Modeling the Thixotropic Behavior of Waxy Crude

Waxy crude shows viscoelastic, yielding, and thixotropic behaviors below the gelation point. In this paper, to depict those complex rheological behaviors a new thixotropic model is proposed. In the model, the total shear stress is considered to be a combination of an elastic stress and a viscous stress. The elastic stress, assumed to be structure- and strain-dependent, is described by a nonlinear damping function together with a structural parameter. The evolution of structural parameter is described by a new kinetic equation, whose breakdown term is assumed to be dependent on energy dissipation rate rather than on shear rate. The proposed model is validated respectively by the test of stepwise increases in shear rate and the hysteresis loop test for waxy crude and further validated by predicting the transient flow curve of hysteresis loop test with the model parameters obtained from the test of stepwise increases in shear rate. The results showed that the model's capability of fitting experimental data and prediction is satisfactory, and the model nicely predicts the transition from an elastically dominated region to a viscous dominated region without a discontinuity in the stress-strain curve.