Asphaltene Subfractions Research Articles

Probing the interfacial behaviors of interfacially active and non-active asphaltenes and their impact on emulsion stability

HypothesisAsphaltenes subfractions with distinct interfacial behaviors may play different roles in stabilizing oil–water emulsions. ExperimentsIn this work, whole asphaltenes were separated into interfacially active asphaltenes (IAA) and interfacially non-active asphaltenes (INAA). Employing advanced nanomechanical techniques, we have explored the compositions, morphologies, sizes, adsorption, and interfacial behaviors of IAA and INAA. FindingsIAA exhibits a high and unevenly distributed oxygen content, distinguishing it from INAA. In toluene, the diameters of IAA and INAA are about 60 nm and 6 nm, respectively. When adsorbed irreversibly on mica surfaces, the thickness of the IAA and INAA film was measured at ∼5.5 nm or 1 nm, respectively; while in a toluene solution, the film thickness reached ∼46 nm and 3.1 nm for IAA and INAA, respectively. IAA demonstrates superior interfacial activity, and elastic/viscous moduli compared to INAA at the water-toluene interface. Quantified surface force measurements reveal that IAA stabilizes water droplets in toluene at a concentration of only 10 mg/L, while INAA requires a higher concentration of 100 mg/L. This work provides the first comprehensive investigation into the adsorption and interfacial behaviors of asphaltene subfractions and provides useful insights into the asphaltenes-stabilization mechanism of emulsions.

Solubility and structural parameters of asphaltene subfractions separated from bitumen via an improved method

Asphaltene is an important component of bitumen, and it is closely related to the performance of bitumen. Asphaltene is a complex mixture, and it is obviously not accurate to simply regard it as the heptane-insoluble part in bitumen. In this paper, an improved reduced pressure column precipitation device was developed to separate the asphaltene sub-fractions in 2 kinds of road petroleum bitumen which had the same Performance Grade (PG) and almost free of wax, but significantly different physical aging degree. The separation process is strictly based on the solubility of asphaltene subfractions in different polar solvents. Based on the extended Hansen solubility parameter (HSP) model, 6 kinds of isolated asphaltenes were tested by turbidimetric titration test to determine their structural stabilities in three solubility directions: dispersion interaction, polar interaction and hydrogen bonding. Furthermore, the structural characteristic parameters (aromatic sheet diameters and aromaticity) of 6 kinds of asphaltenes were obtained using Raman spectroscopy and Fourier-transform infrared spectroscopy. The research showed that asphaltenes exhibited lower dispersion and stronger hydrogen bonding with the increasing of polarity. The aromatic hydrocarbon size and aromaticity also increased with polarity. The research results can explain bitumen phase imbalance (such as thermoreversible aging) caused by the change of asphaltene components.

Aggregation of Asphaltene Subfractions A1 and A2 in Different Solvents from the Perspective of Molecular Dynamics Simulations

The aggregation behavior of model molecules of asphaltene subfractions A1 and A2 dissolved in heptane, toluene, and tetrahydrofuran (THF) were studied using molecular dynamics simulations. The proposed asphaltene molecular models are based on previously studied structures with two new models, including a highly aromatic model with a prominent island-type molecule and another molecule with a prominent archipelago-type architecture. The aggregation mechanisms in toluene, THF, and heptane solvents were studied. The results in heptane and toluene were consistent with the solubility of asphaltenes and their subfractions in these solvents. The size of the aggregates is well-correlated with aromaticity. When considering THF, large aggregates are broken down into smaller aggregates. This could lead to the mixture of high, medium, and low molecular weight distribution bands usually observed when gel permeation chromatography (GPC) analyses are conducted on asphaltene samples in THF. The size distribution extracted from the simulations shows a bimodal distribution with profiles similar to the size distribution profiles usually found in GPC analysis for asphaltene samples in THF. The distributions of dipole moments of the aggregates against the number of molecules in aggregates were constructed in both THF and toluene and reveal that the dipole moment of the aggregates vanishes when the number of molecules increases as a result of a random structure of the aggregates. The contributions of different molecular interactions to the aggregation mechanism, such as π stacking, van der Waals, and hydrogen bonds, are described.

