Asphaltene Aggregation in Oil and Gas Mixtures: Insights from Molecular Simulation

Abstract

The dynamics of asphaltene molecules is highly impacted by both the nature of the solvent and the physical conditions of the system. We performed molecular simulation to investigate the dynamic behavior of asphaltene during gas flooding. We also consulted the experimental observations for validation purposes when available. Two structures representing the archipelago and continental types are used, whose aggregation and interactions are studied in methane (C1), propane (C3), carbon dioxide (CO2), heptane (C7), and toluene as pure solvents, binary mixtures of toluene and either C1, C3, or CO2, and a representative oil composition. The continental structure is used afterward to evaluate the impact of temperature, pressure, and resin content on the aggregation dynamics in CO2 mixtures. Interestingly, the solvating power of CO2 is dependent on the asphaltene structure where inhibitor-like behavior is observed for the continental structure and precipitator-like behavior is observed for the archipelago structure. The solvent quality is highly correlated with the solvent s ability to replace the interactions among asphaltene molecules with interactions between asphaltene and solvent. The aggregate size is reduced by temperature and enhanced by pressure in CO2. However, limited effect is reported for resins on asphaltene dynamics in CO2. The aggregation of asphaltene is impacted by the physical state of CO2 as its solvating power to asphaltene is significantly enhanced in its supercritical state. Nonetheless, this impact is limited when CO2 is introduced to a representative oil mixture.

New Molecular Insights into Aggregation of Pure and Mixed Asphaltenes in the Presence of n-Octylphenol Inhibitor

“…The intra- and intermolecular potentials are assigned to the atoms and molecules based on the selected force-field, and the coordination and velocity changes are calculated based on the Newton motion law. MD has been successfully employed to model asphaltene precipitation ,− and asphaltene deposition on calcite − and silica. , Also, MD is able to model the interfacial properties in the presence of different substances such as asphaltene, ,− asphaltene and resin, emulsifier, and demulsifier of water/oil emulsion . MD has been used to estimate the solubility parameter for asphaltenes in different solvents, , diffusion coefficients, ,,, and hydrate stability and dissociation. − Recently, the MD approach has been used to model asphaltene aggregation during enhanced oil recovery (EOR) processes such as water injection − and gas injection .…”

“…Researchers use different force-fields to simulate hydrocarbon interactions at various thermodynamic conditions while performing MD simulation runs. The common force-fields used for the hydrocarbon systems include polymer-consistent force-field (PCFF), , constant valence force-field (CVFF), ,, condensed-phase-optimized molecular potential for atomistic simulation studies (COMPASS) force-field, ,,,, assisted model building with energy refinement (AMBER) force-field, chemistry at Harvard macromolecular mechanics (CHARMM27) force-field, GROningen molecular simulation (GROMOS96) force-field, ,,,,,,,, and OPLS-AA force-field. ,,,,,,,,,− ,,,− , GROMOS and OPLS-AA are the most common force-fields while dealing with hydrocarbons. In 2011, Fu and Tian assessed a variety of molecular force-fields, including OPT-FF, AMBER 03, general AMBER force-field (GAFF), OPLS-AA, OPLS-CS, CHARMM27, GROMOS 53A5, and GROMOS 53A6 in prediction of the experimentally available thermodynamic properties of liquid benzene; the OPLS-AA was recommended as the best force-field.…”

“…In this study, we use the distance between the closest atoms and apply a cutoff threshold of 0.35 nm as a criterion for asphaltene aggregation. This particular cutoff was used previously for similar asphaltene structures but without hydrogen bonds; , this threshold value is applicable in our study because the range of hydrogen bond length is between 0.27 and 0.33 nm . The gmx clustsize module in GROMACS calculates the number-averaged aggregation number, g n (see eq ).…”

Catalogue of Plausible Molecular Models for the Molecular Dynamics of Asphaltenes and Resins Obtained from Quantitative Molecular Representation

“…These models and modified versions have been used in a number of simulation studies. 73,1974,75,76,77 So called "digital oils", including an in silico production of whole crudes 78,79 , of petroleum fractions 80 , and vacuum residue fractions 81 have also been produced in a similar fashion. A related procedure, labelled "structure-oriented lumping" 82 has been also extended to vacuum residue fractions.…”

Interfacial and Confinement-Mediated Organization of Gas Hydrates, Water, Organic Fluids, and Nanoparticles for the Utilization of Subsurface Energy and Geological Resources

“…With rising interest in storing CO 2 in depleted hydrocarbon reservoirs rich in heavy hydrocarbons, the influence of multiphase fluids in confinement on asphaltene assembly and resulting effects on porosity require further investigation. The thermodynamics and kinetics of asphaltene assembly in confinement differ from that of bulk fluids due to the effects of pore size and surface chemistry. ,,, The self-assembly of asphaltenes is influenced by asphaltene concentrations and solvent compositions, − the presence of injection agents such as water, air, and CO 2 , ,, and temperature and mechanical conditions. , Smaller asphaltene aggregates are observed in confinement compared to in bulk solvents due to adsorption on the interior surface of the confining pores . Further, pores with diameters larger than 5 nm showed higher adsorption capacity for asphaltenes. , Clays such as montmorillonite and kaolinite adsorb asphaltenes preferentially to other surfaces, making pores of mineral clays more prone to pore blockage due to the adsorption of asphaltene molecules. , The dissociation of aggregated asphaltenes is effective in the presence of silica and alumina nanoparticles due to the high adsorption capacity of asphaltenes on the nanoparticles’ surface. − However, the adsorption of asphaltenes on the surfaces of the nanoparticles changes their properties and performance in inhibiting asphaltene assembly.…”