Asphaltene Solutions Research Articles

Evaluation of Asphaltene Degradation on Highly Ordered TiO2 Nanotubular Arrays via Variations in Wettability

Photocatalytic degradation of both aquatic and atmospheric organic pollutants on titanium dioxide has been extensively investigated in the past decades, but research on direct photocatalytic degradation of solid-phase organic pollutants is rather limited. In this work, photocatalytic degradation of n-C(7) asphaltene, which is composed of solid-phase organic substances found in crude oil, on highly ordered TiO(2) nanotubular arrays (TNAs) was studied using the wettability as an indicator. It was observed that the water contact angle rose linearly with increasing the concentration of n-C(7) asphaltene solution up to 0.02 g mL(-1). Further increasing the concentration of n-C(7) asphaltene only caused small augment in the contact angle, which eventually became stable around 98°. It is demonstrated that the water contact angle can be used as an indicator to reflect the residual solid-phase organic pollutants within a certain range of pollutant concentration. As observed, n-C(7) asphaltene film degraded on TNAs under UV illumination for 60 min, showing complete mineralization of ∼80% of n-C(7) asphaltene that was released into air finally. The remaining 20% of asphaltene was partially decomposed into smaller organic molecules, e.g., -C(═O)- and -C(═O)-OH, confirmed by high-resolution X-ray photoelectron spectra analysis. TNAs can be reused to degrade the solid-phase n-C(7) asphaltene for a number of cycles without further treatment.

Adsorption of Asphaltenes in Porous Media under Flow Conditions

Asphaltene adsorption on minerals induces wettability alteration in porous media, which, in turn, affects capillary pressure, relative permeability, and residual oil saturations. The majority of existing work addresses the adsorption of asphaltenes on minerals under static conditions, where adsorption isotherms are measured and modeled. Very little work currently exists on the adsorption kinetics of asphaltenes under dynamic conditions, which are more relevant to laboratory-scale experiments, specifically those involving dynamic wettability alteration of minerals. In this study, we propose to investigate the dynamic adsorption of different asphaltenes on various minerals using UV−vis spectroscopy. The experimental setup consists of a core holder containing crushed minerals with comparable mesh size. Asphaltene solutions in toluene are then flown though the mineral pack with a constant flow rate, and their concentrations are recorded with time at the outlet. Preliminary results indicate that adsorption is largely controlled by the type of mineral rather than the asphaltene itself. The highest adsorption amounts per unit area are found on calcite. Furthermore, the effect of several parameters, such as concentration, asphaltene composition, and flow rate, are considered. The theory of activated adsorption/desorption (TAAD) approach (Rudzinski, W.; Plazinski, W. J. Phys. Chem. C 2007, 111, 15100−15110) is incorporated into the convection-dispersion transport equation to model the flow of asphaltenes inside the porous medium. The resulting equation is then solved numerically using Barakat−Clark finite difference technique (Satter, A.; Shum, Y.; Adams, W.; Davis, L. SPE J. 1980, 20, 129−138). We find that kinetics and not equilibrium governs asphaltene adsorption in porous media. Levenberg−Marquardt’s optimization algorithm is used to derive kinetic parameters, such as Γm, ka, and kd. Once these concentration-independent parameters are found, the model is able to predict with reasonable accuracy the effect of concentration on asphaltene adsorption in porous media.

Membrane Diffusion Measurements Do Not Detect Exchange between Asphaltene Aggregates and Solution Phase

