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.