Asphaltene Interactions Research Articles

Evidence of molecular complex formation between asphaltene and o-chloranil in aqueous micellar medium: A spectrophotometric study

The spectral (both absorption and steady-state fluorescence) studies of interaction of asphaltene with o-chloranil have been carried out in aqueous micellar media of cationic cetyl trimethyl ammonium bromide (CTAB), and neutral triton X-100 (TX-100). Spectroscopic results indicate formation of a complex between asphaltene and o-chloranil. The fluorescence band positions of the complexes are identical in both the medium. The absorption spectrum of the complex has been detected by difference spectral method. The position of maximum absorption of the complex in these micellar media have been compared and found in the order of SDS>CTAB>TX-100. The formation constant (K) has been determined using Benesi–Hildebrand equation utilizing absorption spectral data. Formation of ground state complex involving asphaltene and o-chloranil has also been observed from the study of quenching of asphaltene fluorescence by o-chloranil. The value of K, as obtained by Stern–Volmer plot, agrees well with that obtained by absorption spectrophotometric method.

Nuclear Magnetic Resonance Dispersion of Distributions as a Probe of Aggregation in Crude Oils

We present a new technique—nuclear magnetic resonance dispersion of distributions (NMRDD)—and we demonstrate that it is a particularly sensitive probe of aggregation in asphaltene-rich crude oils. The technique is based on the measurement of the NMR relaxation distributions at different magnetic fields, i.e., at different proton Larmor frequencies. Because the NMR relaxation rates are controlled by molecular fluctuations at the Larmor frequency, these measurements probe the presence of aggregates that tumble at characteristic rates near that frequency. Our experimental results span the Larmor frequencies from 10 kHz to 40 MHz, which is the range relevant for rotation times of asphaltene and resin aggregates in crude oil. The presence of large aggregates is evident by visual inspection of the data, and aggregate sizes for small and medium size aggregates of the order of several nanometers can be extracted without recourse to any specific models of asphaltene interactions. The technique is noninvasive and nondestructive and can be used for monitoring and study of the onset of aggregation at the preflocculation stage in unstable crude oils.

Multi-scale Simulation and Experimental Studies of Asphaltene Aggregation and Deposition in Capillary Flow

Asphaltenes are known as the “cholesterol” of crude oil. They form nanoaggregates, precipitate, adhere to surfaces, block rock pores, and may alter the wetting characteristics of mineral surfaces within the reservoir, hindering oil recovery efficiency. Despite a significant research effort, the structure, aggregation, and deposition of asphaltenes under flowing conditions remain poorly understood. For this reason, we have investigated asphaltenes, their aggregation, and their deposition in capillary flow using multi-scale simulations and experiments. At the colloid scale, we use a hybrid simulation approach. For the solvent, we used the stochastic rotation dynamics (also known as multi-particle collision dynamics) simulation method, which provides both hydrodynamics and Brownian motion. This is coupled to a coarse-grained molecular dynamics (MD) approach for the asphaltene colloids. The colloids interact through a screened Coulomb potential with varying well depth ε. Here, we present results for deposition of asphaltenes in their whole oil in comparison to our previous results on extracted asphaltenes. Imposing a constant pressure drop over the capillary length, we observe that the transient solvent flow rate decreases with an increasing well depth ε. We find that the dimensionless conductivity measured in the experiment can be well-matched by simulation results using an interaction potential well depth of 8kBT. This means that extracted asphaltenes are more sticky than asphaltenes in whole oil. The interactions between the mesoscopic asphaltene colloids can be related to atomistic MD simulations. Molecular structures for the atomistic calculations were obtained using the quantitative molecular representation approach. Using these structures, we calculate the potential of mean force (PMF) between pairs of asphaltene molecules in an explicit solvent. We obtain a reasonable fit using a −1/r2 attraction for the attractive tail of the PMF at intermediate distances. We speculate that this is due to the two-dimensional nature of the asphaltene molecules. Finally, we discuss how we can relate this interaction to the mesoscopic colloid aggregate interaction. We assume that the colloidal aggregates consist of nanoaggregates. Taking into account observed solvent entrainment effects, we deduct the presence of lubrication layers between the nanoaggregates, which leads to a significant screening of the direct asphaltene−asphaltene interactions.