Although the aggregation of petroleum asphaltenes has been measured by a variety of methods, little is known about the rate of exchange between aggregated and dissolved components. This work studied the diffusion of asphaltenes from several heavy oils and bitumens in dilute toluene solutions using a stirred diffusion cell equipped with ultrafiltration membranes (Ultracel YM and Anopore). The pore sizes were varied between 3 and 20 nm to retain aggregates while allowing free molecules to diffuse. The permeate was continuously monitored using in situ UV−vis spectroscopy. These experiments demonstrated that the sizes of the asphaltene aggregates at a concentration of 1 g/L in toluene at 25 °C were between 5 and 9 nm and that rates of exchange of material between the aggregates and free solution were extremely low. An increase in the temperature results in an increase in asphaltene mobility but does not reduce the size of the asphaltene structures below 5 nm. Likewise, a decrease in the concentration to 0.1 g/L did not result in a decrease in size. The origin of the asphaltenes (Athabasca, Safaniya, and Venezuela) did not have a significant impact on the observed behavior; therefore, the above observations are widely applicable to C7 asphaltenes. The diffusion of vanadyl porphyrins (vanadyl meso-tetraphenylporphyrin, vanadyl octaethylporphyrin, and native petroporphyrins) in the presence of asphaltenes showed that the native petroporphyrins were larger than the model vanadyl porphyrins based on the appearance of hindered diffusion within smaller pores. An increase in the temperature resulted in an increase in petroporphyrin mobility as per the Stokes−Einstein relationship, although decreasing the asphaltene concentration did not. The mobility of the vanadyl petroporphyrins varied with the origin of the sample (Safaniya, Venezuela, and Athabasca) and is therefore not universal.

On the Nanofiltration of Asphaltene Solutions, Crude Oils, and Emulsions

It has long been established that crude oils contain a colloidal suspension of asphaltenes where the mass fraction can range from 0 to 15%. The size of the colloidal particles is less well understood. Crude oil properties depend critically on the asphaltene content, yet measurement of these properties can be hindered by emulsified water and inorganic particulates in heavier crude oils. Here we explore the ability to isolate pure and unaltered crude oil samples using Gore-Tex membranes with nominal pore sizes as small as 30 nm where a transmembrane differential pressure no greater than 60 psi is employed. We first demonstrate that conventional crude oils can pass through a Gore-Tex membrane without alteration of their asphaltene content or their transport properties. Second, we demonstrate that the same filtration process can be used to extract crude oil from a water-in-oil emulsion where the extracted crude’s properties are not distinguishable from those of the original crude. Third, we show that these ex...

The Importance of Asphaltene Content in Petroleum II—Multi-peak Viscosity Correlations

It is common knowledge in petroleum science and technology that viscosities of dead crude oil are monotonic functions of oil's asphaltene content. However, analysis of crude oil databases reveals multiple peaking of oil viscosity (up to two orders of magnitude) at specific asphaltene contents, close to structural phase boundaries observed in asphaltene and oil solutions. Due to strong viscosity-density correlation, maxima of viscosity practically coincide with earlier reported maxima of specific gravity in dead crude oils.

The Importance of Asphaltene Content in Petroleum—The Revision of Some Persistent Stereotypes

It is common knowledge in petroleum science and technology that the bulk properties of a crude oil are monotonic functions of its asphaltene content. However, unbiased analysis of the world's crude oil databases reveals common peaking of oil density as well as of a frequency of oil appearance at specific asphaltene contents, close to structural phase boundaries observed in asphaltene and oil solutions. Another result of database analysis is a new analytical expression for the general trend of density-asphaltene relationship in the world's recovered (dead) crude oils.

Measurement of the Refractive Index of Crude Oil and Asphaltene Solutions: Onset Flocculation Determination

In this work, we measure the refractive index of crude oils and asphaltene in toluene solutions using a fiber optic refractometer designed to work with high-viscosity and high optical density samples. The data of the samples were analyzed using the Lorentz−Lorenz theory and simple mixing rules. The refractive index for crude oils without dilution for three crude oils with different American Petroleum Institute (API) degree, asphaltene quantity, and stability were measured. Flocculation onset for crude oils and asphaltene solutions were measured using n-heptane as the precipitant agent. Results showed that medium crude oils and maltenes from medium, heavy, and extra-heavy crude oils follow the Lorentz−Lorenz mixing rule. In the case of Boscan crude oil, a sample without any significant asphaltene precipitation problems, the refractive index is lower than that obtained for toluene. In contrast, Furrial crude oil, a sample with severe asphaltene precipitation problems, gave a refractive index higher than toluene. Finally, asphaltene flocculation onset was clearly followed by refractive index measurement in crude oils and asphaltene in toluene solutions after n-heptane addition.

