Heavy oils are complex fluids and their flow properties are of primary importance to the assessment of their commercial value or to the design of production and transport facilities. Compounds such as asphaltenes and wax crystals, for instance, are known for their complex physical behaviors and interactions, and consequently they highly contribute to the macroscopic flow behavior of the crude oil. In this study, we investigate two particular aspects of the heavy oil flow behavior: the temperature dependence of the viscosity and the rheological and structural properties. The viscosity and viscoelasticity of a set of 13 different natural heavy oils from various origins (Asia and North, Central, and South America) are characterized over a wide range of temperatures. The zero-shear viscosity is measured from −40 °C to 200 °C, and the data are interpreted through the concept of glass transition, experimentally observed by differential scanning calorimetry (DSC) and fitted by the Williams− Landel−Ferry (WLF) model. The fragility of the different oils is found to be very similar throughout the sample set and the WLF constants are similar to the universal values observed in polymers. A detailed rheological characterization of the oils is also undertaken, under steady-shear experiments and dynamic oscillatory tests at temperatures from −50 °C to 50 °C. Independently of their zero-shear viscosities, the heavy oils have different rheological properties ranging from a Newtonian and purely viscous character to a weak gel-like behavior linked to some elastic internal structure. The viscoelasticity is quantified through the relaxation exponent n, which is then matched to some compositional features. For some oils, the viscoelastic character is linked to the presence of paraffinic wax crystals, the amount of which is quantified by DSC. For the other viscoelastic oils, the elastic character seems to be related to their high amount of asphaltenes: there is indeed a trend between the asphaltene content and the relaxation exponent, suggesting that the asphaltenes, when present in high quantities, are linked to the structural elastic properties, which lead to the macroscopic weak gel-like behavior.

When a reservoir is depleted, the lightest components (methane, ethane, etc.) can exsolve from the crude oil and create a gaseous phase. In conventional oils, the bubbles grow and coalesce quickly and the gas usually separates from the oil (slug flow). On the contrary, in heavy oils, bubbles are small, remain dispersed, and flow within the oil for a long time. This "foamy oil" phenomenon can drastically change the flow properties of the crude oil. This paper is devoted to the characterization of the heavy oil foaming behavior through a rheological study. Our objectives are to study the kinetics of bubble evolution in heavy oil and to measure the influence of the bubbles on the heavy oil viscosity. A new experimental method was developed on the basis of rheological measurements under pressure. Several heavy oils containing dissolved gas have been depleted inside the pressure cells of controlled stress rheometers to create foamy oils. Viscosity and viscoelastic properties have been continuously measured using respectively continuous and oscillatory tests from the nucleation to the disengagement of bubbles from oil. Results reveal that, under low shear rates, the presence of bubbles leads to an increase in heavy oil viscosity, as predicted by the hard sphere model or Taylor. A theoretical model describing the viscosity of foamy oil was then established. It takes into account both first-order kinetics of the appearance and release of bubbles in oil and a classic suspension model. Good agreement was obtained between experimental data and model predictions. Several tests reveal the strong influence of the shear rate on the foamy oil behavior and point out the major role of bubble deformation on the viscosity of foamy oils, as shown previously in other viscous materials, such as magmas and polymers. Under high shear rates, we suggest that the stabilization of the elongated bubbles in oil leads to the establishment of an anisotropic material, which can be seen as a sandwich-like structure. As a result, the viscosity appears lower in the direction of the flow.

Résumé -Systèmes dispersés dans les bruts lourds -La diminution des ressources de pétrole conventionnel conduit les producteurs à orienter la recherche vers l'exploitation de bruts difficiles. Parmi ceux-ci, les bruts lourds, caractérisés par leur forte viscosité, constituent des réserves de pétrole abondantes. Des procédés particuliers tels que la production froide ou l'injection de vapeur permettent de récupérer ces bruts malgré leur forte viscosité. Lors de la production froide, l'huile peut avoir l'aspect d'une mousse que l'on nomme « foamy oil » : sous l'effet de la dépressurisation, certains éléments légers comme le méthane ou l'éthane peuvent passer à l'état gazeux sous forme de bulles. L'injection de vapeur implique, quant à elle, une production importante d'eau émulsionnée avec le brut. Le but de cet article est d'étudier l'influence de ces phases dispersées (gaz ou eau) sur les propriétés d'écoulement du brut dans les conditions de production (pression, température). À l'aide d'un procédé novateur, des mousses de brut lourd et des émulsions ont été reconstituées et leurs propriétés d'écoulement étudiées à l'aide d'un rhéomètre équipé d'une cellule sous pression. La première partie de l'article concerne la présence de bulles dans le brut lourd. Nous montrons qu'une forte déformation des bulles peut mener à une diminution significative de la viscosité de la mousse. En revanche, lorsque les bulles sont sphériques, elles contribuent à une augmentation de la viscosité du mélange. Dans la deuxième partie, les émulsions eau-dans-huile sont étudiées. De la même façon, sous une vitesse de cisaillement suffisamment élevée pour déformer les gouttes, on observe une diminution de la viscosité apparente de l'émulsion. La dernière partie décrit quelques essais qui combinent l'influence de la présence de bulles et de gouttes d'eau sur les propriétés d'écoulement des bruts lourds, se rapprochant des conditions réelles de production. Nous étudions notamment le rôle de la présence de gouttes d'eau sur le comportement moussant du brut. Finalement, ce travail expérimental montre que la viscosité des fluides de gisements dépend fortement des conditions de production et qu'il est essentiel d'en étudier le comportement rhéologique en fonction de la température, de la pression et de la présence de phases dispersées. Abstract -Dispersed Systems in Heavy Crude

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