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 flow 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 (↓) is smaller than that related to an equilibrium up-curve (↑). This results in a decrease of the Casson yield stress τc↓ with respect to τc↑. A certain ξ coefficient describing departure from the equilibrium mean axial ratio was introduced into the model. Values of ξ 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.