Reversibility of Asphaltene Aggregation As Revealed by Magnetic Resonance Imaging in Situ

Morozov, Evgeny V.; Martyanov, Oleg N. ENERGY & FUELS, 31 (10):10639-10647; 10.1021/acs.energyfuels.7b01755 OCT 2017

Aggregation of asphaltenes followed by precipitation presents severe problems for existing technologies in the production, recovery, and processing of heavy oils. Better understanding of asphaltene behavior behind the processes of their precipitation and dissolution is vital to address this issue. While investigating the inhomogeneity of different oil systems, the reversibility of the asphaltene aggregation process initiated by flocculant in either asphaltene solution in toluene or crude heavy oil was revealed and investigated using magnetic resonance imaging methods. It was found that the inhomogeneous distribution of the flocculant initiates local spatial-selective asphaltene aggregation registered in a thin layer around the flocculant/oil sample interface. The local excess of flocculant concentration over the threshold of asphaltene precipitation onset is a driving force of this process. As the flocculant diffuses into the volume of the sample, a decrease of the asphaltene flocculated area is observed until it disappears when the equilibrium composition throughout the whole volume of the system is achieved. Depending on the overall flocculant concentration, the asphaltene aggregation may not be reversible and could be followed by subsequent precipitation of the asphaltene aggregates. The similarity of the phenomena observed for the model asphaltene solutions and crude heavy oil samples was established. Partial mechanical stirring of the multicomponent system comprising flocculant and oil or asphaltene solution does not prevent the formation of the local zones with increased concentration of asphaltene aggregates; those sizes evolve depending on the flocculant concentration. The results obtained in this work are consistent with the generally accepted concept of asphaltene precipitation reversibility depending on the system composition and are compatible with the observations obtained by other methods. The approach presented can provide deeper insight into the asphaltene precipitation reversibility issue and can facilitate the understanding of asphaltene behavior in heavy oils.