Interfacial Properties of Skins at the Crude Oil – Water Interface: Effect of Salt Concentration, Temperature and Surface Active Chemicals

Brine chemistry in waterflooding of hydrocarbon reservoirs has gained much attention recently. The composition of injected brine may affect significantly the oil recovery. The low salinity brines and small amount of surface active chemicals may result in high oil recovery. A number of recent studies have focused on determining the mechanisms involved; however full understanding is still lacking. Most authors advocate fluid-rock interactions. Formation of molecular structures at the fluid-fluid interface may be also an important mechanism. Crude oil-water interface shows a viscoelastic character from adsorption of organic compounds which may lead to formation of skins. We observe interfacial skins at low salinity at the crude oil-water interface. Some authors report formation of skins at the interface of the aqueous phase and the oil phase composed of asphaltenes dissolved in toluene.

In this work we present a systematic analysis of the interfacial viscoelasticity and interfacial tension of oil-water interfaces as a function of temperature, and small amount of pentanol added to the aqueous phase. The concentration of pentanol in the aqueous phase is 0.5 wt%. We vary the concentration of NaCl in our experiments.

Viscoelastic measurements are made using a shear rheometer with a DuNoüy ring at a small strain amplitude (0.1%) and frequency of oscillations (0.5 rad/sec). Interfacial tension measurements are made using a Krüss K12 processor tensiometer adapted with a duNoüy ring.

Interfacial viscoelasticity develops relatively fast in oil-water interfaces, and in all cases, stabilizes at 48 hours. At room temperature, the interface is found to be more elastic at low salt concentrations and become more viscous at higher salt concentrations. By increasing the temperature, the interfacial storage modulus (Gʹ) and loss modulus (G") both increase, while the elastic character behavior remains independent of temperature. In another set of experiments, when we add small amount of pentanol (0.5 wt%) to water, both moduli decrease and the interface becomes more viscous.

A skin is observed at the interface below 0.01 M NaCl while there is no interface skin at higher salt concentrations. We do observe the same with the alcohol at low salt concentrations. Interfacial tension measurements reveal a steep decrease at room temperature when the salt concentration is higher than 0.01 M NaCl. Interestingly, the effect of salt on interfacial tension is in line with interfacial viscoelasticity data.

Increase in interfacial viscoelasticity can be explained in terms of electrostatic interactions in the double electrical layer at the water-oil interface. The screening of the charge leads to adsorption of the polar components in the oil onto the interface. At high salt concentrations, the screening is decreased reducing the adsorption of polar components onto the interface. Our study of fluid-fluid interfaces may shed light on a key mechanism in improved oil recovery by optimizing brine chemistry of waterflooding performance.