Waxy Gels with Asphaltenes 1: Characterization of Precipitation, Gelation, Yield Stress, and Morphology

We examine the effects of asphaltenes upon the crystallization behavior of a model waxy oil. Yield stress measurements on the model waxy oils with asphaltenes isolated from Shengli crude oil showed that both the relative amount of wax to asphaltenes and the aggregation state of the asphaltenes affected the crystallization properties of the wax. At very low asphaltene concentrations and high wax concentrations, the yield stress of the waxy gel is not significantly affected. At higher asphaltene concentrations, the asphaltenes significantly degraded the microscopic structure of the wax network and drastically reduced the yield stress. There is a threshold ratio of ∼100 paraffin/asphaltene molecules for such behavior. Asphaltenes produced large decreases in yield stress when they were highly aggregated. Oscillatory testing showed that in such cases asphaltene−asphaltene interactions contributed to the gel strength, in addition to the wax platelet interactions. Asphaltenes increased the wax precipitation temp...

Polarization of Fluorescence of Asphaltene Containing Systems

Formation of supermolecular structures in petroleum disperse systems is determined by interactions of asphaltenes. Petroleum systems are lyophilic oleocolloids with low polar dispersive media which is in dynamic balance with elements of disperse structure. Supermolecular scale of organization is most important for determining the macroscopic parameters. Levshin-Perrin equation for depolarization of fluorescence was modified for polydispersed systems. Interfacial tension coefficient of model asphaltene solution was calculated in a case of Volmer's function. For technogenic mixtures the temperature dependences of the sizes near the point of phase transition are presented. Enthalpy of asphaltene association per one molecule was calculated. Polarization degree together with macroscopic parameters correlates with appearance of asphaltene dispersed phase in system.

Theoretical modeling of molecular interactions of iron with asphaltenes from heavy crude oil

The interaction of metallic iron with a model asphaltene molecule in the presence of water and electron-donor additives was modeled by using a quantum mechanics parametric method, CATIVIC. Results of Fe–asphaltene interactions show the formation of bonds on aromatic rings and directly on heteroatoms and so a decrease of C C, N C, and S C bond energies (bond activations) and the formation of metal–asphaltene complexes. Values of diatomic energies (DE), equilibrium bond distances (EBD), and diatomic bond energies (DBE) show that the most significant bonds arise when the interaction is directly on heteroatoms (N, S) and the highest activation is in the C S bonds. Electronic charge transfer occurs from the metal to hydrocarbon, except when interaction is on the N atom. Important metal O bond is observed when water interacts with Fe–asphaltene complex, leading to some activation of O H bonds. Negative charge on the system, in the presence of H 2O, will decrease Fe O and Fe N bonds and will reinforce Fe S ones. Comparison with previous work shows that, in general, hydrogenation and carbon–heteroatom bond activations due to metal interaction in the presence of water follow the trend: nickel > iron.

Interaction of Asphaltenes with Nonylphenol by Microcalorimetry

This paper shows the work performed in the study of the capability of isothermal titration calorimetry (ITC) to characterize the interaction between petroleum asphaltenes with a model molecule, namely, nonylphenol. ITC is widely used in biochemistry to study the interaction of proteins with ligands. The intention is to transfer the knowledge into the asphaltene field, with the aim of getting a better understanding of the mechanism of interaction, as well as the energies involved in this process. Calorimetric experiments show that nonylphenol has a complex mechanism of interaction with asphaltenes in toluene, including more than one process. Several models have been used to fit the experimental data. The enthalpies calculated with a model based on polymerization are in the order of -1 to -7 kJ/mol, which are very close to the hydrogen bond energies. This shows the capability of ITC to provide experimental data to the modeling of asphaltene behavior. The number of sites of interaction has been inferred by means of a model taken from protein-ligand science. The values obtained are in the range two to five sites per molecule, assuming an average Mw of 1000 units.

