Abstract
An procedure is given for separating surfactant precursors that occur in some crude oils. The effect of the precursors on the properties of the oils are described also. The separations were made by silica gel chromatography on crude oil from which the asphaltenes had been removed. The effect of the precursors on the properties of the crude was evaluated by blending the surfactant precursors into the original oil, a modified oil, or a hydrocarbon solvent such as benzene. Precursors activated and converted to surface active materials by a strong base such as sodium hydroxide are effective in reducing the interfacial tension between the oil and aqueous phase. Occurrence of precursors in crude oils is essential for improved oil recovery by the causticflood process. The procedure for separating the precursors should provide a viable means for evaluating the applicability of a causticflood tertiary oil recovery process to a particular crude or reservoir.
Introduction
Tertiary oil recovery by the causticflood process is inherently dependent on naturally occurring surfactant precursors in the crude. The surfactant precursors react with the caustic (base) in the floodwater to form surface active compounds that reduce the interfacial tension between the crude and aqueous phase, alter the wettability of the mineral surfaces, or reduce rigid film formation at the crude/aqueous interface. In laboratory oil-recovery tests, these mechanisms stimulate oil production characterized by increased production at caustic breakthrough and a high oil/water ratio after breakthrough. An early effort to identify the surfactant precursors in a Rio Bravo (CA) crude concluded that the surfactant precursors were related closely to the asphaltene and resin fractions of the crude. Subsequent studies using an Eichlingen Niedersachen (West German) crude and a Ventura (CA) crude concluded that the surfactant precursors were acids and phenols, respectively. The extensive work of Seifert established that the surfactant precursors of a Ventura crude were carboxylic acids and that the phenolic components of the crude were interfacially inactive. The purpose of our study was to develop a simple and practical method of separating surfactant precursors from crude oil and to evaluate their effect on the interfacial tension, acid number, and other properties of the crude. The separation technique was developed using Smackover Nacatoch crude and the surfactant precursors evaluated were obtained from the same crude.
Description of Smackover Nacatoch Crude
The Smackover reservoir is located in southern Arkansas, and the Nacatoch pay zone is the shallowest of five pay zones. The crude has an API gravity of 21 degrees, a viscosity of 160 cp at room temperature, and is produced from an unconsolidated sand formation about 2,000 ft deep. Preliminary studies showed that the interfacial tension between the crude and an aqueous phase was reduced from about 12 to 0.02 dyne/cm when the pH of the aqueous phase was increased from 7.0 to 12.5 with sodium hydroxide. The significant reduction in interfacial tension at higher pH's indicated that the crude contained a relatively high concentration of surfactant precursors that were converted to surface active materials by sodium hydroxide.
SPEJ
P. 493^
