Molecular Simulations on the Colloidal Behaviors of Model Asphaltenes

  • Author / Creator
    Sun, Xiaoyu
  • Asphaltenes are the heaviest and most polar group of molecules in crude oil. Colloidal behaviors of asphaltenes, including the aggregation and interfacial behaviors, have impacts on many petroleum production processes. For example, the aggregation of asphaltenes can lead to problems such as clogging the pipelines and changing the wettability of oil reservoirs. Also, the interfacially adsorbed asphaltenes contributed to the stabilization of crude oil emulsions, which can cause severe corrosion problems. The colloidal behaviors of asphaltenes were under influence of the molecular structure of asphaltenes, the salinity in the aqueous phase, the solvent type, and the addition of demulsifiers.
    In this dissertation, molecular dynamics (MD) simulations were performed to provide mechanistic understandings of the colloidal behaviors of asphaltenes at atomic level, where the first two topics were on the aggregation behaviors and the last two topics were on the interfacial properties. Firstly, the relations between aggregation and intramolecular deformation of archipelago type molecules were studied. A single archipelago asphaltene molecule favored unfolded state in water, while folded state was preferred in molecules in aggregates. In heptane and toluene, a single archipelago asphaltene or multiple asphaltenes in aggregates were flexible and could easily change between folded and unfolded states. In the co-aggregates of continental and archipelago asphaltenes, the archipelago molecules became less flexible in heptane, where the change was insignificant in toluene. Secondly, the effect of salinity (NaCl concentration) on the aggregation of continental asphaltenes with different side chain length was elucidated. With long side chains, the model asphaltenes molecules formed aggregate mainly through the hydrophobic interactions at the side chains. Due to the effect of salt on the hydrophobic interaction, the aggregation of these model asphaltenes was enhanced at low salt concentration and then hindered at high salt concentration. For model asphaltenes with moderate side chain length, the interactions between core-core, core-chain, and chain-chain were under mutual influence of the salt concentration. The aggregation of model asphaltenes with the shortest side chain length was mainly driven by the core-core interactions, which were significantly affected by the salt concentration.
    Thirdly, the interfacial adsorption of asphaltenes at the water/oil interface with the presence of model demulsifiers (PEO-PPO-PEO copolymer and Brij-93) was studied. A fraction of archipelago model asphaltenes were adsorbed and formed loosely structured aggregates at the water/oil interface. The addition of Brij-93 resulted in the desorption of archipelago model asphaltenes at water/toluene or heptane interface, while adding PEO-PPO-PEO copolymer led to desorption of archipelago model asphaltenes at the water/toluene interface but not at the water/heptane interface. The continental model asphaltenes had more adsorption and formed more compact aggregates at the water/oil interface than archipelago model asphaltenes. Thus, co-aggregation between continental asphaltenes and model demulsifiers, instead of desorption of asphaltenes, was observed at both water/toluene and water/heptane interface. Lastly, with model asphaltenes or model demulsifiers adsorbed at the water/toluene interface, the free energy of interaction between water droplets was calculated. When water droplets were far apart (over twice the droplet diameter), the dispersed asphaltenes contributed to the stability of water droplets that their coalescence was energetically unfavorable. At close proximity and before the merging of water droplets, the steric repulsion between the adsorbate films induced a large repulsive force between water droplets, which prevented the coalescence. The steric repulsion also contributed to the redistribution of adsorbate molecules at the surface of water droplets, and the water droplets in the head-on direction were left with uncovered water molecules. Hydrogen bonds were formed between the uncovered water molecules, which gradually overcame the repulsive force and assisted the merging of water droplets. The copolymer had longer hydrophobic chains than Brij-93 and was more easily redistributed on the surface of water droplets, when the surface coverage on water droplets was high. Thus, the maximum repulsive force for water droplets coalescence was lower when copolymer was the adsorbate compared with Brij-93. Overall, the work in this dissertation provided fundamental understandings of the colloidal properties of asphaltenes, which can further help to solve the related problems in petroleum industries.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.