| Both experimental and theoretical studies are conducted in order to determine the physicochemical mechanisms responsible for lowering of interfacial tension in alkali/acidic oil systems with and without an added surfactant. A well-defined model acidic oil is selected for controlled design of experiments, thus enhancing verification of known and unknown mechanisms. Ionic strength, pH, alkalinity, acid concentration and type, and surfactant concentration and type are investigated for their effect on interfacial tension. The newly developed theoretical model predicts interfacial tension both qualitatively and quantitatively well. Interfacial tension goes through a minimum with pH at constant ionic strength. The ultralow value of interfacial tension is shown to result from the simultaneous adsorption of ionized and unionized acid on the interface, and the lowering results from the formation of mixed micelles of ionized and unionized acid. A complicated acidic crude oil also follows these mechanisms.;Since the unionized acid acts in harmony with the ionized acid, it is advantageous to control the extent of ionization through the pH. This control can be accomplished through the use of inexpensive mixed alkalis, such as carbonates which can buffer in the required intermediate alkaline pH range at concentrations that can survive transport through the reservoir. The minimum in interfacial tension and the region for spontaneous emulsification correspond to a particular pH range, so that by buffering the aqueous pH against changes in alkali concentration, low interfacial tension can be maintained when the amount of alkali decreases due to the influence of external factors such as divalent ions, acids, rock and dispersion. The addition of a surfactant aids in maintaining ultralow equilibrium interfacial tension. A new "pH-gradient" concept for floodwater design is also proposed.;The mechanism for spontaneous emulsification is observed to be interfacial turbulence. The system is characterized by roll cell size and roll cell formation time as a function of pH, ionic strength, and added surfactant. Interfacial turbulence is found to be a necessary but not sufficient condition for spontaneous emulsification. Ultralow interfacial tension is also a necessary condition. |
