The effect of surface-active chemicals on oil-water interfacial tension (IFT) and wettability in crude oil-brine-rock systems at reservoir conditions is important in enhanced oil recovery processes. However, most of the experimental studies on IFT and contact angles have been conducted at ambient conditions and using stocktank crude oils. In this study, live and stocktank crude oils have been used at reservoir conditions to make IFT and dynamic contact angle measurements using the Drop Shape Analysis (DSA) and Dual-Drop-Dual-Crystal (DDDC) techniques, respectively. Yates reservoir rock and fluids and two types of surfactants (nonionic and anionic) in varying concentrations have been used at reservoir conditions of 82o F and 700 psi.

The dynamic oil-water IFT was found to be a strong function of oil composition, temperature and showed a slight dependence on pressure. An attempt has been made to explain the dynamic behavior of IFT using a four-stage mechanistic model involving induction, diffusion, kinetic barrier and equilibrium stages. The significant difference observed between the advancing contact angles of live oil (55 °) and stocktank oil (154°) clearly indicates the need to use live oils at reservoir conditions to determine in-situ reservoir wettability. Anionic surfactant altered the weakly water-wet behavior of live oil to strongly oil-wet (165°). It was also able to alter the strong oil-wet behavior of stocktank oil to less oil-wet (<135°). The nonionic surfactant was able to alter water-wet live oil system to intermediate-wet (82°), while it did not affect the strongly oil-wet behavior of stocktank oil system.

The oil-wet behavior observed with the live oil due to the surfactants used indicates the possibility of these surfactants to develop continuous oil-wet paths for potential mixed wettability development. Thus, this study is of practical significance where the surfactant-induced wettability alterations to either intermediate-wet or mixed-wet can result in improved oil recovery through lowering of both capillary and adhesion forces.

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