Influence of Asphaltenes Content and Dilution on Heavy Oil Rheology

Abstract

Abstract The increasing oil demand is leading to the development of the very large world resources of heavy oils that are of the same order of magnitude than conventional oils. However, production and transport of these oils are challenging because of their very high viscosities. Our aim in this study was first to understand the role of asphaltenes in these high viscosities and secondly to evaluate the viscosity reduction efficiency when blending with different diluents. Many previous works studied asphaltene associations in simple organic solvents like toluene or toluene/heptane mixtures, but these conditions are not representative of heavy crude oils. Our approach has been to characterize the rheological behavior of asphaltenes in their natural environment. Samples coming from the same crude oil have been prepared with different asphaltenes contents, from 0 to 20%, after deasphaltating and recombination. From viscosity measurements two concentration regimes were identified: a diluted regime where the apparent viscosity increases linearly with the weight fraction of asphaltenes and a concentrated regime where the viscosity increases dramatically because of asphaltene particles entanglement. The diluted regime can be described as a solution of solvated asphaltene particles that are independent from one another. Natural heavy crude oils correspond to the concentrated regime and the high viscosities observed are due to this entanglement of solvated asphaltene particles. A strong influence of temperature on the rheology is revealed by measured high activation energies. Dilution of heavy oils has been studied using various solvents, in terms of aromaticity, chemical nature and viscosity. Dilution with low viscosity hydrocarbons, like light crude oil or naphtha, has shown that the viscosity reduction efficiency is controlled by the sole viscosity of the diluent and not by its aromaticity. The aromaticity has no effect up to very high dilution rate, more than 60% in our tests, where flocculation may begin. Results have also shown that blending diluents of different chemical natures and polarities can enhance viscosity reduction. From these observations, the colloidal description of heavy oils can be extended to diluted heavy oils, considered as solutions of asphaltene particles solvated in diluted maltenes. Introduction Heavy oils represent a strategic source of hydrocarbons as their reserves are of the same order of magnitude as the ones of conventional oils. The production of these crudes remains low, in particular because of their very high viscosities. It is worth noticing that new targets are now the production of oil below 20°API and more in the range of 8 to 15°API. With such crude oils are associated production problems that have never been met. On the subsurface point of view, such high density comes along with high viscosity, ranging from a few thousand to million of centiPoises at reservoir temperature. It means that oil is not flowing and needs a thermal or chemical activation. If thermal techniques for recovery have been screened over the past decade, other issues concerning surface facilities and transportation in long pipelines are remaining. Typical solutions are to heat the fluid, or to dilute it with a lighter oil or solvent [1–8ם. But such solutions can be expensive and may be detrimental to projects economics. There is then a urge to improve the existing solutions or to find new ones, in order to reduce or by-pass the viscosity of the heavy oil to be transported. The study presented here has the objective to better appraise the origin of high viscosities of the natural heavy crude oils, in order to explore solutions for transportation. In these regards, chemical and physical properties of heavy oils were examined, on various samples.