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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHeavy 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.

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In case of a prolonged shutdown and an inopportune cooling, the crystallization of a crude oil takes place and hinders the production of these petroleum products. Because of the gel-like network formed by the crystals entrapping the oil, a restarting pressure is required to start-up flow. In order to predict the necessary pressure, the restart process and the corresponding mechanical behavior of the gelled oil must be assessed. In this paper, a set of two experimental approaches is presented and discussed in relationship with the restart process. The solid-like behavior of the gelled bulk oil was investigated thanks to a set of mechanical test cells. The rheological behavior of the gelled waxy crude oil was studied with a vane test apparatus and a controlled-stress rheometer. Eventually, the two experimental methods gave complementary and converging results revealing how a crude oil stuck inside a pipeline would react during a restarting operation. They help the understanding of the physical mechanism of the flow restart and release the data required for prediction.