Heavy Oil Rheology as a Function of Asphaltene and Resin Content and Temperature

Abstract

Abstract Because of very important reserves that are of the same order of magnitudethan conventional oils, heavy oils represent a strategic source ofhydrocarbons. The major difficulty for producing and transporting these crudescomes from their very high viscosities. The success of exploitation of thesepetroleum products requires new treatments to improve their transport. Such atarget implies to better understand the relationship between the composition ofheavy oils, in particular in terms of asphaltenes and resins content and theirflow properties. Influence of asphaltenes content in the crude has beenparticularly studied in our lab : the experimental work revealed the existenceof a critical concentration C* above which the asphaltenes particlesoverlapp. This structural change dramatically increases the viscosity andintensifies the elastic character of heavy crude oils. A further series ofexperiments has focused on the influence of resins: they show that in diluteregime (C<C*), resins increase the viscosity of the crude and inconcentrated regime (C>C*), which corresponds to the case of aheavy oil, they lower the asphaltenes effect on viscosity. Experiments werefirst carried out with asphaltenes and nonylphenol as a model of resins andfollowed by experimental work on real crude that confirmed the trends. Tounderstand the rheological behavior of a heavy crude oil in relation with itsinternal structure, asphaltenes and resins have to be simultaneouslyconsidered. From this study we conclude that the origin of the high viscositiesof heavy oils comes from to the entanglement of solvated asphaltene particlesstabilized by resin molecules. A strong influence of temperature on therheology is also revealed by high activation energy measurements. This thermaldependent rheological behavior shows that the internal organization of theheavy crude oil is partly based on thermal dependent physical bonds. Using thereversibility of these physical links, we should be able to modify thestructure of heavy crude oils to facilitate their transport. Introduction The increasing oil demand is leading to the development of the very largeworld resources of heavy oils that are of the same order of magnitude thanconventional oils. However, production and transport of these oils arechallenging because of their very high viscosities. Reduction of the viscosityof heavy and extra heavy oils is thus necessary to facilitate their productionand their transport. Usual methods of heavy oils transportation include thecrude upgrading, the addition of diluents, the formation of oil-in-wateremulsions and thermal techniques [1–3]. These technologies imply highoperational and investment costs so that less expensive solutions have to befound. It is with this objective that the study presented here was undertaken.It deals with the relationship between the composition of heavy oils and theirflow properties, and specially the role played by the asphaltenes and theresins, the most polar and highest molecular weight components. To determinehow these petroleum macromolecules co-exist and are involved in the viscosityof heavy oils, a rheological investigation has been realized on different setsof samples. The understanding of the structure formed by the association ofasphaltenes and resins inside a crude will help to appraise the origin of highviscosities and to explore new methods of transportation.