Application of the Casson model to thixotropic waxy crude oil

The rheological behavior of a waxy crude oil was investigated using a coaxial cylinder viscometer. Experimental how curves were fitted with the Casson equation. The Casson model was modified to interpret the hysteresis between upward and downward curves obtained in a series of consecutive runs. At the same shear rate, the mean axial ratio of the flow unit related to a down-curve (down arrow) is smaller than that related to an equilibrium up-curve (up arrow). This results in a decrease of the Casson yield stress tau(c) down arrow with respect to tau(c) up arrow. A certain xi coefficient describing departure from the equilibrium mean axial ratio was introduced into the model. Values of xi calculated from the Casson yield stresses agreed satisfactorily with the theoretically predicted ones. Deviations from the Casson model at low shear rates were also explained.