Effects of Temperature and Pressure on the Phase State of Oils and Asphaltene Solutions Observed Using Dielectric Spectroscopy

The dielectric spectra of Caspian Oil were measured at atmospheric pressure in temperature range from 123 to 203 K. Also dielectric responses of Kumkolskaya Oil and asphaltene solutions were obtained in pressure range up to 1 GPa and in temperature interval 250−320 K. The dependences of glass transition temperature (Tg) on pressure for oil samples were obtained for the first time by dielectric spectroscopy, they increase approximately from 125 at atmospheric pressure to over 280 K at 1 GPa. The phase diagrams of oil samples were constructed using Williams−Landell−Ferry equation. For asphaltene solutions, the relaxation times depend on viscosity only prior to the beginning of a crossover region nearby crystallization pressure of solvents. It is shown that the dielectric strength is not proportional to total concentration of asphaltenes in polydisperse solutions. The values of the thermal expansion coefficient of the free volume (αf), the isothermal compressibility of the free volume (βf), and the relative free volume at Tg (fg) were obtained for the examined oils. The received data are in agreement with the data obtained by independent thermophysical methods.

Composition of Asphaltene Solvate Shell at Precipitation Onset Conditions and Estimation of Average Aggregate Sizes in Model Oils

Macroscopic properties of petroleum disperse systems are functions of its particle size. Asphaltene particles in model oils are in fact building agents. Compositional changes of disperse medium have an effect on the internal structure of the dispersed system. Optical methods are informative enough for analysis of structural evolutionary processes. Near-infrared (NIR) spectroscopy was used to determine asphaltene precipitation onset conditions in model oils (asphaltenes were dissolved in binary solvent: toluene and n-alkane precipitant n-hexane/n-heptane). Asphaltene precipitation onset was determined as a point where the deviation from Beer’s law takes place. Solvate shells of asphaltenes in binary solvents are formed according to differences between intermolecular interaction potentials of components of the model oil. The model of non-uniform distribution of components in solvate shells of asphaltene aggregates at the moment of aggregation stability loss (first stage of precipitation onset) is proposed. On the basis of experimentally determined asphaltene precipitation onset conditions, average aggregate sizes in toluene solutions, set of assumptions concerning geometry of solvent and precipitant molecules, and interaction energy of molecule pairs, the proposed model allows for estimation of the precipitant fraction threshold value in solvate shells of asphaltene aggregates at the moment of stability loss. Polarization of fluorescence was used for the determination of the concentration dependence of average sizes of aspahltene aggregates in toluene solution. The linear aggregation model was used for characterization of asphaltene solutions in toluene; the fraction of n-mers was determined. The concentration dependence of the asphaltene aggregation rate constant was calculated within this model.

Asphaltene Aggregates Fractal Restructuring Model, A Population Balance Approach

A new population balance model is developed to predict asphaltene fractal aggregates size distribution. Asphaltene aggregates fractal dimension is increased when shear is induced to them. Fractal dimension evolution is a dynamic process. This model predicts aggregate size distribution more accurately in comparison to a previously proposed model, which considered that aggregates have a constant fractal dimension. The fractal dimension increases from an initial value of 1.6 to final steady-state values. The final value depends upon the shear rate and is described by an exponential function. This function contains some constants that are calculated from experimental data. The asphaltene size distribution of Iranian crude oil asphaltene solutions in mixtures of toluene and n-heptane are predicted by the developed model accurately.

