Asphaltene Solutions Research Articles

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.

Molecular Thermodynamic Approach to the Formation of Mixed Asphaltene−Resin Aggregates

The influence of resins on asphaltene aggregation is studied using a molecular thermodynamic approach. The formation of mixed resin−asphaltene aggregates is described using a model previously developed for asphaltene aggregates. In this model, the aggregates are described as aromatic cores composed of stacked aromatic sheets surrounded by aliphatic chains. Using this simple description for the aggregates, an analytical expression is developed for the free energy of mixed aggregation. This expression incorporates solubility, mixing, and interfacial and steric effects. The proposed model predicts that the presence of resins decreases the size of the aggregates and also increases the cmc of the asphaltene solutions in agreement with the experimental observed behavior. The effectiveness of different resins to split the asphaltene aggregates depends upon the solubility of their polyaromatic ring. The lower the solubility, the larger the effectiveness of the resins because more resins are able to be incorporated inside the aggregates. In addition, the thermodynamic molecular model developed does not use any information derived from experiments on asphaltene/resin solutions, and therefore, it is strictly predictive.

Interactions between Asphaltenes and Water in Solutions in Toluene

Binding of water by asphaltenes dissolved in toluene was investigated for two asphaltene samples, OMV1 and OMV2, from the same reservoir deposit. Solubility of water in asphaltene solutions in toluene was found to increase with an increasing asphaltene concentration, indicative of solubilization of water by asphaltenes. Fourier transform infrared (FTIR) spectroscopy of stretching modes of OH groups in the region of 3800−3100 cm−1 was used to obtain insight into the state of water in water-unsaturated asphaltene solutions in toluene. The number of water molecules bound to one asphaltene molecule was determined for water-saturated solutions (OMV1 and OMV2) and for water-unsaturated solutions (OMV1 only). These numbers were found to decrease from several water molecules per asphaltene molecule to below unity upon an increase of the asphaltene concentration in toluene from 0.1 to 20 g/L, suggesting that binding of water by asphaltenes dramatically depends upon the aggregated state of asphaltenes and that the mechanism of water solubilization by asphaltenes differs significantly from water solubilization in reverse micelles formed by conventional surfactants.

Aggregation−Dissociation Studies of Asphaltene Solutions in Resins Performed Using the Combined Freeze Fracture−Transmission Electron Microscopy Technique

Solutions of asphaltenes in resins (3%) were studied combining the freeze fracture and transmission electron microscopy techniques (FF-TEM). These studies were performed by analyzing samples prepared at different temperatures ( T) from 25 up to 250 °C. In this temperature range, mean particle diameters ( D p) were found to drop from about 5 to about 3.5 nm. The smallest diameter measured at each temperature ( D S) reached a constant value equal to 2.5 nm in the range between 100 and 250 °C. Frequencies f( D) of all but D S values were found to decrease at some temperatures reveling that diameters D > D S correspond to aggregates (Ag). Moreover, during this heating, we found that whereas for big (B) and smallest diameters frequencies f( D) either decrease (big) or increase ( D S) continuously, maxima were found for intermediate (I) diameters. This suggests that under the conditions of this experiment (temperature and time) conversion from intermediate to smallest diameter meets some energy barrier high enough for the accumulation of intermediate size diameters. These results were interpreted in terms of the consecutive process A g ( B ) → T k 1 A g ( I ) → T k 2 A 1 M where A1M stand for A1-type molecules and k 1 > k 2; consequently, because of the low solubility of A1M, conversion from intermediate aggregates to molecules is a difficult step. The above results and many others in the literature are in agreement with the colloidal, solubility, and aggregates hypothesis related to A1 and A2 fractions of asphaltenes.

Study of Asphaltene Precipitation by Calorimetry

Can calorimetry bring new input to the current understanding of asphaltene precipitation? In this work, two types of precipitation were studied by means of calorimetry: addition of n-heptane into asphaltene solutions and temperature/pressure variations on a recombined live oil. The first series of experiments showed that weak forces determine precipitation. Indeed, isothermal titration calorimetry could not detect any clear signal although this technique can detect low-energy transitions such as liquid–liquid equilibrium and micellization. The second series of tests proved that precipitation caused by T and P variations is exothermic for this system. Furthermore, the temperature-induced precipitation is accompanied by an increase in the apparent thermal expansivity. Therefore, it seems that these two phase transitions exhibit different calorimetric behaviours and they may not be as similar as expected.

