Surfactant (in situ)-Surfactant (Synthetic) Interaction in Na^sub 2^CO3/Surfactant/Acidic Oil Systems for Enhanced Oil Recovery: Its Contribution to Dynamic Interfacial Tension Behavior

The effect of synthetic surfactant molecular structure on the dynamic interfacial tension (DIFT) behavior in Na^sub 2^CO3/surfactant/crude oil was investigated. Three surfactants, a nonionic (iC^sub 17^(EO)^sub 13^), an alcohol propoxy sulfate (C^sub 14-15^(PO)^sub 8^SO^sub 4^), and sodium dodecyl sulfate (SDS) were considered in this study. Sodium tripolyphosphate (STPP) was added to ensure complete compatibility between brine and Na^sub 2^CO3. In Na^sub 2^CO3/iC^sub 17^(EO)^sub 13^/oil and Na^sub 2^CO3/C^sub 14-15^(PO)^sub 8^SO^sub 4^/oil systems, a strong synergistic effect for lowering the dynamic interfacial tension was observed, in which the dynamic IFT are initially reduced to ultralow transient minima in the range 1.1 × 10^sup -3^-6.6 × 10^sup -3^ mNm^sup -1^ followed by an increment to a practically similar equilibrium value of 0.22 mNm^sup -1^ independent of Na^sub 2^CO3 concentration (for iC^sub 17^(EO)^sub 13^) and to decreasing equilibrium values with increasing alkali concentrations (for C^sub 14-15^(PO)^sub 8^SO^sub 4^). The observed difference in the equilibrium IFT for the two systems suggest that in both systems, the mixed interfacial film is efficient in reducing the dynamic interfacial tension to ultralow transient minima (~10^sup -3^ mNm^sup -1^) but the mixed film soap-iC^sub 17^(EO)^sub 13^ is much less efficient than the mixed film soap-C^sub 14-15^(PO)^sub 8^SO^sub 4^ in resisting soap diffusion from the interface to the bulk phases. In both systems, the synergism was attributed, in part, to the intermolecular and intramolecular ion-dipole interactions between the soap molecules and the synthetic surfactant as well as to some shielding effect of the electrostatic repulsion between the carboxylate groups by the nearby ethylene oxide (13 EO) and propylene oxide (8 PO) groups in the mixed interfacial monolayer. SDS surfactant showed a much lower synergism relative to iC^sub 17^(EO)^sub 13^ and C^sub 14-15^(PO)^sub 8^SO^sub 4^, probably due to the absence of ion-dipole interactions and shielding effect in the mixed interfacial layer at the oil-water interface.