Trujillo, Edward M.; SPE; Marathon Oil Co.
Abstract
One method to achieve EOR is chemical alteration of the reservoir environment so that previously trapped oil cam 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 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.
Introduction
Several investigators have studied the reaction of caustic with crude oils. In one of the earliest publications, Reisberg and Doscher in 1956 measured IFT's between a California crude and various sodium hydroxide solutions by the pendant-drop method. The IFT was lowered by a factor of 1,000 with a 0.5% NaOH solution but increased at higher and lower concentrations. The pendant-drop ages were on the order of 5 seconds. They observed a change pendant-drop ages were on the order of 5 seconds. They observed a change in IFT with time, but no model for such a change was proposed. Jennings et al. determined a minimum IFT with a North American crude at about 0.1% NaOH, also with the pendant-drop technique. Several of their values were too low to be mea.sured (0.003 dyne/cm). Their data showed that only a small amount of calcium (25 ppm) increased the IFT between caustic and crude considerably. At 247 ppm calcium, sodium hydroxide was ineffective in reducing IFT at all concentrations up to 1%. Sodium chloride reduced the amount of caustic required to give maximum surface activity. All IFT measurements were made at 74F and at an interface age of 20 seconds. Jennings stated, "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." Measurements were made on 164 crudes from 78 fields. An attempt to relate the interfacial properties to crude acid number was not very successful. One article stated that the "interfacial tension must fall below about 0.01 dynes/cm if oil recovery is to show a significant increase due to caustic injection." Cooke et al. proposed that wettability alteration plus IFT reduction was a factor in oil recoveries with caustic. They suggested a less restrictive criterion in IFT for oil recovery, stating that "No combination in which the interfacial tension was greater than 2 dynes/cm was ever found to be successful in an alkaline water flood." They also confirmed that sodium chloride is beneficial but calcium is detrimental.
SPEJ
p. 645
