The Yielding of Waxy Crude Oils

The yielding process of statically cooled waxy crude oil was examined in detail. Three direct measurements a controlled stress test, a creep−recovery test, and an oscillatory test were employed using a controlled stress rheometer to investigate the yielding of two distinctly different statically cooled waxy crude oils. The results showed that yielding of waxy crude oil occurs by an initial elastic response, followed by viscoelastic creep and a final fracture. A model with an elastic-limit yield stress, a static yield stress, and a dynamic yield stress was introduced to describe the yielding process. The three yield stresses were determined by means of different techniques. The effect of the time scale in different measurements on the three yield stresses was studied. It was shown from both the creep−recovery and oscillatory tests that the elastic-limit yield stress was independent of the time scale. The static yield stress was found to be dependent on the time scale in the three tests. The dynamic yield stress measured only with the controlled stress test was also found to be dependent on the time scale of the yielding. Good reproducibility can be obtained in these tests by strictly controlling the thermal and shear history of the samples.