Advantages of a Millifluidic Approach to the Characterization of Long-Time Solvency Effects on Bulk Asphaltene Precipitation

The colloidal and aggregation behavior of asphaltenes in bulk dispersions is largely dependent on the solvent environment. Increasing the aliphatic fraction in these systems drives the aggregation of asphaltene clusters, leading to precipitation out of the bulk that can result in complications in the oil recovery and refining process. In this work, a millifluidic device is used to characterize the precipitation behavior of two asphaltene subfractions in good and poor solvent conditions to characterize the long-time solvency effects. Advantages to this approach are high discretization of the concentration gradient as well as shorter path lengths, which allow for better resolution of optical measurements in these extremely opaque systems. To measure precipitation, the critical aliphatic volume fraction (CAVF), defined in this work as the highest fraction of aliphatic solvent at which there is no visible precipitation in the sample, functions as a compositional breakpoint between stable and unstable dispersions. Results from this work show that asphaltene dispersions will reach a steady state after long times, on the order of months, of sample aging. Further, precipitation is visible at lower aliphatic fractions than what was determined in experiments using shorter aging times. This demonstrates how long-term effects should be considered when determining the stability of asphaltene dispersions.

Influences of asphaltene subfractions with different polarities on hydrate growth at water/oil interface

As a kind of typical natural surface-active substance, asphaltenes can not only lead to deposition and plugging accidents, but also affect the formation and slurry flow of hydrate during deep-sea oil and gas transportation process. This paper investigates the polarity effects of the asphaltene subfractions on the growth process of hydrate particles at oil–water interface. First of all, four asphaltene subfractions with different polarities were extracted by the solvent precipitation method. Then, the dynamic interfacial tension and dilational viscoelastic properties were measured to characterize the oil–water interfacial films adsorbed with the different asphaltene subfractions. Following that, the hydrate growth experiment was carried out using a self-designed atmospheric visualization unit. Specifically, the transformation process of a single droplet into a hydrate particle in oil phase containing asphaltenes was studied to determine the influence of asphaltene subfractions on hydrate growth. The change degree of particle morphology and the growth rate of hydrate shells were used as two analysis indexes. The results show that the change degree of particle morphology and the growth rate of hydrate shells were both affected by the dispersion state and interfacial adsorption amount of the asphaltene subfractions. Under the combined action of migration and surface covering, the asphaltene subfractions have different effects on hydrate growth process at various concentrations. At the concentration of 0.01 mg/g and 0.1 mg/g, a looser interfacial adsorption layer will result in a large effective interfacial area, i.e., a weaker mass transfer hindrance. At this time, the structural strength of the interfacial film is low. At the concentration of 0.2 mg/g, however, the transformation rate of the hydrate shells is accelerated owing to the trapped oil phase, so the structural strength of the interfacial film is enhanced. Therefore, with the increase of the asphaltene concentration, the hydrate particles are easier to maintain their original state.

In-depth characterization of light, medium and heavy oil asphaltenes as well as asphaltenes subfractions

Asphaltenes, and their related issues, have been the focus of many literature investigations. However, in-depth analysis of asphaltenes structure and its relation to asphaltenes stability has been considered by fewer studies. In this research, extensive analysis of the structure of asphaltenes extracted from light, medium, and heavy oils is provided, together with analysis of three subfractions of the medium oil asphaltene having the least, intermediate, and highest solubilities. To this end, elemental analysis, EDX, mass spectroscopy, FTIR, NMR, XRD, and SEM results were collected. Higher hydrogen content and hydrogen/carbon atomic ratio, lower aromatic nature and olefinic entities were observed in heavy oil asphaltenes as well as the most soluble subfraction. Moreover, the molecular structure of heavy oil asphaltenes and the most soluble subfraction displayed broader mass to charge (m/z) distribution range, longer alkyl side chains as well as higher number of stacked aromatic sheets within their nanoclusters. Asphaltenes hydrogens in α position followed similar trend as their heteroatom content, which was higher for the less soluble subfraction and medium oil asphaltenes. Generally, heavy oil asphaltenes possessed a molecular structure that was closer to the most soluble asphaltenes subfraction, whereas light oil asphaltenes possessed a molecular structure similar to the least soluble subfraction. Asphaltenes precipitation/destabilization was attributed to primarily higher attractive π-π interactions coupled with lower steric hindrance.

