This paper presents the results of an experimental study performed to investigate the effect of reduced interfacial tension (IFT) on oil recovery by spontaneous imbibition and gravity segregation. The results of the static imbibition experiments, in which a matrix block was surrounded by the imbibing fluid, showed that a minimum Bond number (NB-1) was required to increase the final oil recovery. However, the reduction of the IFT may increase or decrease the rate of imbibition substantially depending on the relative contribution of capillary and gravity forces. The results of the experiments performed on three ranges of permeabilities with four different surfactant solutions (different IFT values) showed that imbibition can occur in three different regimes: capillary dominated, gravity dominated, and a regime in which both forces affect the imbibition process. In order to examine the effect of IFT on oil recovery on the same core, core wettability has to be restored after each test. Several cleaning procedures were investigated to reproduce original core wettability. As an outgrowth of this research, a core cleaning technique for restoring core wettability has been suggested. Firing the core after toluene cleaning for seven hours at 950 F was found to be an effective procedure to restore core wettability after solvent extraction. The same procedure also proved effective for restoring the original core wettability after surfactant solution imbibition tests. Imbibition has been recognized as an important factor in recovering oil from water-wet fractured reservoirs subjected to waterflood or water drive. Water imbibition is the process by which water flows into porous media by capillary forces. These forces allow the water to flow from the fractures into the water-wet, oil-saturated matrix blocks, which causes the oil to be displaced from the matrix into the fracture. Unfortunately, water imbibition is inefficient in recovering oil, often leaving more than half of the original oil-in-place unrecovered. A static imbibition by surfactant solution imbibition process recovers residual oil by reducing the surface forces between the oil and the imbibing fluid. On the other hand, by reducing the interfacial tension, the capillary forces that cause imbibition are reduced. Objective: The objective of this static imbibition work is to investigate the effect of reducing IFT in different oil/water/rock systems on final oil recovery. Methods of Investigation: Sets of experiments were performed by using a specially designed static imbibition apparatus. Each experiment consisted of exposing an oil-saturated Berea sandstone core to water imbibition (base case), cleaning and saturating the core with the same oil, and exposing it to surfactant solution imbibition. Oil recovery by water and surfactant solutions imbibition from each core are compared. P. 107

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