The Stabilization of Water-in-Hydrocarbon Emulsions by Asphaltenes and Resins

Abstract

The role of asphaltenes and resins in stabilizing water-in-crude oil emulsions was investigated by measuring the interfacial composition and stability of model emulsions composed of water with mixtures of toluene, heptane, asphaltenes, resins, and native solids. The interfacial composition (mass surface coverage) was determined from a combination of emulsion surface area measurements and concentration measurements of both the continuous and the emulsion phases. The emulsion surface area was calculated from drop size distributions measured with optical microscopy. The concentrations were found from gravimetric analysis. The stability of the model emulsions was assessed from the amount of water resolved after heating and periodic centrifugation of the emulsions. Asphaltene surface coverage was found to increase with an increase in the asphaltene bulk concentration until a limiting surface coverage was achieved. Surprisingly, while asphaltenes always tend to stabilize these emulsions, the stability of the emulsions decreased as asphaltene surface coverage increased. This change in stability was attributed to a change in the asphaltene configuration on the interface. The addition of a good solvent was found to reduce both the amount of adsorbed asphaltenes and the emulsion stability. The addition of resins always destabilized model emulsions. It appears that resins act as a good solvent for the asphaltenes and, at sufficiently high concentrations, are able to replace asphaltenes on the interface. Naturally occurring solids that coprecipitate with asphaltenes had little or no effect on asphaltene adsorption but dramatically increased emulsion stability. The results suggest that the combination of asphaltenes and native solids causes the most stable emulsions.