Asphaltene instability in the presence of emulsified aqueous phase

Water cut of high salinity, which is reported to be on the rise in many reservoirs, may profoundly exacerbate asphaltene related problems. In this work, the impact of the emulsified aqueous phase in synthetic oils on asphaltene precipitation has experimentally been studied. The oil phase of each emulsion was divided into three sub-fractions of oils 1, 2, and 3, based on their asphaltene’s affinity to interact with the oil/water interface. These three sub-fractions were then contacted with n-heptane as precipitator. The influence of ion types, temperature, and finally the presence of tetrahydrofuran (THF) in emulsified aqueous phases were further investigated for two different asphaltene samples. Moreover, the role of ions on asphaltene interaction with the oil/water was examined using interfacial tension, interfacial rheology, and droplet size analysis. The results showed that precipitation behavior of oils 1, 2, and 3 would be controlled by changes in relative distribution of asphaltene sub-fractions and their respective concentration. IFT measurements showed that asphaltenes in oils 1, 2, and 3 have different interactions with de-ionized water. Ions of higher valence, like Fe3+, also would profoundly decrease the size of water droplets in emulsion and also cause more asphaltene to remain in emulsions. When THF was added to the injected aqueous phase, then it got partitioned between aqueous and oleic phases. Thus, its addition to the emulsified aqueous phase firstly stemmed emulsion formation, and secondly, it alleviated asphaltene precipitation by diluting the oil phase at the oil/water interface. IFT measurements showed that asphaltenes in oils 1, 2, and 3 had different interactions with the de-ionized water. It was shown that brines did not directly affect the asphaltene precipitation. Instead, their presence had a noticeable impact on the distribution of asphaltene between the bulk of the oil and emulsion phases. This, in turn, altered the precipitation of asphaltene of the oil. The findings of this study address the main concerns about asphaltene destabilization by aqueous phase injection to petroleum reservoirs.

Asphaltene formation modeling using vapor-liquid-liquid equilibrium calculations by PC-SAFT for reservoir and surface conditions

Asphaltene precipitation can occur during the different stages of oil production, including an underground reservoir, wellbore, pipelines, and surface facilities. An accurate model to predict precipitation in these conditions is still a challenge, which is addressed in this work. In this study, we first utilize the lumped characterization method of Panuganti et al. 1 in a vapor-liquid-liquid equilibrium calculation approach using PC-SAFT, and by making use of a new parameter fitting method and genetic algorithm, we predict the Asphaltene instability conditions due to the pressure, composition and temperature variations in reservoir and surface. Titration experiments of the dead oils have indicated that Asphaltene is comprised of several sub-fractions with different molecular weights that precipitate under different thermodynamic conditions. Therefore, understanding the polydisperse nature of Asphaltene is crucial in order to better estimate the Asphaltene yield during the composition change. In the next step, Asphaltene is split into three sub-fractions according to the titration data of different precipitating agents. The parameters of Asphaltene sub-fraction are then adjusted with respect to the initial monodisperse Asphaltene parameters. In addition, the amount of precipitated Asphaltene at different solvent ratios of titrant to degassed oil is calculated. The results are compared with the model of Buenrostro‐Gonzalez et al. 2 and the experimental data of two Mexican crude oils (C1 and Y3). Regarding the outcomes, from a numerical perspective, this study's approach has resulted in accurate predictions for both the live and dead (degassed) oil. The average estimation error for sample C1 is equal to 3.46% for this study, while this number is 5.13% for the Buenrostro‐Gonzalez et al. 2. Regarding the sample Y3, the sum of errors are nearly at 3.60% and 5.10% for this study and Buenrostro‐Gonzalez et al. 2, respectively.

