Dynamic Asphaltene-Stearic Acid Competition at the Oil–Water Interface

Abstract

Interfacial tension (IFT) is one of the major parameters which govern the fluid flow in oil production and recovery. This paper investigates the interfacial activity of different natural surfactants found in crude oil. The main objective was to better understand the competition between carboxylic acids and asphaltenes on toluene/water interfaces. Dynamic IFT was measured for water-in-oil pendant drops contrary to most studies using oil-in-water drops. Stearic acid (SA) was used as model compound for surface-active carboxylic acids in crude. The influence of concentration of these species on dynamic IFT between model oil and deionized water was examined. The acid concentrations were of realistic values (total acid number 0.1 to 2 mg KOH/g oil) while asphaltene concentrations were low and set between 10 and 100 ppm. In mixtures, the initial surface pressure was entirely determined by the SA content while asphaltenes showed a slow initial diffusion to the interface followed by increased adsorption at longer times. The final surface pressure was higher for asphaltenes compared to SA, but for binaries, the final surface pressure was always lower than the sum of the individuals. At high SA concentration, surface pressures of mixtures were dominated entirely by the SA, although, Langmuir isotherm analysis shows that asphaltenes bind to the interface 200-250 times stronger than SA. The surface area/molecule for both SA and asphaltenes were found to be larger than the values reported in recent literature. Various approaches to dynamic surface adsorption were tested, showing that apparent diffusivity of asphaltenes is very low, in agreement with other works. Hence, the adsorption is apparently under barrier control. A possible hypothesis is that at the initial phase of the experiment and at lower concentration of asphaltenes, the interface is occupied by stearic acid molecules forming a dense layer of hydrocarbon chains that may repel the asphaltenes.

Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface

“…In addition, it is apparent that the ability to predict the effect of water chemistry on oil recovery reservoir requires not only knowledge of the interfacial chemistry of inorganic ions, but also the range of surface-active species present in the crude oil; here, we have only examined the effects of naphthenic acids. For real crude oils, however, the increased complexity and competition resulting from the presence of other interfacially active species, such as asphaltenes and resin components, would be expected to produce a different interfacial behavior [104]. This has been borne out in studies on crude oil fractions to be reported elsewhere.…”

Model Asphaltenes Adsorbed onto Methyl- and COOH-Terminated SAMs on Gold

“…The adsorption and positioning of natural asphaltene molecules at interfaces have been debated from having large aromatic cores parallel to interfaces to molecules positioned perpendicular to the interface. , Some of this apparent discrepancy is mainly due to the large geochemical variability in composition, molecular structure, and the observation that the molecular nature of adsorbed asphaltenes is different from the asphaltenes in bulk . Much interpretation has been based on interfacial tension analysis using the Gibbs–Langmuir approach estimating an average occupied area/molecule and the analogy with simple molecules .…”

Molecular Transport across Oil–Brine Interfaces Impacts Interfacial Tension: Time-Effects in Buoyant and Pendant Drop Measurements

“…In the case of interfacial phenomena that are of relevance to the upstream oil and gas sector, the two phases involved are a complex aqueous solution of electrolytes and what is arguably one of the most complex mixture of organic species ever imagined, namely, crude oil. − There are primarily two processes that can come into play to impact the tension at the interface: first, diffusion of electrolytes in the brine to the interface with eventual adhesion, and, second, diffusion of naturally occurring surfactants from the bulk of the oil toward the interface. Here, they will interact with the electrolytes and perhaps even react. − The most interfacially active components of crude oil are carboxylic acids and basic compounds of various sortsone example is pyridine derivatives. For carboxylic acids, the interaction with the electrolytes of the brine could lead to deprotonation and an interfacial environment that is significantly impacted .…”

“…The pH is kept above 8 in Brandal’s studies to ensure that a sufficient fraction of carboxylic acids is being deprotonated for stronger chelation to the divalent metal ions . Apart from this reactive approach to explaining the time dependence, Andersen et al have found that the IFT decreases as a function of time in a system where the focus is on asphaltene impact. The setup in this case was a water droplet surrounded by a bulk oil phase, whereas it was the other way around in the studies conducted by Brandal et al, where the setup was that of an oil droplet submerged in a bulk brine phase. ,, For diffusive effects, the relative order of appearance may actually be quite important because it will matter what is in absolute excess, electrolyte or surfactant.…”