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 countercurrent 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.
The transfer of fluid by some combination of capillary imbibition and gravity-driven flow is significant in a variety of oil recovery processes. In fractured reservoirs, for example, capillary imbibition has long been regarded as a primary mechanism for oil recovery during waterflooding. The high IFT associated with oil/water systems induces a correspondingly high capillary pressure which, in turn, provides the driving force for spontaneous imbibition into matrix blocks. Capillary crossflow during waterfloods in heterogeneous reservoirs is the basis for the vertical equilibrium assumption sometimes used to calculate pseudorelative permeability functions . Some combination of capillary and gravity-driven flow undoubtedly operates in enhanced oil recovery (EOR) processes in heterogeneous reservoirs, though scaling of those contributions is largely uninvestigated. In EOR processes such as miscible or surfactant/polymer flooding, for instance, IFT's vary with the composition of the equilibrium phases generated during the flow process, as will density difference between phases. Thus, in such processes the transfer process' will be more complex because the fluid properties that drive the flow will change during the displacement.