VISCOSITY AND ADSORPTION STUDIES OF AUSTRALIAN CRUDE OILS IN THIN FILMS

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Abstract
Properties of crude oils in thin films have been studied with a surface force apparatus. Four crude oil samples from different Australian reservoirs were used. The forces between mica surfaces immersed in the oils were measured and the refractive index of the films determined. The viscosity of the oils was measured for film thicknesses in the range 10-400 nm. Asphaltenes and other high-molecular weight components adsorb to the surfaces, forming 2-5 nm thick layers which cannot be displaced even under high pressures. Dissolved water has a minimal effect on the measured interactions and does not remove the adsorbed layers. We suggest that the very long-range interactions measured in an earlier investigation [Christenson and Israelachvili, J. Colloid Interface Sci. 119.194 (1987)] are due to the heterogeneous adsorption of very large aggregates and are not a general feature of the surface forces in a crude oil system. The viscosity of the oils was found to be equal to or only slightly higher (< 20%) than the bulk values in all cases. The discrepancies found in the previous study are most likely due to evaporation of low-molecular weight components. The adsorption of asphaltenes to the surface shifts the plane of shear by an amount equal to the thickness of the adsorbed layers.

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