Diffusion of asphaltene molecules through the pore structure of hydroconversion catalysts

When hydrotreatment of heavy cuts by heterogeneous catalysis is carried out in liquid phase the, molecules' state of containment in the porous network is very high. Moreover, at that state of containment, the size of asphaltenes and resins, from various origins, can be the cause for the different hydrotreatment yields. Consequently, volume constraints are added to the kinetic and thermodynamic ones (adsorption equilibrium): a given species can penetrate in the solid only if the necessary volume is available within the network. Hindered diffusion and adsorption of asphaltene molecules inside hydrotreatment catalysts' carriers were studied. The system's kinetics was investigated by visible absorption spectroscopy. Asphaltenes were prepared by n-heptane separation and solubilized in toluene at a known concentration and put in contact with a given amount of catalyst support. The evolution of the concentration in the asphaltene's solution was followed, as a function of time, by measuring the absorbance of a monochromatic visible radiation (750 nm) through the asphaltene suspension. A model based on the Stefan-Maxwell equations, that takes into account the volume constraints by the Fornasiero's formulation, which supposes that the molecules collide only by equivalent volume, was developed. The parameters estimation has been performed and discussed. The results show that the diffusional limitations are important in the catalyst used for heavy oil hydrotreatment and the asphaltene adsorption is very strong in this type of material. (C) 2009 Elsevier Ltd. All rights reserved.

Relation between Nanoscale Structure of Asphaltene Aggregates and their Macroscopic Solution Properties

Some peculiar macroscopic properties of crude oils are ascribed to their densest, heaviest and most polar fraction: the asphaltenes. The comprehension of the origin of these properties relies on a fine structural description of these fractions. We present a nanometre length scale characterisation of asphaltene solutions based on new or recent scattering experiments (X-rays and neutrons) from which structural parameters of asphaltene aggregates (M w , R g and A 2 ) are extracted. The mutual dependence of these parameters is consistent with a mass fractal model. This single model accounts for very different macroscopic solution properties. Solvent trapping in fractal aggregates enables asphaltene solution viscosity to be fully predicted as a function of asphaltene concentration. Asphaltene adsorbs at liquid-liquid and liquid-solid interfaces as a monolayer whose thickness is of the same order of magnitude as the characteristic size of the asphaltene aggregate in bulk. Adsorption isotherm analysis, neutron reflectivity experiments and contrast-matching SANS allow one to highlight a densification of the asphaltene aggregates in the adsorbed layer in comparison with the solvated aggregates in bulk. Finally, the repulsive/attractive interaction balance between aggregates can be related to the relative stability of water in oil emulsions and to the stability of hydro-conversion effluents. This structure/property approach allows one to assess the mass fractal model which is discussed.

Water Enhances the Aggregation of Model Asphaltenes in Solution via Hydrogen Bonding

The self-association properties of two 2,2′-bipyridine derivatives, 4,4′-bis(2-pyren-1-yl-ethyl)-2,2′-bipyridine (PBP) and 4,4′-bis[2-(9-anthryl)ethyl]-2,2′-bipyridine (ABA), in water-saturated solvents were studied as models for petroleum asphaltenes. 1H NMR spectroscopy titration experiments in chloroform established that both compounds self-associate in solution. The addition of water (ranging from 0.43 to 0.86 g/L) increased the thermal stability and the aggregation behavior of PBP and ABA at low concentration (3.06 × 10−4 and 1.28 × 10−3 M, respectively). The chemical shift, line-width, and spin−spin (T2) relaxation time of water protons in these solutions showed that the O−H···N hydrogen bonds between the pyridyl nitrogens of PBP/ABA and water were responsible for their aggregation at low concentration, although FTIR spectroscopy suggested that the H-bond between PBP and water was relatively weak. The solubility of water in PBP solutions showed that the number of water molecules per PBP molecule varied between 1 and 2 in water-saturated CHCl3 and that the Gibbs free energy for the exchange of water between the aqueous phase and the solution of PBP was −16 kJ/mol at 22 °C. Water likely enhanced the stability of aggregates by reinforcing π−π interactions via water-bridged intermolecular H-bonding between the pyridyl nitrogen.

