The stability of water-in-oil emulsions formed during oil slicks or petroleum production operations is ensured by natural surfactant molecules (principally asphaltenes) that are present in the crude oil. These persistent emulsions may be broken by adding a suitable demulsifier at the proper concentration to attain a so-called optimum formulation at which the stability of the emulsion is minimum. In this report, the concentration of asphaltenes is varied by diluting the crude oil with a solvent such as cyclohexane, toluene, or mixtures of them. The experimental evidence shows that there exists some critical asphaltene concentration at which the interfacial zone seems to be saturated. Beyond this threshold, which is typically around 1000 ppm of asphaltenes, the demulsifier concentration necessary to attain the emulsionʼs quickest breaking is constant. Below it, e.g. when the crude is highly diluted with a solvent, the optimum demulsifier concentration is found to be proportional to the asphaltene concentration. The map of emulsion stability versus asphaltene and demulsifier concentrations exhibits a typical pattern for different demulsifiers and diluents, which contributes to improving the interpretation of the demulsifying action.

Water-in-oil emulsions formed during oil slicks or petroleum production are known to be stabilized by surfactant molecules that naturally occur in the crude oil, e.g., asphaltenes, which are quite lipophilic in nature. Demulsifier substances combine with naturally occurring surfactants to attain a so-called optimum formulation at which the stability of the emulsion is minimum. The attainment of this formulation is related to the hydrophilicity and concentration of the added demulsifier, and a general phenomenology of the demulsification process is outlined.

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Static and dynamic tensiometries show that a newly prepared water/asphaltenated cyclohexane interface behaves as expected: the mean area occupied per asphaltene molecule is 2 nm2, and variations of interfacial tension and dilatational elastic modulus with time indicate that equilibrium is reached more slowly than that for usual surfactants. The use of the time/temperature superposition principle allows a detailed rheological study of a 2 day old interface of the same type which has reached equilibrium. It is found that the two-dimensional asphaltene network exhibits a glass transition zone, behaves as a gel near its gelation point, and is built by a universal process of aggregation.

The emulsion stabilizing properties of a low-total-acid-number (TAN) crude oil, which had initially been attributed to asphaltenes and calcite precipitation, were re-analyzed with regard to the role of organic acids. Despite high asphaltenes content, this crude oil exhibits features classically observed with acidic oils, such as the increase in emulsion stability upon pressure decrease/pH increase or the poor efficiency of demulsifiers. The potential for a significant role of organic acids was confirmed by the high interfacial activity of indigenous acids, as extracted from the crude oil by means of an ion-exchange resin. This was further addressed analyzing the molecular chemistry of the interfacial layer and its rheology. The interfacial material was found to be composed of a mixture of asphaltenes and organic acids. These acids exhibit a wide range of structures (mono- versus dicarboxylic, fatty versus naphthenic and benzoic) and molecular weights (from 200 to 700 g/mol), contrary to the medium molecular weight fatty monocarboxylic acids that are generally believed to cause “soap emulsions”. The interfacial rheology is indicative of a 2D gel, with an assumed glass transition temperature of approximately 40 °C. In conclusion, this study shows that a co-precipitation of asphaltenes and organic acids can promote the build up of a very cohesive interface. The disruption of this interface not only requires the drainage of individual molecules but also a collective yield of the gel. This paper is part one of two: it confronts physical and chemical data, the latter being further detailed in an associated paper.

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The interfacial material (IM) from four different crude oils with different capabilities to form stable water-in-oil (w/o) emulsion was extracted with the wet silica method and analyzed by different techniques. In the first of a series of papers, we report the use of gel permeation chromatography inductively coupled plasma high-resolution mass spectrometry (GPC ICP HR MS) to analyze the size distributions of sulfur-, vanadium-, and nickel-containing compounds present in the IM. The analysis of replicate samples demonstrated the reproducibility of the wet silica extraction method, and successive extractions of the same crude oil concentrated larger and more insoluble IM aggregates containing S, V, and Ni. The analysis of the IM from different crude oils revealed that there is a similar, selective adsorption of high-molecular-weight compounds containing Ni and V at the w/o interface. Conversely, the sulfur profiles for all of these IMs were unique, and given their widely varying ability to stabilize emulsions, it suggests that these species may play a role in the stability of water-in-crude oil emulsions.

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