The rheological behavior of heavy oils is critical for oil exploitation in different stages, such as extraction, transportation, and refining; during this process, the oil undergoes temperature changes that directly affect the viscosity. For light oils, the viscosity decreases around 1 order of magnitude when the temperature increases 100 K, whereas for heavy oils, this change could be of more than 3 orders of magnitude for the same temperature increment. Furthermore, the heavy oils exhibit a viscoelastic behavior, usually characterized by a viscosity reduction with the increment in the shear rate, the presence of elasticity, and time-dependent rheological behavior. As the temperature increases, the oils acquire a Newtonian behavior. This change is illustrated with the rheological characterization of five heavy oils with an American Petroleum Institute (API) gravity around 12°, different compositions, and zero-shear viscosity that varies up to 2 orders of magnitude among oils. The measurements were carried out in a controlled stress rheometer using a 40 mm parallel plate geometry with a gap of 1 mm. The fluids were tested in rotational and oscillatory modes at temperatures from 5 to 100°C. In the oscillatory experiments, the loss and storage moduli, associated with viscosity and elasticity, respectively, are presented. Above a transition temperature, not only does the viscosity decrease but also the normal force and the elastic modulus tend to vanish, suggesting that the non-Newtonian behavior is also strongly related to the temperature. Furthermore, the departure from linearity of the logarithm of zero-shear viscosity versus the inverse of the temperature appears to correlate with the transition from non-Newtonian to Newtonian behavior.

In this work, the rheological behavior of heavy and light stabilized (dead) oils is presented. The viscosity measurements were carried out in two different apparatuses to ensure the repetitiveness and accuracy: a Stabinger viscometer and an Ares G2 strain-controlled rheometer. The samples were previously conditioned with an established methodology. For heavy oils, shear thinning was observed. The zero shear viscosity appears to correlate with the American Petroleum Institute (API) gravity for the case of light oils. However, for heavy oils, the viscosity definitively does not correlate with the API gravity. For example, the viscosity of fluids with an API gravity of around 12° may differ up to a full order of magnitude because of differences in the molecular structure. Saturate, aromatic, resin, and asphaltene analysis is also reported. Viscosity master curves were used to characterize oils. They were produced by applying the time–temperature superposition principle to the isothermal shear rate sweeps. The Williams, Landel, and Ferry equation was used to correlate the shift factor with the temperature. Rheological curves for all of the oils were quite reproduced with this method in all of the operational range.

The processing, production, and transport of heavy crude oils are big challenges for the petroleum industry. Central to this challenge is the fluid viscosity: the key variable responsible for the oil fluidity throughout the entire production process. From the reservoir to delivery conditions, oils undergo large variations in temperature and pressure, which may cause important phase behavior and physicochemical changes, directly affecting the fluid’s thermophysical properties. In the case of heavy oils, such a broad change of conditions categorically results in several orders of magnitude viscosity span, including the possible Newtonian to non-Newtonian rheological behavior transitions. It is, therefore, of primary importance that heavy oils be rheologically well-characterized to ensure their production process is successful and viable. The viscosities of heavy and extra-heavy crude oils in extreme conditions (high pressure, high to low temperature, high to low shear rate) are, however, difficult for most service laboratories to fully measure directly. Several lapses may occur when measuring the full range of required conditions using traditional rheometers; for example, for such viscous fluids, the development of laminar-flow structural anomalies (eddies) and magnetic decoupling in the high-pressure cell are common practical problems. In an attempt to pragmatically address these problems, in this work, a methodology that may allow for the rheological characterization of heavy and extra-heavy oils within the full field operational range, but based on limited laboratory measurements, is proposed. The proposed approach does not follow from a simple extrapolation but is rather derived from the concept of control-variable shifting. For achieving this, the superposition principle is applied to shear-temperature and shear-pressure reliable measurements to construct master curves to rheologically characterize the fluid within conditions that may be too severe for direct laboratory measurements. This methodology has been successfully applied to a database of 20 Mexican fluids, going from extra-heavy to light fluids. The rheologies of the samples were originally studied using three different types of equipment: (1) a strain-controlled rheometer (for the measurement of the fluid rheology at ambient pressure and different temperatures), (2) a sliding piston viscometer for high-pressure and low-shear-rate viscosity measurements, and (3) a hybrid rheometer coupled with a pressure cell for the estimation of the fluids rheological behavior under pressure and high shear rate. The rheological behavior of crude oils could then be obtained at conditions as severe as the equipment allowed (up to 1000 bar and, in some cases, shear rate up to 1000 s–1). The master curves allowed, however, to extend the rheological characterization of the fluids within conditions that were beyond the laboratory capabilities.

The presence of precipitated wax or even just low temperatures may induce non-Newtonian rheological behavior in crude oils. Such behavior can be found at operating conditions, for instance, in reservoirs at deep-water conditions. Therefore, reliable rheological models for crude oils applicable over the wide range of conditions the fluid may encounter are essential for a large number of oil technology applications. Such models must also be composition dependent, as many applications require predicting the rheological behavior of the fluid under strong compositional changes, e.g., recovery applications such as vapor extraction (VAPEX) processes or blending of fluids for improved rheological characteristics for piping, among many other applications. In this study, a comparative analysis between some published models applicable to the description of the non-Newtonian behavior of crude oils is carried out. Emphasis is placed on the stability of the model predictions within the wide range of conditions that may be encountered.

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