Dep’t of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada

The stability of water-in-crude oil emulsions is frequently attributed to a rigid asphaltene film on the water/oil interface. While it has been shown that asphaltenes initially adsorb as a monolayer at the interface, this relatively thin monolayer is insufficient to explain the long-term stability of many oilfield emulsions. It is suspected that the interfacial film has visco-elastic properties that strengthen with time. This work focuses on the elasticity of the interfacial film. The interfacial tension, elastic, and viscous moduli were measured using a drop shape analyzer. The elastic and viscous moduli were determined for model oils consisting of asphaltenes dissolved in toluene for concentrations varying from 0.002 to 20 kg/m³. The effects of asphaltene concentration, interface aging time, and frequency of oscillations were examined.

The films exhibited time dependent visco-elastic behavior. The total modulus increased as the interface aged at all asphaltene concentrations. An attempt was made to model the interfacial rheology for the full range of asphaltene concentrations. The instantaneous elasticity was modeled with a Surface Equation of State (SEOS) and the elastic and viscous moduli with the Lucassen-van den Tempel (LVDT) model. It was found that only the early time data could be modeled using the SEOS-LVDT approach; that is, the instantaneous, elastic and viscous moduli of interfaces aged for ten minutes. At longer interface aging times, the SEOS-LVDT approach was invalid, likely due to irreversible adsorption of asphaltenes on the interface and the formation of a network structure.