The Static and Dynamic Interfacial Tensions Between Crude Oils and Caustic Solutions

Abstract

Abstract One method to achieve EOR is chemical alteration of the reservoir environment so that previously trapped oil can begin to flow freely. Under certain conditions, caustic or alkaline solutions can do this. The work reported here shows that interfacial tension (IFT) between various crudes and caustics increases with time because of desorption of the surface-active species from the interface. The desorption rate is temperature-dependent. Four kinds of crude oil were used—a California crude, a Wyoming crude, an Illinois crude, and an Alaska crude. Only with crude oils with a high concentration of crude acids, such as the California crude, is the ultralow IFT maintained for any reasonable period of time, namely 24 hours. The presence of calcium ions at concentrations of 200 ppm or more destroys the capability of caustic to reduce the IFT's, even for the California crude. Mass-action relationships are presented that describe the equilibrium IFT at constant ionic strength between crude oils and sodium hydroxide solutions as a function of pH and calcium. Techniques are presented for evaluating time-dependent IFT's obtained by the spinning-drop apparatus. A transient mathematical model shows that IFT can increase by several orders of magnitude over a period of several days. Good agreement between the model and experimental data is obtained. The parameters obtained from these mathematical models describe crude reactivity to caustic more accurately than conventional crude acid numbers. The transient effects observed in the laboratory may or may not be significant in the field.