Aggregation and solubility behavior of asphaltenes and their subfractions

Asphaltenes from four different crude oils (Arab Heavy, B6, Canadon Seco, and Hondo) were fractionated in mixtures of heptane and toluene and analyzed chemically, by vapor pressure osmometry (VPO), and by small angle neutron scattering (SANS). Solubility profiles of the asphaltenes and their subfractions indicated strong cooperative asphaltene interactions of a particular subfraction that is polar and hydrogen bonding. This subfraction had lower H/C ratios and modestly higher N, V, Ni, and Fe contents than the less polar and more soluble subfraction of asphaltenes. VPO and SANS studies indicated that the less soluble subfractions formed aggregates that were considerably larger than the more soluble subfractions. In general, asphaltene aggregate size increased with decreasing solvent aromaticity up to the solubility limit, beyond which the aggregate size decreased with heptane addition. The presence of a low wavevector Q feature in the scattering curves at 25 °C indicated that the individual aggregates were flocculating; however, the intensity of the feature was diminished upon heating of the samples to 80 °C. The solubility mechanism for Canadon Seco asphaltenes, the largest aggregate formers, appears to be dominated by aromatic π-bonding interactions due to their low H/C ratio and low nitrogen content. B6 and Hondo asphaltenes formed similar-sized aggregates in heptol and the solubility mechanism is most likely driven by polar interactions due to their relatively high H/C ratios and high nitrogen contents. Arab Heavy, the least polar asphaltene, had a H/C ratio similar to Canadon Seco but formed the smallest aggregates in heptol. The enhancement in polar and π-bonding interactions for the less soluble subfraction indicated by elemental analysis is reflected by the aggregate size from SANS. The less soluble asphaltenes contribute the majority of species responsible for aggregation and likely cause many petroleum production problems such as pipeline deposition and water-in-oil emulsion stabilization.

Asphaltene Aggregation under Vacuum at Different Temperatures by Molecular Dynamics

Asphaltene aggregation under vacuum at different temperatures was obtained using classical molecular dynamics (MD) simulations under nonperiodic boundary conditions in a monodisperse system of 96 hypothetical asphaltene molecules. Identical asphaltenes were originally set as an array, where the separation between each other was ∼40 A. Simulations under the canonical ensemble at NVT conditions, using the Verlet numerical method to solve the motion equations, were conducted. Aggregated systems formed by several asphaltene monomers after 100 ps of classical MD simulations were found. The structure of the solution was analyzed using the radial distribution function. Simulations at four different temperatures (273, 312, 342, and 368 K) were accomplished. Another similar MD simulation at a temperature of 310 K for 300 ps was performed, to validate the stability in the previous systems. After this run, good structures for explaining asphaltene interactions were also observed; these structures were never before p...

Effects of petroleum resins on asphaltene aggregation and water-in-oil emulsion formation

Abstract Asphaltenes from four crude oils were fractionated by precipitation in mixtures of heptane and toluene. Solubility profiles generated in the presence of resins (1:1 mass ratio) indicated the onset of asphaltene precipitation occurred at lower toluene volume fractions (0.1–0.2) than without resins. Small-angle neutron scattering (SANS) was performed on solutions of asphaltene fractions in mixtures of heptane and toluene with added resins to determine aggregate sizes. Water-in-oil emulsions of asphaltene–resin solutions were prepared and separated by a centrifuge method to determine the vol.% water resolved. In general, the addition of resins to asphaltenes reduced the aggregate size by disrupting the π–π and polar bonding interactions between asphaltene monomers. Interaction of resins with asphaltenic aggregates rendered the aggregates less interfacially active and thus reduced emulsion stability. The smallest aggregate sizes observed and the weakest emulsion stability at high resin to asphaltene (R/A) ratios presumably corresponded to asphaltenic monomers or small oligomers strongly interacting with resin molecules. It was often observed that, in the absence of resins, the more polar or higher molecular weight asphaltenes were insoluble in solutions of heptane and toluene. The addition of resins dissolved these insolubles and aggregate size by SANS increased until the solubility limit was reached. This corresponded approximately to the point of maximum emulsion stability. Asphaltene chemistry plays a vital role in dictating emulsion stability. The most polar species typically required significantly higher resin concentrations to disrupt asphaltene interactions and completely destabilize emulsions. Aggregation and film formation are likely driven by polar heteroatom interactions, such as hydrogen bonding, which allow asphaltenes to absorb, consolidate, and form cohesive films at the oil–water interface.