A novel and efficient chemometric approach to identifying oil families by saturate biomarker data and FTIR spectroscopy of asphaltene subfractions

The genetic classification is commonly performed by the utilization of a few interpretive biomarker ratios (e.g., 14 peak ratios) in chemometric methods. In this paper, a novel workflow with higher efficiency in revealing subtle differences between samples was suggested to solve the limitations of traditional methods. The peak areas of selected source related biomarker compounds were employed to generate 342 peak ratios. The use of higher peak ratios in the comparisons provides higher sensitivity regarding dissimilarities among studied samples. These peak ratios were utilized as an input in PCA and HCA techniques. Also, a new similarity coefficient measurement was suggested for the HCA technique. In this procedure, the moving window correlation coefficient technique was implemented on generated data series. The dissimilarity matrix for each pair of samples was constructed by the calculated minimum correlation coefficient in all moving windows. A complete linkage algorithm was then exerted on the dissimilarity matrix to relate different samples and define oil families. This new procedure provides higher efficiency regarding commonly used procedures in the PCA and HCA techniques for disclosing dissimilarities between samples. Furthermore, structural characteristics of asphaltenes due to their similarity with the precursor kerogen were applied to complement GC-MS data. Asphaltenes were fractionated into three subfractions for comprehensive comparison among studied samples. The proposed technique was carried out on the 18 oil samples from the Dezful Embayment, SW Iran. The studied oils comprise three distinct oil families, including group I (Rag-e Sefid and Shadegan samples), group II (Ahvaz samples), and group III (Qale Nar samples). Both FTIR and GC-MS corroborate these three oil families.

Evidence of selective asphaltene subfraction adsorption on SiO2 nanoparticles studied by UV-vis absorbance and fluorescence spectroscopy

The study of asphaltene adsorption onto nanoparticles is an area of great interest due to its ability to mitigate problems caused by asphaltenes in the oil industry. In this work, UV-vis absorbance and fluorescence spectroscopy were used to study asphaltene adsorption onto silica nanoparticles prepared from rice husk ash. The adsorption process followed a Dubinin-Radushkevich adsorption behavior, which suggested a progressive adsorption of larger aggregates onto the nanoparticles. From the Gaussian decomposition of the fluorescence emission spectra, it was determined that species of high molecular weight were preferentially adsorbed on the nanoparticles. These results have both academic and industrial importance since they help to propose more suitable thermodynamic models and methodologies to mitigate the issues related to adsorption and deposition of asphaltenes on surfaces.

Molecular Cartography of A1 and A2 Asphaltene Subfractions from Classical Molecular Dynamics Simulations

The physical chemistry of the heavyweight fraction of crude oil is still a subject of vivid discussions due to the complications which arise from the tendency of this fraction to aggregate and fina...

Crude oil fingerprint heterogeneity assessment by investigation of asphaltene subfractions: Implementation for reservoir continuity evaluation

Asphaltenes are being a proxy of kerogen in the reservoir oil, which they have structural similarity. This characteristic provides an invaluable tool for obtaining oil fingerprints from the structural composition of asphaltenes. Since these compounds are classically defined as a solubility class, they constitute a large number of compound types with similar solubility features, instead of chemical characteristics. An effective strategy for the comprehensive characterization of these complex compounds is the fractionation of asphaltenes into several solubility subfractions. In this paper, asphaltenes were fractionated into three subfractions via adsorption of asphaltene onto CaCO3. The three obtained subfractions were entirely distinguishable, in which the FR-3 has the highest aromatic content, lowest aliphatic content, and the highest polarity among three subfractions. The FTIR spectroscopy was used for achieving structural composition information from each subfraction. Ninety peak ratios (10 × 9) based on ten characteristic bands were calculated for each subfraction, and a total of 270 peak ratios (90 × 3) were generated for each asphaltene sample. The modified moving window correlation coefficient technique was employed on generated peak ratios to obtain oil fingerprints for each sample and then the pairwise comparison between different samples. As exemplified, ten samples from the Asmari and Bangestan reservoirs in the Rag-e Sefid field were studied by the proposed procedure. The thresholds of MMWR and AMWR were determined as 0.9494 and 0.9894, respectively, in the Rag-e Sefid field. The similarity matrix was constructed based on a comparison of calculated MMWR and AMWR for all pairs of samples and thresholds to distinguish homogeneous and heterogenous oil fingerprints. However, the Bangestan reservoir has fingerprint homogeneity, some heterogeneities were observed in the Asmari reservoir. The investigation of migration pathways revealed initial heterogeneity in the reservoir, in which these fingerprints dissimilarities were removed in connected wells during geological times. The examination of heterogeneities revealed three compartmentalized regions in the Asmari reservoir.

