recovery

Carbonate reservoirs contain a significant fraction of the world’s remaining oil. However, carbonate rock tends to be oil-wet as a result of long-time contact with oil in most cases. As a result, wettability alteration is found to be the other major EOR mechanism besides IFT reduction. Some studies have shown that the contribution of wettability alteration is affected by rock permeability. The main objective of this work is to clarify how the rock permeability affects the relative oil recovery contribution of wettability alteration and IFT reduction in carbonate rock. Indiana limestone cores of three different permeability ranges were cut into plugs and slices. They were aged in filtered crude oil under high temperature in the oven for two weeks. A locally synthesized cationic gemini surfactant was used to alter rock wettability towards the water-wet condition. Both ambient contact angle measurement and spontaneous imbibition experiments were conducted for comparison. Results showed that permeability is negatively correlated with the oil recovery contribution of wettability alteration. In spontaneous imbibition tests, lower permeability samples produced a larger fraction of oil in place. This was because capillary pressure helped the imbibition of water phase when rock wettability became water-wet. Lower permeability is related to a smaller pore throat radius and thus implies larger capillary pressure. As a result, in lower permeability rock, the relative oil recovery contribution of wettability alteration was enhanced compared to that of IFT reduction. In the ambient contact angle test, however, slices of various permeability showed comparable contact angles before and after wettability. This observation can be explained by the negligible effect of capillary pressure under such cases. This study shows that when rock wettability is altered from oil-wet to water-wet condition, the contribution of wettability alteration on oil recovery rate is negatively correlated with rock permeability. Besides, this study also suggests the locally synthesized cationic gemini surfactant as an effective wettability modifier. Conclusions obtained in this study indicate that more attention should be given to wettability alteration when optimizing the EOR fluid formula for low permeability carbonate formations.