Frequency Dependence of Oil Conductivity at High Pressure

Electric conductivity of materials is defined by a competition of several mechanisms of charge carrier movement: phoretic, ionic and electron‐hole jumping. The relative contribution of them is not constant when thermodynamic conditions are changing. Paraffine and naphtene hydrocarbons are typical dielectrics. Formation of supermolecular structures is accompanied by ordering of asphaltene‐resin and polyaromatic molecules. Solvate shells contain the molecules with less molecular masses. The disperse particle from the centre to periphery is characterized by reduction of conductivity from the values peculiar to “good” semiconductors in a nucleus, “average” in intermediate phase to typical for dielectrics in the disperse media. Threshold of the mobility of charge carriers is determined by temperature, pressure, strength of the external field, width of the forbidden zone, and nature of components of the disperse media and phase. Spectra of conductivity of model asphaltene solutions and Kumkolskaya oil in frequency range from 10−3 up to 106 Hz are measured for pressures up to 1 GPa in temperature interval from 250 to 320 K. Particularly at the analysis of spectra of conductivity the technique offered by Sheu and Mullins is used. Dependences were approximated by power‐law functions according to the concept of Jonscher's “the universal response.” Influence of pressure and temperature to the exponents is analyzed. Values of activation energy are determined. Received data can be useful for prognosis of oil phase behavior at high pressures.

A New Procedure for Direct Precipitation and Fractionation of Asphaltenes from Crude Oil

A new precipitation and fractionation procedure was established where asphaltenes were precipitated successively by stepwise addition of pentane. The first fraction was obtained by mixing 3∶1 volumes of n‐pentane/crude oil followed by filtration. The following fractions were obtained by addition of more pentane to the crude oil with filtration between each step. Four fractions were precipitated with this procedure giving fractions based on a pentane‐to‐crude oil ratio of 3∶1, 10∶1, 15∶1, and 20∶1. Near infrared light at 1,600 nm was used to detect precipitation of asphaltenes in solutions of heptane and toluene. The asphaltenes precipitated showed an increasing solubility in heptane/toluene mixtures from the first precipitate to the last one. Interfacial tension was determined using the pendant drop technique. These measurements showed that the 10∶1 intermediate fraction exhibited a considerable reduction of the interfacial tension between toluene and water compared to the other fractions. Obviously the surface activity varies very much (almost by a factor of 2) between the different solubility classes within the asphaltene family. This may have large technical consequences within crude oil production and processing.

Surface characterization of adsorbed asphaltene on a stainless steel surface

X-ray photoelectron spectroscopy was used to characterize a single layer of adsorbed asphaltene on a metallic surface. The deposits were created by immersing a stainless steel disc into a dilute asphaltene solution with either toluene or dichloromethane as the solvent, although the toluene solution allowed for better control of the adsorbed asphaltene layer and less atmospheric oxygen contamination. The analyses for C 1s, S 2p3/2, N 1s and O 1s photoemission peaks indicated that different functional groups are present in the asphaltene layer including carboxylic, pyrrolic, pyridininc, thiophenic and sulfite, with slight differences in their binding energies.

T-C Phase Diagram of Asphaltenes in Solutions

Comparative analysis of previously available and new experimental data reveals a complex structure of ambient pressure T-C phase diagram of asphaltenes in solutions, as well as in native petroleum. One of the characteristic properties of the diagram is an apparent series of temperature-controlled transitions between structural states in primary asphaltene nanoaggregates, presumably related to different types of intermolecular bonding. Hence, various earlier models of primary aggregates may be more closely related than conventionally believed.

Characterization of the Charge Carriers in Bitumen

This work aims to identify the nature of the charge carriers in Athabasca bitumen to better understand the mechanisms of emulsion stabilization and electrically induced separations. A quartz crystal microbalance (QCM) with nanogram sensitivity is used to determine the electrodeposited mass in toluene-based systems. The use of low voltages for the first time allows a small amount of deposited material to be determined accurately. The method consists of applying a DC voltage between two gold-coated QCM crystals immersed in toluene solutions of asphaltenes, maltenes, or solid-free bitumen. Application of a 250 V/cm electric field strength produces an electrocollection of cationic and anionic species at the negatively and positively charged QCM crystals, respectively. A small fraction of the electrocollected material eventually electrodeposits and forms a transparent colorless film at the electrode surfaces. The specific mass (i.e., mass over charge) is found to be higher for the positive species than it is for the negative species. With this method, the characteristic time of relaxation of the bituminous solutions is estimated during the early stage of voltage application. This time could be used to establish a minimum frequency criterion for the critical field methods of water electrocoalescence in oil. Solids separated from bitumen are mainly negatively charged, and their specific mass is relatively high. This study shows that electrodeposition can efficiently separate solids and solid-stabilized water droplets from bitumen. Thus, an electrostatic treatment processes could be successful in removing solids and water from diluted bitumen systems.

