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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.

Published by the The Society of Rheology Related ArticlesEffects of dispersion and deformation histories on rheology of semidilute and concentrated suspensions of multiwalled carbon nanotubes J. Rheol. 57, 1491Rheol. 57, (2013 Rheology of sedimenting particle pastes J. Rheol. 57, 1237Rheol. 57, (2013 Microstructure in sheared non-Brownian concentrated suspensions J. Rheol. 57, 273 (2013) Ageing, yielding, and rheology of nanocrystalline cellulose suspensions J. Rheol. 57, 131 (2013) Wall slip and flow of concentrated hard-sphere colloidal suspensions SynopsisThe slump test, initially developed to determine the flow properties of fresh concrete, has been adopted as a means of accurately measuring the yield stress of strongly flocculated suspensions. The slump test offers a quick and easy way of measuring yield stress without the need for sophisticated electronic equipment, thereby giving plant operators an effective tool for determining yield stress. The model used to predict the yield stress from the conical slump test was devised by Murata ͑1984͒ and corrected by Christensen ͑1991͒. In the present case the theory has been adapted for a cylindrical geometry. Yield stress measurements obtained with the vane for numerous mineral suspensions under known surface chemistry conditions are compared to the slump measurements and theoretical prediction. Good agreement is obtained.

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