Rheological evidence of structural phase transitions in asphaltene-containing petroleum fluids

The aim of this paper is to provide data on the rheological/structural prop erties of "synthetic oils," composed of light hydrocarbons (toluene) and a heavy fraction, containing asphaltenes (vacuum residue, VR). Samples with a sphaltene concentrations 20-85 g/l have been studied at temperatures 0-60 d egreesC and shear rates up to 1500 s(-1). The non-Newtonian flow curves wer e approximated by the Bingham and the Herschel-Bulkley models to determine the apparent yield stress and the shear-rate exponent as functions of the a sphaltene concentration and the temperature. Sharp variations of these para meters were attributed to formation/destruction of extended ordered structu res in asphaltene colloid suspensions. Structural changes were observed in the temperature range 20-30 degreesC, particularly important for industrial processes of reservoir development and pipeline transportation. A molecula r model of the observed macroscopic effects takes into account possible fir st-order structural phase transitions in the nanometer-size resin/asphalten e colloid microparticles. (C) 2001 Elsevier Science B.V. All rights reserve d.