Circular dichroism of crude oil and its derivatives. Role of permolecular structures

Petrols with different octane numbers, diesel fuel and solutions of crude oil in toluene have been studied by the methods of absorption spectroscopy, circular dichroism spectroscopy and correlation spectroscopy of scattered light. Circular dichroism signal was registered for crude oil solutions in the spectral ranges corresponding to the measured earlier resonance absorption of asphaltene solutions in toluene. We show that the optical activity of crude oil solutions is due to the aggregation of asphaltene molecules in one spectral range and is intensified with the aggregation of asphaltene molecules in another spectral range. Petrols have no optical activity. The optical activity registered for diesel fuel is possibly due to the aggregation of asphaltene molecules.

Equipartitioning of Precipitant Solubles between the Solution Phase and Precipitated Asphaltene in the Precipitation of Asphaltene

It is shown by sequential reprecipitation that both n-C5 and n-C7 Athabasca asphaltene contain significant amounts of precipitant solubles, comprising resins and low-molecular-weight asphaltene-like substances in about the same percentage amounts relative to the residual precipitated insoluble asphaltene. All of the several hundred resinous molecules detected in the n-C5 solubles, with some minor exceptions, were also detected in the resin fraction of the maltene. From an estimation of the concentrations of several compound classes in the solution and solid asphaltene phases, values were derived for the equipartitioning coefficients. The results implicate aromatic stacking as the dominant mode of association of resins and asphaltene. The aggregate amount of n-C5 solubles associated with the n-C5 insoluble plus the aggregate amount of n-C7 solubles associated with the n-C7 insoluble residual asphaltene obtained in the sixth reprecipitation is more than 50% of the latter, and in the native oil, it is estimated to exceed 70%. This fraction, representing a fifth compound class of crude oil, plays a paramount role in the stabilization of the colloidal solution of asphaltene and should be investigated more extensively.

A simplified thermodynamic modeling procedure for predicting asphaltene precipitation

In this work, a simplified thermodynamic modeling procedure for determination of the amount of asphaltene precipitation is presented. The onset of asphaltene precipitation in the presence of different solvents is predicted by applying the molecular solubility model. Since asphaltene molecular weight varies in different solvents, the molecular weight of asphaltene has been tuned by experimental data. The tuned molecular weight is used to predict the amount of precipitated asphaltene. The Flory–Huggins statistical thermodynamics theory is applied to derive the Gibbs free energy relationship. A necessary and sufficient condition is defined to minimize the Gibbs free energy relation. By solving the set of equations derived from the minimization, the weight percent of precipitated asphaltene has been determined. The advantage of this model is that one does not need to perform expensive and time consuming experiment in order to obtain molecular weight data. The other improvement of this model compared to the other models is the requirement of a minimum amount of experimental data to find the unknown parameters. It also has the advantage of obtaining solubility parameters through thermodynamics relationships rather than using the relation for regular solution when the asphaltene solution is a non-regular solution. The results of this model are in good agreement with the experimental data.

High-molecular-mass components of pyrolysis tar as photosensitizers of decomposition of aliphatic alcohols and liquid paraffins

Photodecomposition of aliphatic alcohols and liquid paraffins sensitized by asphaltenes and resins of heavy pyrolysis tar (HPT > 400°C) was studied by ESR and low-temperature photolysis. It was shown that the photoirradiation of frozen solutions of asphaltenes and resins in alcohols and paraffins generated alcohol and hydrocarbon radicals. The photosensitizing properties of resin-asphaltene compounds was found to facilitate the photodegradation of organic compounds via both one-and two-photon mechanisms with a more efficient use of solar energy.

Solvent Entrainment in and Flocculation of Asphaltenic Aggregates Probed by Small-Angle Neutron Scattering

While small-angle neutron scattering (SANS) has proven to be very useful for deducing the sizes and masses of asphaltenic aggregates in solution, care must be taken to account for solvation effects within the aggregates so as to not err in the characterization of these important systems. SANS measurements were performed on solutions of asphaltenes dispersed in deuterated solvents in which a broad spectrum of solute and solvent chemical compositions was represented. Fits to the scattering intensity curves were performed using the Guinier approximation, the Ornstein-Zernike (or Zimm) model, a mass-fractal model, and a polydisperse cylinder model. The mass-fractal model provided apparent fractal dimensions (2.2-3) for the aggregates that generally decreased with increasing aggregate size, indicating increased surface roughness for larger aggregates. The polydisperse cylinder model provided typical values of the particle thicknesses from 5 to 32 angstroms, the average particle radius from 25 to 125 angstroms, and approximately 30% radius polydispersity. Subsequent calculation of average aggregate molar masses suggested a range of solvent entrainment from 30 to 50% (v/v) within the aggregates that were consistent with previous viscosity measurements. Additional calculations were performed to estimate the proportion of microparticle to nanoparticle aggregates in the solutions. The results indicate that the inclusion of solvation effects is essential for the accurate determination of aggregate molecular weights and fractal dimensions.

