The Effect Of Caustic Concentration On Interfacial Charge, Interfacial Tension And Droplet Size: A Simple Test For Optimum Caustic Concentration For Crude Oils
Authors: Bansal V.K. , Chan K.S. , McCallough R. , Shah D.O. 
Published: March 2010

Abstract
From electrophoretic mobility arid interfacial tension measurements of crude oil/caustic systems, it was found that the minimum in the interfacial tension corresponds to the maximum electrophoretic mobility. The minimum in the interfacial tension for crude oil caustic systems was explained on the basis of Surface charge density at the crude oil/caustic interface. The maximum charge density (i.e. maximum electrophoretic mobility) at the crude oil/caustic interface corresponds to the minimum interfacial tension. A simple optical absorbance lest was developed to determine the optimum caustic concentration necessary to produce the minimum interfacial tension and the maximum electrophoretic mobility.

Introduction
The importance of alkaline flooding (caustic flooding) was recognized by Squires(l) as early as 1917. Nutting (2,3) under the title of "soda process", described the use of alkaline salts such as sodium carbonate and sodium silicate for improving the performance of the waterflooding process, although he dismissed the use of a stronger base such as sodium hydroxide. Since the first patent issued on caustic flooding in 1927(4), extensive research and field tests have been carried out on caustic flooding. Recently, Johnson(5) has reviewed the status of caustic and emulsion methods for oil recovery. Several mechanisms have beenproposed to explain the role of caustic in improving the oil recovery. Reisberg and Doscher(6) placed primary importance on lowering the interfacial tension, which will result in the formation of oil-in-water emulsion and the oil will be produced as an emulsion. They also felt that the ability of caustic to prevent adherence of oil to reservoir rock and suppression of semi-solid film formation at the oil-water interface played an important role in improving oil recovery by caustic flooding. Subkow(7) suggested that in-situ emulsification of the crude oil and its entrainment into alkaline water is responsible for the improved oil recovery. Ehrlich et al.(8) also pointed out that if oil-water interfacial tension is below a critical value, caustic flooding can improve oil recovery by the emulsification and entrainment mechanism. In 1959, Wagner and Leacht D1 suggested that reversing wettability of a porous material (from oil-wet to water-wet) by controlling the salinity and pH of the water phase will result in an increase in oil recovery. Mungan(30) in 1966 demonstrated that relative permeability of water decreased after caustic flooding because reversal from an oil-wet to a water-wet state produced a more favourable water-oil mobility ratio, which in turn improved oil recovery efficiency. Recently, Cooke er al-(11) reported that if the composition of the crude oil is favourable, some crude oils and porous media systems can be converted from a water-wet to an oil-wet state and oil/water interfacial tension can be simultaneously lowered to very low values by the right combination of caustic and salt. Recently, Jennings el al.(12) proposed that if the interfacial tension is low enough, residual oil in a preferentially water-wet system could be emulsified in situ and will move downstream with caustic, and these oil droplets could be entrapped again by very small pore throats.