Variation of gas/oil IFT during gas injection processes from immiscible to miscible conditions has been observed in lab and field experiences. The possible effects of variation of IFT on kr have been investigated by several researchers. Although there is some conflict with the findings of different researchers, in general, all agree that any reduction in IFT causes an increase in kr. It has been observed that as IFT decreases toward zero, the kr increases, its curvature reduces, and less hysteresis effect is observed. Also, most of the research studies have introduced a critical IFT value above which there are no significant effects of IFT variations on relative permeabilities while below the critical value IFT significant changes are observed in relative permeabilities. A summary of the literature review is presented in the following paragraphs.
Bardon and Longeron (1980) performed a series of coreflood (gas injection) experiments on Fontainebleau sandstone core using a binary mixture of two pure hydrocarbons (C1-nC7/C1-nC10). The IFT was controlled by varying the equilibrium pressure of the mixture. For the range of IFT from 0.001 to 12.6 mNm-1, they introduced a critical IFT value of 0.04 mNm-1. As shown in Figure 5-1, for the IFT values greater than the critical IFT, they obtained a single curve for gas relative permeability (krg) but a family of curves for oil relative permeability (kro) with kro increasing with decreasing IFT. However, for the IFT values less than the critical IFT, there is a great variation in the shapes of both kro and krg curves. The variation is mainly in the kro and krg curvature towards a straight-line as IFT decreases. i.e., both kro and krg increase, as IFT decreases.
Harbert (1983) performed coreflood experiments on outcrop and reservoir rock samples using an alcohol, brine, and oil fluid system to investigate the effect of low IFT on oil and water relative permeabilities. They found that IFT reduction had more pronounced effect on the non-wetting phase kr than on the wetting phase kr.
Fulcher (1985) conducted a series of steady-state oil/water kr measurements on fired Beria sandstone, to determine whether the capillary number or its constituents cause any changes in the two-phase relative permeabilities. They introduced a critical oil/water IFT value of 2 mNm-1, below which value both oil and water relative permeabilities increased with decreasing IFT, and the curves straightened out at very low IFT values. Moreover, the increase in oil (non-wetting) kr was observed to be more significant than the increase in water (wetting) kr as IFT reduced.