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The mechanism and ability to stabilize water-in-oil (w/o) emulsions have been compared for different emulsifiers, including asphaltenes extracted from a crude oil sample, hydrophobic silica particles, and a model acidic surfactant, N-(1-hexylheptyl)-N 0 -(5-carbonylicpentyl)perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe). The results indicated that C5Pe describes the behavior of asphaltenes relatively well, although the interfacial activity was substantially higher than for the real asphaltenes at high pH. The silica particles produced rather different results. The particles did not appear to desorb from the oilÀwater interface, leading to a timeindependent situation without any visible signs of coalescence. Additionally, the transition from unstable to stable emulsions happened at a "critical" particle concentration. For asphaltenes and C5Pe, the influence of other parameters has been investigated, such as solvent composition and pH. In both cases, the interfacial activity increased when a poor solvent was added, because the new oil phase made it more energetically favorable for the molecules to assemble at the interface. C5Pe became a better emulsion stabilizer at high pH, because of dissociation of the carboxylic acid groups and enhanced interfacial activity of the molecule. The asphaltenes had the lowest ability to stabilize around pH 6, while their ability increased for both higher and lower pH values because of ionization of acids and bases. Finally, the asphaltene concentration in the residual oil phase was measured by ultraviolet spectroscopy, to compare to results obtained by other authors. All of the results suggested that only a subfraction of the asphaltenes is responsible for emulsion stabilization.

The tetrameric acid ARN is present in crude oil and can form deposits that can cause plugging in oil production facilities during oil production in the presence of calcium ions. It has been previously shown that BP-10, a compound designed to mimic the properties of ARN, can form gel at the oil/water interface by reacting with Ca 2+ if the pH is high enough to ionize BP10s carboxylic acid functions. In this study, the BP-10 and ARN are compared. Although BP-10 forms a gel at the interface, ARN creates rather a viscoelastic material perhaps due to the presence of impurities like naphthenic acids in the sample analyzed. The amount of BP-10 required to form a gel at the interface was found to be ca. 0.45−0.76 mg·m −2 . The influence of the presence of asphaltenes on the gel formation has then been studied. It has been found that the gel is weakened in the presence of asphaltenes and G′ is not measurable at high enough concentrations. This inhibition could result from either a competition between ARN and asphaltenes to adsorb at interface or complexation of ARN and asphaltenes in the oil bulk phase. The influence of naphthenic acid (NA) on the gel formation was also studied by considering a commercial mixture of NA from Fluka. It is shown that NA can inhibit the gel formation of ARN or BP-10 at the oil/water interface most likely by acting as a cross-linking termination point agent. Finally, the effect of commercial calcium naphthenate inhibitors on gel strength has been studied. It is found that the inhibitors delay the formation of tetrameric acid gels and reduce their elastic modulus G′. However, results show that the order of inhibitor efficiency depends on the tetrameric acid used (ARN or BP-10). This could be linked to different specific inhibitor/TA interactions between ARN and BP-10. In conclusion, interfacial shear rheology appears to be a promising method to screen calcium naphthenate inhibitor.

The interfacial reaction between ARN tetrameric acid and Ca 2+ which leads to the formation of calcium naphthenate deposits was studied using a co-axial capillary device fitted to a profile analysis tensiometer. This device allows to continuously exchange the oil droplet subphase by a fresh ARN containing solution, thus mimicking the flow of oil as in an industrial separator.The effect of pH was first studied, and it was found there was a formation of a crosslinking structure by reaction between ARN and Ca 2+ at pH comprised between 7 and 8.Then, the mechanism of growth of the interfacial gel was studied by determining the influence, on the interfacial shear rheology properties, of the exchange time and the contraction of the volume of the droplet. Two mechanisms can explain interfacial gel growth: formation of multilayers due to the constant input flow of ARN and coalescence between droplets reducing the interfacial area.

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