1 PREPRINT of the paper published in: Journal of Petroleum Science and Engineering, Vol. 30. Iss.3-4, pp. 199–211 (2001) Rheological evidence of structural phase transitions in asphaltene- containing petroleum fluids I.N. Evdokimov, N.Yu. Eliseev, D.Yu. Eliseev Department of Physics, Gubkin Russian State UniХersity of Oil and Gas, Leninsky Prospekt, 65, Moscow B-296, GSP-1, 119991, Russia Abstract The aim of this paper is to provide data on the rheological/structural properties of “synthetic oils”, composed of light hydrocarbons (toluene) and a heavy fraction, containing asphaltenes (vacuum residue, VR). Samples with asphaltene concentrations 20–85 g/l have been studied at temperatures 0–60 o C and shear rates up to 1500 1/s. The non-Newtonian flow curves were approximated by the Bingham and the Herschel–Bulkley models to determine the apparent yield stress and the shear-rate exponent as functions of the asphaltene concentration and the temperature. Sharp variations of these parameters were attributed to formation/destruction of extended ordered structures in asphaltene colloid suspensions. Structural changes were observed in the temperature range 20–30 o C, particularly important for industrial processes of reservoir development and pipeline transportation. A molecular model of the observed macroscopic effects takes into account possible first-order structural phase transitions in the nanometer-size resin/asphaltene colloid microparticles. 1. Introduction Rheological parameters of petroleum fluids are very important for all processes where these fluids are transferred from one place to another. For example, description of petroleum migration in the source/reservoir rocks requires the viscosity of the migrating fluids (Werner et al., 1996). Petroleum viscosity is important in technological applications, such as the production processes of hydrocarbon reservoirs and the processes of petroleum refining. In the refining units, rheological properties of crude oils determine the head losses and, thus, the pressure within a unit. Fluid mechanics problems in the heavy petroleum industry frequently involve nonlinear rheological behaviour (Werner et al., 1998a,b; Pedersen and Rønningsen, 2000). Despite the demand for oil viscosity/rheology data as a function of composition and external parameters, only a limited number are available, particularly, for very heavy fluids, rich in resins and asphaltenes. Most authors focus on the effect of only one parameter, usually either pressure or temperature, neglecting the effect of the others (Al-Besharah et al., 1987; Amin and Beg, 1993). It has been repeatedly stated that studying viscosities of fluids with heavy oil components requires close monitoring of the phase behaviour of the mixture (Werner et al., 1996). Such studies are important also because flow behaviour (rheology) of these fluids is enormously sensitive to structural transformations far too subtle to be detectable by other experimental techniques. Rheological characterisation often becomes the method of last resort, as have been demonstrated, e.g. in studies of polymer-containing fluids (Hussein and Williams, 1999; Janzen and Colby, 1999). Hence, rheometers/viscosimeters may be regarded as probes not only of the macroscopic flow parameters, but also of the subtle molecular-structure features. Shear-induced structural evolution of nonlinear fluids is an extensively studied subject. Polymeric liquids typically shear-thin, branched polymers may exhibit shear thickening (Janzen and Colby, 1999). Colloidal suspensions in clays usually shear-thicken (van Kessel and Blom, 1998; Abend and Lagaly, 2000), solutions of surfactants can shear-thicken or shear-thin. In some suspensions, the flow-induced structural transformations are dramatic and have many features in common with equilibrium phase transitions (Annable et al., 1993; Wolthers et al., 1997; Mellema