Thermal Flow Properties of Heavy Oils

Abstract

Abstract A major flow assurance challenge in the near future is the production and transport of heavy oils: although these petroleum products represent very important reserves, their exploitation is limited by their high viscosity. The paper discusses the origin of this high viscosity in order to ultimately develop operating guidelines. It focuses on the relationship between the composition of heavy oils, in particular their asphaltenes and resins contents, and their flow properties. The structural analysis and rheological measurements that were performed revealed how strongly the transportability of heavy oils depends on thermal conditions, especially in deepwater ones. From this study, we conclude that the high viscosity of a heavy oil comes essentially from the overlapping of its asphaltenes. We observed that as temperature decreases, not only the viscosity increases but also the rheological behavior of heavy oils becomes non Newtonian. Dynamic tests carried out on several crudes show that at low temperatures, they develop a shear thinning behavior with no yield stress appearance. It was checked that this phenomenon comes from the particular structure of asphaltenes and that it is not due to the presence of waxy crystals. It is therefore recommended to take into account this non Newtonian behavior if heavy oils are to be transported in deepwater conditions. Introduction. A major flow assurance challenge in the near future is the production and transport of heavy oils. Despite very large reserves, their exploitation is limited by their high viscosity. Actually, the very low mobility of these petroleum products is making an as such pipeline transportation impossible. Advanced methods are necessary. They include upgrading, dilution, formation of oil-in-water emulsions and heating. These technologies imply high operational and investment costs so that optimized transport conditions have to be found. With this aim in view, a detailed study of the rheology of heavy oils was carried out. It deals with the link between the constitution and the flow properties of the crude, and lays emphasis on the role played by temperature. The understanding of the origin of the high viscosity of heavy oils will help to improve their methods of transportation, especially at low temperatures like in expected deepwater discoveries. The overlapping of asphaltenes inside a heavy oil and its consequence on viscosity. One main chemical characteristics of a heavy oil is its large content in asphaltenes. These molecules constitute a class of substances defined on the basis of their solubility in organic solvents: they are soluble in toluene but insoluble in alkanes such as n-pentane. Asphaltenes are the heaviest and most aromatic and polar fraction of a crude oil. These particular components are described as molecules composed of polycondensed aromatic rings carrying aliphatic chains that contain acid-base and polar groups at their edge. It is well recognized that thanks to these chemical characteristics, asphaltenes can self assemble through physical interactions and increase the viscosity of a medium in which they are added. Most of these studies were realized with simple organic solvents [Reerink and Lijzenga (1973), Sheu et al. (1991), Yudin et al. (1998), Acevedo et al. (1999).].