The Properties of Asphaltenes and Their Interaction with Amphiphiles

The functional groups on asphaltene surfaces of two kinds of Chinese residue oil were analyzed by X-ray photoelectron spectroscopy (XPS). The ζ potential and electrophoretic mobility of asphaltene solutions and residue solutions were measured through phase analysis light scattering (PALS) technique. The ability to stabilize asphaltenes of two typical ionic amphiphiles, dodecyl benzene sulfonic acid (DBSA) and dodecyl trimethyl ammonium bromide (DTAB), were investigated. Karamay asphaltenes contain large amount of carboxyl and calcium and are negatively charged; whereas Lungu asphaltenes are rich in nickel, vanadium, and pyrrolic structures and are positively charged. DBSA has good ability to stabilize Lungu asphaltenes but has no effect on Karamay asphaltenes. Differently, DTAB has good ability to disperse Karamay asphaltenes but has no obvious effect on Lungu asphaltenes. It is concluded from these results that the charges might derive from the dissociation of metal ions and the deprotonation of acid groups (such as COOH, OH, and SH) or basic groups (such as pyridinic groups) on asphaltene surface. The electric property of asphaltenes plays an important role in the interaction between asphaltenes and amphiphiles. The negatively charged asphaltenes tend to be dispersed by cationic amphiphiles, whereas the positively charged asphaltenes tend to be dispersed by anionic amphiphiles.

Study of Solvent−Bitumen−Water Rag Layers

A major operational issue in the crude oil industry is the formation of intermediate rag layers, (primarily water-in-oil emulsions) in oil−water separation processes that limit the amount and quality of recoverable oil. In this study, the formation of rag layers is evaluated as a function of solvent−bitumen−water ratios, solvent aromaticity, and temperature, with various imaging techniques. Using these techniques, it is possible to obtain an estimate of the amount of oil, water, and asphalthenes in the rag layer and excess phases. On the basis of these material balances, it was observed that when bitumen is diluted with a more paraffinic (poor) solvent, such as Heptol 80/20 (80% heptane and 20% toluene), the asphaltenes in solution tend to adsorb/segregate at exposed oil−water interfaces, impacting the extent of rag layer formation. Diluting similar systems with a more aromatic solvent (Heptol 50/50) reduces the surface activity of the asphaltenes, and the stability of rag layers, as evidenced by lower asphaltene and oil losses to the rag layer. Furthermore, it was observed that increasing the temperature of the system minimizes rag layer formation and the fraction of oil lost to the rag layer. The better separation at high temperature could be explained by the lower viscosity of the oil, which results in improved oil drainage from the rag layer.

Interactions between Mica Surfaces across Crude Oil and Asphaltene Solutions

The behavior of adsorbed surface active molecules at mica surfaces from crude oil and from their extracted asphaltenes dissolved in different solvents was investigated. The interactions between mica surfaces across a crude oil and across asphaltene solutions using different solvents and humidity conditions were measured using the surface forces apparatus. The nature of the interactions between the adsorbed layers strongly depends on their composition and on the presence of dissolved water. The results clearly indicate that the adsorbed layer/oil interface for a crude oil is significantly different from that for an asphaltene/toluene interface.

Oil chemistry and its impact on heavy oil solution gas drive

A series of laboratory depletion experiments are conducted at reservoir temperatures with three different live crude oils, as well as mineral oils. These tests examine the role of oil composition on the heavy-oil solution gas drive process. The morphology of gas bubbles, pore pressure, critical gas saturation and oil recovery is compared. Core-level depletion indicates that oil composition plays a role in determining metastability of dispersed gas bubbles in foamy oil. Generally, systems that exhibit effluent gas bubbles on the order of the size of pores tend to recover more oil, all other factors being equal. The concentration of organic acid and base groups as well as asphaltene content of crude oil is measured to characterize each crude-oil sample and develop an understanding of the influence of oil chemistry. Results suggest that significant asphaltene content as well as substantial acid number (the amount of potassium hydroxide in milligrams that is needed to neutralize the acid groups in one gram of crude oil) and base number (the amount of potassium hydroxide in milligrams needed to neutralize the acid titrant for one gram of crude oil) are indicators of whether oil is foamy. The partitioning of acid and base groups between the asphaltene fraction and deasphalted oil is also studied. Organic acid and base groups are clearly present in the asphaltene fraction. The role of such functional groups on oil foam stability is investigated by measuring the lifetime of single foam films formed from crude oil and asphaltene solutions. Transparent micromodels etched with a sandstone pore network and containing gas dispersed within the oil are also used to investigate the correlation of acid number, base number, and asphaltene content on gas-bubble coalescence. The results show that a high concentration of asphaltene that exhibits acid and base functional groups tends to increase foamability and film lifetime of gas/crude-oil dispersions. The deasphalted fraction is not foamy despite possessing significant acid and base number. We conclude that acid and base groups within asphaltene, and their interaction at the gas–oil interface, are a source of interfacial stability.

