Rheological and Structural Properties of Heavy Crude Oils in Relation With Their Asphaltenes Content

Abstract

Abstract 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. The asphaltenes which they contain are known to be responsible for this situation. Actually, many studies have shown that these high molecular weight polar components self-associate more or less severely depending on different parameters like temperature, concentration and solvent quality. The experimental work was usually performed using simple organic solvents (toluene, heptane…) which are not representative of complex heavy crude oils. Therefore, we decided to investigate the rheological behavior of asphaltenes in their natural environment and in relation with their structure. Samples coming from the same crude oil were prepared with different asphaltenes contents, from 0 to 20% in weight. The apparent viscosity and the oscillatory modulus (G' and G") were measured with a controlled stress rheometer and SAXS were undertaken. Two domains were identified. The first one concerns the dilute samples for which the relative viscosity increases linearly with the weight fraction of asphaltenes. In this domain, the aggregates of asphaltenes stay independent from one another and have the same radius of gyration. For the more concentrated samples, the viscosity increases dramatically because of the aggregates entanglement as was observed by SAXS. Concerning the oscillatory experiments, the elastic character increases from the first domain to the second one, which confirms that a structural change occurs. Combining the rheological measurements and SAXS reveals an analogy between heavy crude oils and concentrated colloidal systems of polymer solutions. All these results help the understanding of the flow properties of heavy crude oils and aim to contribute to the improvement of their transport. Introduction When looking for new reserves, oil companies are shifting from conventional to non-conventional reserves. Among these new reserves, heavy crude oils are a hot topics due to large quantity of oil in place (ranging from 1000 to 1800 billion barrels according to authors). If subsurface production remains a technical issue, options are currently limited for surface production and it can be required to have an upgrader on-site to improve crude oil quality to a commercial grade. This costly solution, with investment as high as 1 billion dollars, should be compared to transporting heavy crude oils to existing refineries. Yet the most employed method is to dilute the crude with a lighter oil, that results in high associated operating costs. It is of prime importace to further our knowledge on heavy crude oils to be able to suggest more efficient and less costly solutions. Therefore, rheological and structural experiments were performed on these petroleum products. They both focused on the role plaid by the asphaltenes that are the highest molecular weight and polar components of heavy crude oils. The results obtained with these two techniques are successively presented in this paper. We show how their combining helps the understanding of the flow properties of heavy crude oils and aims to contribute to the improvement of their transport.