| Asphaltenes are the heaviest and most polarizable fractions of crude oils. As a result, they have a high tendency to aggregate and flocculate under certain conditions. Asphaltene precipitation and subsequent deposition in production tubings and topside facilities present significant cost penalties to crude oil production. Therefore, it is highly desirable to predict their phase behavior and the efficiency of dispersants in preventing or delaying deposition. Several thermodynamic models have been developed for the description of asphaltene precipitation from petroleum fluids. However, the lack of reliable and coherent data on the energy of association and aggregation size of asphaltenes has imposed severe limitations on these models. Indeed, these models either consider asphaltenes as non-associating components or use fitting parameters to characterize the association. This research was undertaken to address this issue and the main objectives are three-fold: 1) to understand asphaltene aggregation mechanism and the effect of resins on their stability using impedance analysis, 2) to estimate association parameters for asphaltenes and the effect of dispersants on their aggregation behavior using Molecular Dynamic (MD) simulations, and 3) to predict asphaltene precipitation in the presence of dispersants using a novel PC-SAFT thermodynamic model. In the first part of this study, asphaltene charge carriers were used as tracers to measure asphaltene association into nanoaggregates and clusters in organic solvents. Impedance analysis through DC conductivity measurements was validated with centrifugation experiments on the same samples. Our data suggest that resins are unlikely to coat asphaltene nanoaggregates within the concentration range investigated. Thus, the long time-standing Nellensteyn hypothetical model where resins adsorb on asphaltenes to provide a steric stabilizing layer is not valid. In the second part, MD simulations confirmed that the interactions between aromatic cores of asphaltene molecules are the major driving force for association and are enhanced with the presence of heteroatoms. Moreover, the length and number of aliphatic chains do not have a noticeable effect on asphaltene dimerization, however they have a profound effect on asphaltene aggregation size since steric repulsion can prevent asphaltenes from forming T-shape configurations. The mechanism of nonionic dispersant - asphaltene association includes H-bonding, aromatic, electrostatic, and Van 2 der Waals interactions. While nonionic dispersants can reduce the size of asphaltene aggregates; ionic dispersants generate larger but more stable aggregates. Finally in the third part, we were able to determine molecular association parameters by combining MD to PC-SAFT equation of state, in order to predict the macroscopic phase behavior of asphaltenes and the effects of nonionic dispersants of known structure on asphaltene precipitation. |