Asphaltene multilayer growth in porous medium probed by SANS

Presence of suspended particles such as asphaltene in crude oils could significantly affect the production by means of deposition in porous media especially near the well bore. We investigate this phenomenon using the ability of Small Angle Neutron Scattering technique to probe directly the asphaltene adsorption process in a porous medium at the nanometer length scale under flow conditions. A device based on a quartz tube filled with SiC particles constitute the porous medium in which an asphaltene solution in a mixture of good (toluene)/bad (heptane) solvent is injected under controlled flow. The contrast matching technique enables to match the porous medium scattering contributions and to measure the signal of the deposit. Such a device can be used for curves surface measurements on a setup originally designed for bulk studies and permit thus the direct comparison with measurements on flat surfaces (neutron reflectivity) and indirect adsorption measurements (adsorption isotherm). We show here that asphaltene in good solvent leads to a monolayer whereas addition of bad solvent results in a multilayer growth which is consistent with the deposition behaviour described in the literature.

Study of Asphaltene Nanoaggregation by Nuclear Magnetic Resonance (NMR)

The colloidal nature of asphaltenes impacts various physical properties of crude oils and exhibits a series of aggregation phenomena. Optical diffusion experiments at very low concentrations have measured the size of asphaltene monomers; ultrasound experiments have observed a nanoaggregation transition in toluene. In this work, we use a suite of nuclear magnetic resonance (NMR) methods to study the molecular dynamics and thermodynamics of aggregation in dilute asphaltene solutions. The molecular size of the single asphaltenes and aggregates is determined from NMR diffusion measurements. We observe an abrupt drop of the asphaltene diffusion constant at the onset of aggregation. We also measured the hydrogen index and observed a reduction in the spin−echo signal because of restricted motion of the alkyl chains upon aggregation. The enthalpy of aggregation is found to be negative, most likely because of π-stacking interactions. The entropy of aggregation is found to be positive, which is unexpected for a nonpolar solution. We also discuss the interpretation of these results.

A combined QCM and XPS investigation of asphaltene adsorption on metal surfaces

To investigate asphaltene–metal interactions, a combined quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS) study of asphaltene adsorption on a gold surface was conducted. Adsorption experiments were conducted at 25 °C with solutions of asphaltenes in toluene at concentrations ranging from 50 to 1500 ppm. QCM measurements yielded information on the kinetics of adsorption and further assessment of the data allowed the estimation of equilibrium adsorption levels. XPS analysis of adsorbed and bulk asphaltene demonstrated the presence of carboxylic, thiophenic, sulfide, pyridinic and pyrrolic type functional groups. The intensity of the main carbon (C–H) peak was related to surface coverage of adsorbed asphaltene as a function of asphaltene concentration by a simple mathematical model. The mass adsorption data from the QCM experiments also allowed estimation of the surface coverage, which was compared to those from XPS analyses. Surface coverage estimates as a function of asphaltene concentration could be described by a Langmuir (type-I) isotherm. The free energy of asphaltene adsorption was estimated to be − 26.8 ± 0.1 and − 27.3 ± 0.1 kJ / mol from QCM and XPS data, respectively assuming asphaltene molar mass of 750 g/gmol. QCM and XPS data was also analyzed to estimate adsorbed layer thickness after accounting for surface coverage. The thickness of the adsorbed asphaltene estimated from both XPS and QCM data analyses ranged from 6–8 nm over the entire range of adsorption concentrations investigated.