Influence of sequential changes in the crude oil-water interfacial tension on spontaneous imbibition in oil-wet sandstone

Crude oil-water interfacial tension in petroleum reservoir is reduced or increased due to surfactant injection or surfactant retention, respectively. Changes in the interfacial tension crucially attribute to a governing capillary pressure and hence an oil displacement in spontaneous imbibition process. While a reduction in the interfacial tension has been highlighted as one of the underlying mechanisms for enhanced oil recovery, fluctuated surfactant concentration within reservoir promptly disturbs such interfacial phenomenon. The current study therefore attempts to elucidate an influence of such changes on spontaneous imbibition by replacing surfactant concentration consecutively with two approaches: sequential decrease and sequential increase in the interfacial tension. Two fluid flow directions were examined simultaneously: co-current and counter-current flows. Dimensionless numbers were analyzed to emphasize the fluid displacement. With strongly oil-wet wettability (contact angle >= 123 degrees), capillarity was a resisting element to oil displacement and therefore controlled by the oilwater interfacial tension. The sequential-reduced interfacial tension was found to weaken such resisting capillary force gradually and resulted in consecutive incremental oil production. On the contrary, the sequential-increased interfacial tension initiated the lowest interfacial tension fluid that produced an immediate large amount of oil, but did not much displace further oil. The current study also observed a greater oil recovery obtained from a sequential reduction in the interfacial tension scheme (26.9%) compared to a conventional single reduction scheme (22.4%), with both schemes attaining same interfacial tension at last. Variation in pore-filling events was believed to attribute to such discrepancy since an inertia hindrance to oil displacement developed differently. In counter-current imbibition, same characteristics of oil displacement were observed as in co-current imbibition, with less oil produced (<= 17.6% ultimate recovery) and less sensitive to fluid changes due to negligible gravitational contribution. The results emphasized how the sequential-reduced interfacial tension exhibits a greater oil recovery by imbibition as analogy to secondary oil production by surfactant injection after water flooding, while increasing interfacial tension is likely attributed from surfactant retention could produce less oil.