Experimental Evidence of Enhanced Adsorption Dynamics at Liquid-Liquid Interfaces under an Electric Field.

In this work, we investigate the adsorption of surface-active compounds at the water-oil interface subjected to an electric field. A fluid system comprising a pendent water drop surrounded by an asphaltene-rich organic phase is exposed to a DC uniform electric field. Two subfractions of asphaltenes having contrasting affinities to the water-oil interface are used as surface-active compounds. The microscopic changes in the drop shape, as a result of asphaltene adsorption, are captured and the drop profiles are analyzed using our in-house code for axisymmetric drop shape analysis (ADSA) under an electric field. The estimates of dynamic interfacial tension under different strengths of the field (E0) and concentrations of the asphaltene subfractions (C) are used to calculate adsorption dynamics and surface excess. The experimental observations and careful analyses of the data suggest that the externally applied electric field significantly stimulates the mass-transfer rate at the liquid-liquid interface. The enhancement in mass transport at the water-oil interface can be attributed to the axisymmetric electrohydrodynamic fluid flows generated on either side of the interface. The boost in mass transport is evident from the growing decay in equilibrium interfacial tension (γeq) and increased surface excess (Γeq) upon increasing strength of the applied electric field. The mass-transfer intensification does not increase monotonously with the electric field strength above an optimum E0, which is in agreement with the previous theoretical studies in the literature. However, these first explicit experimental measurements of adsorption at an interface under an electric field suggest that the optimum E0 is determined by characteristics of the surface-active molecules.

Fractionation of Crude Oil Asphaltene by Adsorption onto Silica Particles in a Fixed-Bed Column: Tensiometry Study and Interfacial Behavior of Different Asphaltenes Subfractions

One of the most important characteristics of asphaltene molecules is their interfacial properties. These are amphiphilic molecules with polar and nonpolar portions. Consequently, asphaltenes have a...

Aggregation Behavior of E-SARA Asphaltene Fractions Studied by Small-Angle Neutron Scattering

Using the extended-SARA method to fractionate asphaltenes based on their interfacial activity, the current study reports the first results on the estimated size and shape of interfacially active asphaltene (IAA) and remaining asphaltene (RA) nanoaggregates. These fractions have been reported to exhibit distinctly different chemical architectures that influence the size of asphaltene clusters in good and poor solvents. However, little is known about the building blocks, commonly referred to as nanoaggregates, which form these clusters and how those subtle differences in chemical architecture impact aggregation of asphaltenes. The nanoaggregate size and shape of IAA and RA was measured using small-angle neutron scattering (SANS). The characteristic length and asymptotic power-law exponent of whole asphaltenes (WAs) extracted from heavy crude oil and dispersed in deuterated toluene were 28.0 ± 0.2 Å and 2.86 ± 0.01, respectively, showing negligible variations with changing asphaltene concentration, source of asphaltenes (bitumen and heavy crude oil), and solvent aromaticity. For RA fractions, which account for 98.5 wt % of WA, the characteristic length and power-law exponent of 28.8 Å and 2.86 were comparable to that of WA but in contrast to 59.7 Å and 2.20 for IAA. A ∼100% increase in the characteristic length and reduced power-law exponent of the IAA fraction confirms that these two asphaltene subfractions form dissimilar nanoaggregate structures.

Advances in Asphaltene Petroleomics. Part 4. Compositional Trends of Solubility Subfractions Reveal that Polyfunctional Oxygen-Containing Compounds Drive Asphaltene Chemistry

