A Study of Caustic Solution-Crude Oil Interfacial Tensions

Abstract

Abstract This paper presents the results of a study of caustic solution-crude oil interfacial tension measurements on 164 crude oils from 78 fields. Of these crude oils 131 showed marked surface activity against caustic solutions. Surface activity of crude oil against caustic solution correlates with the acid number, gravity, and viscosity. Almost all crude oils with gravities of 20 degrees API or lower produced a caustic solution-crude oil interfacial produced a caustic solution-crude oil interfacial tension less than 0.01 dyne/cm. Of the interfacially active samples, 90 percent reached maximum measurable surface activity at a caustic concentration of close to 0.1 percent by weight. The dissolved solids content of the water bas a marked influence on the surface activity. Sodium chloride in solution reduces the caustic concentration required to give maximum surface activity. Conversely, calcium chloride in solution suppresses surface activity. Introduction The oil-production technology literature contains a number of papers that indicate that the addition of sodium hydroxide to the flood water beneficially affects oil recovery. Although the proposed recovery mechanisms differ in detail, a variable common to almost all is the interfacial tension between caustic solutions and crude oil. A study of the factors influencing caustic solution-crude oil interfacial tensions is fundamental to an understanding of the proposed mechanisms and their optimum utilization. proposed mechanisms and their optimum utilization. We have obtained interfacial tensions against caustic solutions of 164 crudes. These crudes come from all major oil-producing areas in the free world. In addition to determining the correlation of interfacial tension with crude oil properties of acid number, gravity, and viscosity, we have also determined the effect of certain dissolved solids in the water. We define acid number as the number of milligrams of potassium hydroxide required to neutralize the acid in one gram of sample. The interfacial tension data were obtained by the pendent-drop method. pendent-drop method. EXPERIMENTAL PROCEDURE The caustic solutions used in this study were prepared by adding reagent-grade sodium hydroxide prepared by adding reagent-grade sodium hydroxide to laboratory distilled water. Our standard solutions were made from a 50 percent by weight reagent-grade sodium hydroxide solution. For convenience in relating our laboratory data to possible field application, the data were recorded and plotted in terms of weight percent sodium hydroxide. The pH of the caustic solutions was determined experimentally using a Coming expanded-scale pH meter; and the densities of the caustic solutions were measured experimentally using a Chainomatic Westphal balance. CRUDE OILS The crude oils were protected from the atmosphere and were collected in carefully cleaned glass or, when practicable, in plastic-lined containers. The crude oil samples were free of chemical additives, such as emulsion breakers and corrosion inhibitors. If the oil contained suspended solid material it was dehydrated and filtered. The densities were determined by the Westphal balance; and the viscosities were determined as a function of temperature using a glass capillary viscometer. APPARATUS AND EXPERIMENTAL PROCEDURE The interfacial tension measurements described in this study were made by the pendent-drop method. The pendent-drop method is based on the formation of a drop of liquid on a tip, the drop being slightly smaller than that which will spontaneously detach itself from the tip. The profile of this drop is magnified by projection and can be recorded on a photosensitive emulsion. The interfacial tension is photosensitive emulsion. The interfacial tension is calculated from the dimensions of the drop profile, a knowledge of be densities of the liquid forming. the drop, and the bulk phase surrounding the drop. All interfacial tensions described in this paper were recorded at a temperature of 74 degrees F and at an interface age of 10 seconds. Most systems were studied as a function of temperature; but temperature was found to be a second-order effect, so we selected 74 degrees F in order that all correlations would be at constant temperature. We selected 10 seconds because a study of the time variable showed that most of the decay of interfacial tension with time in these systems had occurred by the end of 10 seconds. SPEJ P. 197