Effect of pH on Interfacial Films and Stability of Crude Oil-Water Emulsions

Abstract

Oilfield emulsions are stabilized primarily by film-forming asphaltenes and resins containing organic acids and bases. Adding inorganic acids and bases radically changes the physical properties of these films and their emulsion-stabilizing properties. Interfacial films show one of three different physical characteristics when retracted in pendent drop tests. These compressed films have rigid, soap-type or minimum film properties depending on the quantity of asphaltenes and resins in the crude oil and on the pH and composition of the brine. There exists for each crude oil-brine system an optimum pH range over which the adsorbed film exhibits minimum contracted film properties. In this range, interfacial tension is high - frequently near its maximum value - indicating the absence of highly surface-active species; crude oil-brine emulsions generally show minimum stability: and surfactant requirements for breaking these emulsions are significantly reduced - sometimes no surfactant is needed. Introduction: About two-thirds of the world's crude oil is produced as emulsions that must be treated (demulsified) to be marketed. Approximately three-fourths of all domestic crude oil produced must be treated. The percentage of crude requiring treatment increases as fields become older and more water is produced. Demulsification most frequently is attained through use of surfactants, heat and electric treaters. It is estimated that over 75 percent of all oilfield emulsions are broken by chemicals generally containing alkylene oxide condensates. Although acids and bases are known to affect emulsion stability, they rarely are used in the oil field except in batch treatment of the most severe emulsions. Nevertheless, pH adjustment by using acids or bases in demulsification has some advantages over other methods. Mainly, it is cheaper. Acids and bases frequently are cheaper to use than surfactants, or can appreciably reduce the amount of surfactant required for demulsification. Secondary advantages are that less oil is lost by emulsification into the aqueous phase, and that less corrosion is encountered when the optimum pH is basic. Limitations to the use of pH includeuse in brines containing high concentrations of reactive ions,closer supervision for pH control than is required for surfactants andneed for corrosion protection when the optimum pH is sufficiently acidic. These factors indicate that the economics of pH control in demulsification will be maximized when relatively large treating facilities are needed, when relatively large surfactant concentrations are required to break an emulsion, when a relatively low concentration of the aqueous phase is emulsified in the oil and when the emulsified aqueous phase contains relatively low concentrations of reactive ions. Because there are no broad guide lines for using pH control in breaking oilfield emulsions (and because the limited guides available are highly specific and frequently contradictory), this paper will attempt to clarify the principles involved in using acids and bases to break emulsions. These principles also should be useful in refinery separations and in other oilfield applications where stable or unstable petroleum emulsions are involved. Principles of Emulsion Stability: Oilfield emulsions generally occur as water droplets dispersed in oil (w/o) and occasionally as oil droplets in water (o/w). These emulsion droplets are usually in the 0.1- to 10-micron range. However, more coarsely dispersed water droplets frequently are present in the emulsion. Pure hydrocarbons and water will not form stable emulsions except in extreme dilutions where the large distance between droplets inhibits coalescence. An emulsion breaks because interfacial tension acts to minimize the surface area of the dispersed drops by coalescence. However, emulsions can be stabilized for long periods of time by the presence of a third phase that adsorbs at the oil-water interface and inhibits coalescence of the dispersed phase. Emulsions generally are stabilized byrepulsive charges on the surfaces of the dispersed phase andadsorbed films that, being preferentially wetted by the continuous phase, act as a physical barrier to inhibit contact between the dispersed droplets. Both these factors generally are important in stabilizing and in breaking o/w emulsions. However, repulsive charges are less effective in stabilizing w/o emulsions which generally are stabilized by adsorbed films. For this reason the nature and importance of these films at oil-water interfaces were examined closely; as will be shown, pH changes were found to affect the films significantly, and this work led to laboratory tests of pH adjustment in emulsion breaking. JPT P. 303ˆ