Asphaltenes have traditionally been conceived as highly aromatic, alkyl-deficient compounds enriched in pericondensed aromatic “island” motifs. This structural definition evolved into the general notion that aromatic core-dominated interactions (π-stacking) drive asphaltene aggregation, and heteroatom-based intermolecular forces have no significant effect on the overall solubility and aggregation behavior. However, the exclusion of heteroatoms in asphaltene chemistry is inconsistent with the Boduszynski continuum and known asphaltene properties, such as increased heteroatom content relative to maltenes, interfacial activity, and strong adsorption to polar stationary phases. Thus, to determine whether or not heteroatoms are involved in solubility, we have separated asphaltene fractions enriched in single-core (island) or multicore motifs (archipelago) according to their partitioning in n-heptane by two fractionation methods. In the first separation procedure, the acetone fraction from Wyoming deposit n-heptane asphaltenes (island-enriched) was adsorbed on polytetrafluoroethylene powder and Soxhlet extracted with n-heptane. Subfractions were collected after one day, one week, one month, and three months of extraction, and the residue, n-heptane insoluble material, was recovered with a mixture of toluene and dichloromethane. In the second method, the acetone fraction from Athabasca bitumen n-heptane asphaltenes (archipelago-enriched) was fractionated by differential precipitation in mixtures of n-heptane and toluene. The molecular composition of the asphaltene subfractions was accessed by positive-ion atmospheric pressure photoionization coupled to 9.4 T Fourier transform ion cyclotron resonance mass spectrometry and structures were determined by infrared multiphoton dissociation. The compositional trends for heteroatom content, double bond equivalents, and alkyl substitution suggest that the Boduszynski continuum can be extended to asphaltenes. In particular, the compositional range of polyoxygenated asphaltene compounds shifts toward lower aromaticity, whereas oxygen-depleted species are more aromatic. Moreover, the results demonstrate that polyoxygenated species (e.g., O₃ and S₂O₃ classes) are pivotal in asphaltene solubility, as they concentrate in the most polarizable and insoluble asphaltene subfractions. Therefore, the results support the existence of atypical asphaltene species with remarkably low aromaticity that reside in the most insoluble asphaltene subfractions because of their high heteroatom content. Such asphaltene compounds preferentially ionize as protonated molecules rather than radical cations and overlap the compositional range of interfacially active species, consistent with their tendency to participate in hydrogen bonding. Collectively, the results highlight the need for an asphaltene molecular model based on the existence of polyfunctional species capable of interacting with neighboring asphaltene molecules through several intermolecular forces, including London dispersion forces between aliphatic moieties, π-stacking of aromatic cores, hydrogen bonding between nitrogen and oxygen-containing functionalities, and acid/base interactions between carboxylic acids and pyridine rings.

Polarity effects of asphaltene subfractions on the stability and interfacial properties of water-in-model oil emulsions

Asphaltenes can adsorb at the interface between crude oil and produced water and form a defensive interfacial layer to stabilize emulsions. This paper details the effects of the polarities of asphaltene subfractions on emulsion stability and interfacial properties. Asphaltene subfractions with different polarities were first obtained by a solvent-precipitation method, followed by the characterization of their molecular structures and dispersive states in the model oil. The results show that the subfractions with lower polarity contain a higher proportion of aliphatic groups, which benefits oil compatibility, thus leading to a better dispersive state and weaker aggregation tendency. Then, the stabilities of the emulsions prepared with these subfractions were explored at different concentrations. The emulsion stability worsens as polarity weakens at 500 ppm. However, at a concentration of 5000 ppm, the emulsion stability increases first with a polarity decrease, reaches a peak, and then decreases. The adsorption behavior and interfacial structural strength are detected through dynamic interfacial tension and dilational viscoelastic experiments. The subfractions with lower polarities exhibit better dispersion in the model oil, allowing for higher rates of absorption. The dilational modulus and emulsion stability depend on the migration rate and surface coverage of the subfractions and share similar trends with changes in polarity. With an increase in the polarity of the subfractions, the surface coverage and migration rate have reverse contributions on the dilational modulus and emulsion stability. Additionally, the contributions become more obvious after asphaltene aggregation.

Influence of Co-based catalyst on subfractional composition of heavy oil asphaltenes during aquathermolysis

The chemical changes in the composition of asphaltene subfractions of heavy oil under the influence of oil-soluble Co-based catalyst were investigated. The results of SARA-analysis and MALDI-spectroscopy revealed a decrease in absolute content of asphaltenes and their molecular masses in the presence of catalyst. Moreover, the sulfur content was decreased under the catalyst influence and it was mostly observed in the subfraction with the highest molecular weight (A5). The main catalytic reaction was a destructive hydrogenation, directed to break the –С–S–C– bonds in the asphaltene molecules. The 13C NMR spectra revealed the decrease in average length of chain (MCL) after catalytic treatment by 15 times in A1 subfraction, which was accepted as a solvate layer and almost by 13 times in A2 subfraction. The C–O and C–S bonds break in the side chains of aromatic and alicyclic hydrocarbons. The content of primary (Cp) and the sum of secondary and quaternary (Csq) carbons was increased, while the content of tertiary (Ct) and aromatic carbons (Car) declined. The high-molecular poly-alicyclic and polyaromatic components had lost the straight and branched aliphatic hydrocarbons, and then were transformed into coke like substance – carbene-carboids.

