Elasticity of a Model Oil/Water Interface

Introduction Water-in-crude oil emulsions are often created during the recovery, treatment, and transportation of crude oil. Some oilfield emulsions are difficult to break into separate phases using conventional methods. In order to develop more effective treatments for these emulsions, it is necessary to determine the properties of the interfacial film that contributes to their stability. In many cases and, in particular with heavy oils, this film is believed to consist of surface-active materials such as resins, asphaltenes, and native solids. This work focuses on asphaltenes and their role in the formation of interfacial films and emulsion stability. It has been shown that asphaltenes can adsorb as a monolayer at the water-oil interface(1). When the monolayer is present, emulsion stability is significantly enhanced. One possible explanation for the strong stabilizing effect of the monolayer is that it is highly elastic and therefore provides resistance to coalescence. To test this idea, the interfacial rheology of asphaltene films on hydrocarbon/water interfaces is measured using drop shape analysis of an oscillating hydrocarbon droplet in an aqueous medium. The hydrocarbon phase consists of asphaltenes dissolved in mixtures of toluene and heptane. The aqueous phase is distilled water. Elasticity is measured as a function of the frequency of the oscillations, the asphaltene concentration, and the solvent composition. Emulsion stability is assessed for model emulsions prepared from the same phases that were tested in the elasticity experiments. A correlation between elasticity and emulsion stability is sought. Experimental Methods Asphaltenes were precipitated from Athabasca bitumen with n-heptane in a 40:1 solvent to bitumen (cm3 /g) ratio. The mixture was sonicated for 45 minutes, then left to stand for a period of 24 hours. The supernatant was filtered through Whatman #2 filter paper and the filtrate was further diluted to a 4:1 n-heptane to bitumen cm 3 /g) ratio. This mixture was sonicated for 45 minutes, left to stand overnight, and poured through the filter paper. The asphaltene filter cake was allowed to dry until the mass was invariant. The yield of asphaltenes was 15.0%. Precipitated asphaltenes contain non-asphaltenic solids (NAS) such as sand and clay. These solids can introduce error into interfacial tension and elasticity measurements. The solids were removed as follows: an asphaltene sample was dissolved in toluene in a 100:1 toluene:asphaltene (cm3/g) ratio. The mixture was sonicated for 20–40 minutes to ensure complete asphaltene dissolution. The mixture was allowed to stand for one hour, after which it was centrifuged at 4000 rpm (1640 RCF) for six minutes. The supernatant was decanted, and the solvent evaporated until only dry asphaltenes remained. NAS made up 3.1% of the asphaltenes. All emulsion and elasticity experiments were performed with asphaltenes free of NAS. Model emulsions were prepared with asphaltenes, toluene, heptane and water. A known mass of asphaltene was dissolved in toluene and then heptane was added to make up mixtures ranging from 0 to 50 vol% heptane. Water was added to the hydrocarbon phase while the mixture was homogenized with a C A T - 5 2 0 D homogenizer for five minutes.