High-Q Ultrasonic Determination of the Critical Nanoaggregate Concentration of Asphaltenes and the Critical Micelle Concentration of Standard Surfactants

Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water interfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenes in good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the critical micelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate for asphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration (CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q, ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shown to exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods, we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with the literature. Using density determinations, we obtain micelle compressibilities consistent with previous literature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocity versus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L, although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, using asphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar to micellar compressibilities obtained with standard nonionic surfactants in toluene. These results strongly support the contention that asphaltenes in toluene can be treated roughly within the micelle framework, although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore, our extensive results on known surfactants agree with the literature while the asphaltene CNACs reported here are one to two orders of magnitude lower than most previously published results. (Previous work utilized the terminology "micelle" and "CMC" for asphaltenes.) We believe that the previously reported high concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they refer to higher levels of aggregation or perhaps to a particular surface structure.

Asphaltene Deposition on Metallic Surfaces

The potential for asphaltene deposition in wellbores and flowlines is a major concern during design of oil production and transportation facilities, especially in deep‐water environments. Understanding the processes that control asphaltene deposition, especially the relationship between precipitation and deposition, can help to reduce the risk and cost. Stainless steel capillary tubes were used to study the influences of factors including temperature, degree of asphaltene instability, and precipitant molar volume on asphaltene deposition from mixtures of stock‐tank oils and n‐alkanes. Temperature varied from 20°C to 60°C. Pressure drop across the capillary tube was used to estimate the amount and distribution of deposit formation. Existing asphaltic particles in stock‐tank oil samples did not create deposits. Below the wax appearance temperature, intermittent pressure spikes indicated deposition of wax. Above the wax appearance temperature, deposition occurred gradually from near‐onset mixtures created by co‐injection of oil and n‐alkane precipitants. Asphaltenes flocculated by addition of higher molar volume n‐alkanes deposited more material than those flocculated by lower molar volume n‐alkanes. Examination of the deposited materials showed that they contained not only asphaltenes but also waxes. Much different depositional characteristics were observed for solutions of asphaltenes in an aromatic solvent than for the stock‐tank oil from which they were derived.

Methodology for the Characterization and Modeling of Asphaltene Precipitation from Heavy Oils Diluted with n-Alkanes

A regular solution model, previously used to model asphaltene precipitation from Western Canadian bitumens, was tested on four international heavy oil and bitumen samples. The input parameters for the model are the mole fraction, the molar volume, and the solubility parameter for each component. Heavy oils and bitumens were divided into four main pseudo-components, corresponding to the SARA fractions (saturates, aromatics, resins, and asphaltenes). Asphaltenes were divided into fractions of different molar mass, based on a gamma molar mass distribution. The molar volumes and solubility parameters of the pseudo-components were calculated using solubility, density, and molar mass measurements and previously developed correlations. Model predictions were compared with the measured onset and the amount of asphaltene precipitation for solutions of asphaltenes in toluene and n-heptane and for heavy oils diluted with n-alkanes, all under ambient conditions. The overall average absolute deviations (AAD) of the pr...

Effects of Temperature and Pressure on Asphaltenes Agglomeration in Toluene. A Light, X-ray, and Neutron Scattering Investigation

Small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and dynamic light scattering (DLS) are used to study the temperature dependence of asphaltene aggregate sizes in toluene solutions. A large range of aggregate sizes is covered by combining the three scattering methods. The effect of temperature on aggregate size is also investigated over a considerable temperature range. A huge modification of asphaltene macrostructure is observed. At high temperatures, reversible aggregation of asphaltene leads to stable small entities. When decreasing the temperature, irreversible aggregation of asphaltene occurs, corresponding to a large increase of the aggregate size. We have also investigated the effect of pressure on asphaltene solution in toluene as a function of temperature. Our results indicate that pressure has a minor effect, which is much less important than that of temperature, on the weight of the asphaltene aggregates.

Asphaltene Flocculation, Precipitation, and Liesegang Ring

Precipitations of Ratawi and Kuparuk asphaltene in toluene/heptane mixed solvents were investigated using a newly developed dripping test method. Precipitation of asphaltene solution at a critical heptane volume ratio can be readily identified. In addition, for the first time, a Liesegang-like phenomenon [Liesegang, R. E. Naturwiss. Wochenschr. 1896, 11, 353] was observed for the Ratawi asphaltene system. Similar but less-profound Liesegang rings were observed for asphaltene derived from Kuparuk crude oils. Clear crossover and a diffusive hyperfine structure were observed in the Ratawi asphaltene precipitation pattern, whereas asphaltene derived from Kuparuk crude oil showed only marginal crossover phenomenon. The patterns of the rings from Ratawi asphaltene seem to fit the space law of the Liesegang pattern. Both the post-nucleation Ostwald−Prager process and the pre-nucleation spinonal decomposition phase separation process are possible precipitation mechanisms.