Interfacial and colloidal behavior of asphaltenes obtained from Brazilian crude oils

In this work, we present new data on the interfacial and colloidal behavior of asphaltenes in model aromatic solvents and crude oils and discuss the implications of these data on the aggregation, adsorption on solid surfaces, inhibition of deposition and emulsion stabilization capacities for two types of asphaltenes obtained from Brazilian crude oils. Surface tension measurements in solutions formed by any of these two types of asphaltenes, pentane insolubles (C5I) or heptane insolubles (C7I), in aromatic solvents suggested the occurrence of an aggregation phenomenon of asphaltenes in each of the solvents studied. Viscosity measurements in these same solutions revealed yet another type of aggregation at higher asphaltene concentrations. Both aggregation processes were found to be a function of temperature and type of asphaltenes. For the asphaltenes investigated, C7I was always more prone to aggregation, a characteristic ascribed to its higher asphaltene and lower resin content as revealed by SARA chromatographic analysis, and to its larger average molecular weight as determined by VPO. The effect of temperature on theses two processes confirmed the exothermic nature of both. A number of block copolymers, ionic and non-ionic surfactants, were tested for their effectiveness as asphaltenes stabilizers in crude oil. The results revealed different and distinct mechanisms for solubilization/dispersion of asphaltenes in aromatic (aliphatic) solvents and for the inhibition of asphaltene precipitation in crude oils. The two types of asphaltenes (C5I and C7I) were found to be effective in the stabilization of water/oil emulsions as well as water/toluene emulsions, with larger effects for C7I in water/toluene emulsions, consistent with its greater surface activity and its larger tendency to self-associate. Finally, the interaction of asphaltenes with solid surfaces was evaluated by determining the adsorption isotherms of asphaltenes on silica and activated carbon. The organic adsorbent was found to be much more efficient in removing asphaltenes due, perhaps, to the similar chemical nature of the adsorbing species and adsorbent. In all cases, evidence points to multilayer formation, in agreement with the previously detected tendency of asphaltenes to aggregate.

On the interfacial properties of micrometre–sized water droplets in crude oil

The remarkable stability of water–in–crude oil emulsions is due to the presence of a complex adsorbed layer at the surfaces of the dispersed droplets. Except for its role as a steric barrier, little is known about the in situ properties of this interfacial structure. In this study, new insights into the adsorbed layer are provided by direct micrometre–scale measurements. At low crude content in the bulk where, according to interfacial tension isotherms, there should be little or no surfactants on the droplet surface, the adsorbed layer displays pronounced rigidity and is capable of preventing coalescence and coagulation of the droplets. Such interfaces are highly dissipative and can be well described by the Boussinesq–Scriven model. As the supply of surface active materials in the bulk (i.e. the crude content) increases , the adsorbed layer transforms from a rigid structure to a fluid interface. This fluid layer continues to inhibit coalescence, although signs of weak interdroplet adhesion begin to appear. Under area compression, the fluid interface will discharge micrometre–sized emulsion droplets into the oil phase via a ‘budding’ mechanism.

Interatomic Interactions in the Adsorption of Asphaltenes and Resins on Kaolinite Calculated by Molecular Dynamics

Molecular dynamics calculations of the energy of interaction of model asphaltene and resins on neutral surfaces of kaolinite in a vacuum showed that the van der Waals interaction provides the largest contribution (60−70%), with a smaller one (30−20%) from the Coulombic interaction. The H bonds provided 10% or less of the energy of interaction in the adsorption of these fractions onto kaolinite. It was found that the variations in the polarity of the resin introduced only minor changes in the energy of interaction with the surfaces. It was also determined that the resin interacts more strongly with the surfaces of kaolinite than with the asphaltene molecule, indicating that these surfaces may perturb the aggregates formed by the organic molecules that are found in crude oil.

