The effect of reduced interfacial tension (IFT) on oil recovery by spontaneous imbibition and gravity segregation is investigated experimentally for a wide range of IFT’s in cores oriented vertically. The experimental results presented indicate that a crossover from capillary to gravity driven imbibition occurs. For cores with low permeability and high IFT between the wetting and nonwetting phases, capillarity initiates coun-tercurrent imbibition and oil is produced from all faces. For moderately low IFT’s and permeabilities above about 100 md, gravity segregation is the controlling mechanism. In this case, the rate of oil recovery may actually be greater for low IFT’s. Gravity induced imbibition is cocurrent (oil produced from the top face only), which helps prevent snap-off and entrapment and allows high total recoveries. In addition, data are presented that indicate that surface tension gradients arising from composition gradients can substantially increase or decrease the rate of imbibition depending on the direction of the surface tension gradient.

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