Impact of aqueous phase in emulsified form on distribution and instability of asphaltene molecules

Asphaltene extracted from a crude oil covers a wide range of molecules that would similarly act in terms of solubility in general but they may differ in terms of precipitation behavior due to different molecular structures. For this reason, they are commonly represented as a solubility class, not a particular molecule though each sub-fraction of asphaltene may have its own precipitation tendency. In recent years, the impact of water presence on the precipitation of these molecules has attracted attention due to increased production of water, although the reported results are somewhat inconsistent. In this study, a systematic experimental study was performed to investigate the impact of aqueous phase, which composed of deionized water and ions of single, double, and three valence with net positive and negative charges on asphaltene precipitation from a synthetic oil composed of toluene and asphaltene. The aqueous phase was emulsified within this synthetic oil phase using ultrasonic mixer. Then, the oil phase was divided into three fractions named oil 1, 2, and 3, respectively depending on the degree of their desire to remain in the emulsion. The results showed that oils 1, 2, and 3 have different precipitation curves. The precipitation became more intensified even up to four fold from oil 1 to 3. Moreover, the presence of ions with higher valence in the aqueous phase drew extra asphaltene molecules from oil 1 to oil 2 and then oil 3 (i.e. from bulk to the interface) which, in turn, changed the precipitation curve. The experimental results also showed that the relative concentration of asphaltene sub-fractions in oils 1, 2 and 3 determines the shape of the final precipitation curve and the onset point of precipitation.

Experimental and mathematical model evaluation of asphaltene fractionation based on adsorption in porous media: Dolomite reservoir rock

In this study, a whole asphaltene sample was first aged with five dolomite core samples. Then the core samples were washed with toluene; this asphaltene was named the bulk sub-fraction. Thereafter, they were washed with tetrahydrofuran, so that the n-adsorbed sub-fraction was dissolved in solvent. At the last step, the core sample was washed with an azeotrope of methanol/chloroform, which corresponded to the h-adsorbed sub-fraction, and the residual asphaltenes within the porous media were irreversibly adsorbed. According to the elemental analysis of whole asphaltene and its sub-fractions, sulfur was the most abundant element in the molecular structure of asphaltene. The presence of sulfur near dolomite increased the interaction between asphaltene and the rock surface; thus, the h-adsorbed asphaltene had the highest sulfur content. Nitrogen and oxygen content did not show a clear trend in the sub-fractions; therefore, their placements in the molecule were examined using more precise tools, such as Fourier-transform infrared spectroscopy (FTIR). Oxygen was found in asphaltene fractions in the form of a carboxylic acid functional group. From the calculation of the value of the RO–H index, the h-adsorbed asphaltene had more ability to establish a hydrogen bond (RO–H = 26.88) than the other fractions. Furthermore, by calculating the aliphatic and aromaticity indices, the sub-fraction adsorbed to the dolomite surface (h-adsorbed) had the least aliphatic compounds and the highest amount of aromaticity among the sub-fractions. In addition, the h-adsorbed sub-fraction had shorter side alkyl chains than the bulk and n-adsorbed asphaltenes. Carbon number analysis was also performed to determine the number of carbons present in each molecule and to measure the molecular size of each asphaltene sub-fraction. The results showed that the C29+ fractions in the bulk sub-fraction were high (41.39%), while the C14–C26 and C15–C24 fractions had the highest frequencies in the n-adsorbed and h-adsorbed sub-fractions, respectively. A new mathematical model was tuned to determine the porosity and permeability of a resulting asphaltene deposition. The main reason for the high accuracy and innovation in the proposed relationship was the use of the fluid velocity parameter in the porous media and the volume of fluid injected into the core. In addition, the dependence of porosity and permeability reductions on parameters such as the ratio of the contribution of each asphaltene sub-fraction and the amount of the heteroatoms in each sub-fraction was studied for the first time.