Study of Asphaltene Solutions by Electrical Conductivity Measurements

The asphaltene interactions in model solutions were studied using a technique based on the electrical conductivity measurement. Interactions with n-heptane, resins, surfactants, water, phenol and NaCI were investigated. The conclusions drawn from this study confirmed previous opinions on aggregation mechanism of asphaltenes in solutions. They confirmed also the interpretation of asphaltene behaviour in terms of colloidal solution theories.

Effects of Asphaltene Aggregation in Model Heptane–Toluene Mixtures on Stability of Water-in-Oil Emulsions

As part of an ongoing investigation into the stability of water-in-crude oil emulsions, model oils have been utilized to further probe the effects of crude solvency as well as specific resin–asphaltene interactions on emulsion stability. These model oils were constructed by dissolving varying amounts of resins and/or asphaltenes in a mixture of heptane and toluene. The resins and asphaltenes used in this study were isolated from four different crude types—Arab Berri (AB), Arab Heavy (AH), Alaska North Slope (ANS), and San Joaquin Valley (SJV)—and characterized in a previous study using heptane precipitation of the asphaltenes followed by an extrographic separation of the resins from silica gel. Asphaltenes dissolved in heptol at concentrations of just 0.5% were shown to generate emulsions which were even more stable than those generated from their respective whole crude oils. Some types of resins (e.g., from AH and SJV) also demonstrated an ability to stabilize emulsions although these resin-stabilized emulsions were considerably less stable than those prepared with asphaltenes. The primary factors governing the stability of these model emulsions were the aromaticity of the crude medium (as controlled by the heptane:toluene ratio), the concentration of asphaltenes, and the availability of solvating resins in the oil (i.e., the resin/asphaltene or R/A ratio). The model emulsions were the most stable when the crude medium was 30–40% toluene and in many cases at small R/A ratios (i.e., R/A ≤1). This strongly supports the theory that asphaltenes are the most effective in stabilizing emulsions when they are near the point of incipient precipitation. The types of resins and asphaltenes used to construct these model oils also played a role in determining the resultant emulsion stability which indicates the importance of specific resin-asphaltene interactions. The interfacially active components that stabilized these model systems were the most polar and/or condensed portions of the resin and asphaltene fractions as determined by elemental and neutron activation analyses. All of these results point to the significance of the solubility state of the asphaltenes in determining the emulsifying potential of these crude oils.

VISCOMETRIC STUDY OF AGGREGATION INTERACTIONS IN HEAVY OIL

ABSTRACT Physical interactions of asphaltenes in the heavy oil environment are studied. The approach consisted of following the variation of viscosities of toluene solutions of Heavy Arab vacuum resid containing asphaltenes as a function of: (a) concentration and (b) temperature of the solutions. Based on the results, a mechanism of aggregation consisting of step-wise aggregation followed by cluster formation is proposed. The aggregation is reversible by dilution and by increasing the temperature of the solution.

Reaction of Athabasca asphaltene with tetralin

Interaction of Athabasca asphaltene with a hydrogen-donating solvent such as tetralin at temperatures between 195 and 390 °C has been investigated. The reaction resulted in lowering the molecular weight of the treated asphaltene. At 390 °C, 50% of the original asphaltene was converted into pentane-solubles and the sulphur and oxygen contents were each depleted by about 40%. The results, which were interpreted in terms of thermal cleavage of sulphide bonds, are complementary to those obtained in the radical-ion electron-transfer reduction of the asphaltene with potassium in tetrahydrofuran. The negligible amount of coke formation at 390 °C proves the excellent stabilizing properties of tetralin in thermal reactions.

Reactions of Athabasca asphaltenes and heavy oil with metal chlorides☆

Abstract The interactions of Athabasca asphaltenes and heavy oil with metal chlorides are described. The asphaltenes are converted to insoluble, presumably higher molecular weight, products which contain substantial amounts of chemically bound chlorine. The heavy oils were converted partly into insoluble products; both this fraction and residue oils contained chemically bound chlorine. Statistical structure analysis aided by infra-red spectrographic and nuclear magnetic resonance examination of the soluble portion of the reacted heavy oils suggests that inter- and intramolecular condensation by dehydrogenation and dealkylation accompany chlorination.