  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. The
current study 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. With strongly oil-wet wettability, capillarity was a
resisting element to oil displacement and therefore controlled by the oil-water
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 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. In counter-current imbibition, same
characteristics of oil displacement were observed as in co-current imbibition,
with less oil produced and less sensitive to fluid changes due to negligible
gravitational contribution.
