Role of Asphaltene in Stability of Water-in-Oil Model Emulsions: The Effects of Oil Composition and Size of the Aggregates and Droplets

Abstract

Water-in-oil (W/O) emulsions are the most common type of emulsions handled in petroleum processes. It is thought that the field emulsions are primarily stabilized by asphaltene–resin micelles, and several research studies have examined the stability mechanisms of asphaltene in crude emulsions. However, there are still plenty of research gaps and unanswered questions in this area due to the complexity of the problem and difficulty of access and crude emulsion processing. These challenges can be addressed by investigating the effect of asphaltene on model emulsions’ kinetic stability. This study introduces a model W/O emulsion prepared by a new stabilizer (gilsonite) that contains asphaltenes and resins in combination with a non-ionic surfactant (Span 83). The colloidal characterization of the asphaltene aggregates in the mineral oil and oil blend of toluene and mineral oil was carried out. The size of the asphaltene aggregates in the mineral oil was found to increase with the added gilsonite concentration because of asphaltene precipitation and the process of smaller aggregates clumping together, forming flocs. Gilsonite was also found to precipitate and stabilize the W/O emulsions with the mineral oil as the external phase where the asphaltene precipitation was the most severe. The emulsions’ least kinetic stability was measured when toluene was added in the oil blend (50% volume fraction), where 100% water phase separation was observed with 0.25% gilsonite concentration. However, the dilute emulsion (10% water content in the emulsion) samples with 25% toluene revealed higher stability in terms of water phase separation than the case with 12.5% toluene with 20.83% less water separation in a 3 day storage period. This observation contradicts the expected outcome in a thermodynamic perspective where the W/O emulsion stability is thought to be merely dependent on asphaltene precipitation. The dilute emulsion with 12.5% toluene contained asphaltene aggregates larger than the emulsion droplets, which cannot contribute to the stability process. The ratio of the mean aggregate size to the mean droplet size was 133% larger for the dilute emulsion with a smaller fraction of toluene in the oil blend for this case. Therefore, the aggregates’ size to droplet size misalignment resulted in less stability for this emulsion than for the emulsion with higher aromaticity of the oil blend despite the very high precipitation rate. This paper presents observations of the effects of the asphaltene precipitation rate, size of the aggregates, and size distribution of the emulsion droplets on the model W/O emulsions’ stability. The significance of the results is in revealing the importance of integrating the thermodynamical and colloidal viewpoints to describe the role of asphaltene in stabilizing emulsions. This approach leads to the conclusion that besides asphaltene precipitation, the aggregate size distribution in relation to the size of the emulsion droplets is a critical factor in stabilizing the emulsions. Results presented in this study can be used in the design of solvents in enhanced oil recovery, producing model emulsions, replicating oil reservoir in situ emulsion features, synthesis of demulsifiers for the emulsions stabilized with asphaltene–resin micelles, and other industrial applications. Additionally, gilsonite was introduced as a new additive that can be used to study the role of asphaltene in stabilizing model emulsions.

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