MOLECULAR MECHANICS OF AGGREGATES OF ASPHALTENES AND RESINS OF THE ATHABASCA OIL

An analysis of the effects of an almost continuous chemical distribution of asphaltenes and resins on the molecular recognition processes occurring in crude oil indicates that their aggregates will have a broad distribution both in the chemical composition and in the strength of the intermolecular interactions responsible for the aggregation. Then, crude oil cannot be described just as a sol formed by solid asphaltene particles dispersed by resins or as a simple micellar system of asphaltene and resin molecules. The molecular aggregates may vary from solid particles formed by asphaltenes and resins to loosely bound micelles with quite short lifetimes. These different aggregates may coexist within the crude oil and many will exchange components with others. The entropic contributions to the changes in free energy upon aggregation were also discussed. Molecular mechanics calculations showed that a model asphaltene aggregate from Athabasca exhibits stronger interactions with its resins than with solvents such as toluene and n-octane. The resins showed a considerable selectivity for the different adsorption sites of the asphaltene aggregate. This selectivity was stronger than that found for the solvent molecules, indicating that it is enthalpically more favorable for them to form aggregates with the asphaltenes. The selectivity may also help to explain the specificity of some resins that are able to disperse only the asphaltenes of certain types of crude oils while failing to do the same for others.     

黏土稳定剂和稠油降黏剂对稠油乳状液破乳性能的影响

为研究三次采油过程中黏土稳定剂和稠油降黏剂对破乳的影响机理,采用界面膨胀流变仪考察了阳离子型黏土稳定剂NT-1A、阴离子和非离子复配型稠油降黏剂JN-1A对滨南原油及四组分破乳体系界面膜强度的影响,应用原子力显微镜观察了NT-1A、JN-1A对沥青质聚集结构和形貌的影响。结果表明:原油中沥青质有较高的扩张模量;NT-1A通过静电吸引作用促进沥青质分子交联聚集体的形成,使聚集体粒径由489 nm增至665nm,导致界面膜强度增强,原油界面扩张模量由3.23 m N/m增至5.93 m N/m,脱水率由77.27%降至7.80%;JN-1A在油水界面上的吸附能力较强,部分顶替原油中的活性组分,抑制沥青质分子在油水界面上的聚集,聚集体粒径由489 nm降至215 nm,导致界面膜强度降低,原油界面扩张模量由3.23 m N/m降至1.68 m N/m,脱水率由77.27%增至87.50%。NT-1A、JN-1A主要通过改变原油中的活性组分沥青质的交联强度,影响界面扩张模量,从而影响稠油乳状液的破乳效果。     

Quantifying the Role of Ultrasonic Wave Radiation on Kinetics of Asphaltene Aggregation in a Toluene–Pentane Mixture

Abstract

Recently, ultrasonic wave technology has received much attention as a method for removal of asphaltene deposits from the near wellbore region. However, very little is known about another feature of this technology on the kinetics of asphaltene molecules aggregation. In this work, the kinetics of asphaltene flocculation in several crude oil samples exposed to ultrasonic waves for different time intervals is studied by confocal microscopy. The colloidal structural evolutions of flocks are described by analysis of size distribution of flocculated asphaltene particles. The results show that for the first 90 min of flocculation time, the size of aggregates increases rapidly, and a reaction-limited aggregation model matches well with the experimental data for all samples. But, after 90 min, a reduction in aggregate size of sonicated samples is observed, whereas the aggregate size of nonsonicated oil samples increases in close agreement with the diffusion-limited aggregation model. It has been found that asphaltene flocculation of sonicated samples cannot obey classic Derjaguin-Ladau-Verwey-Overbeek (DLVO) theory of colloidal dispersions due to partial reversibility of flocculation. An optimum value for ultrasonic radiation time, at which the viscosity and flocculation rate of asphaltenic crude oils reduce to a minimum, is found to be close to 10 min for Sarvak crude oil. The results of this study illustrate two different behaviors associated with asphaltene aggregation in the case of sonicated oil samples.     

Metal porphyrin occlusion: Adsorption during asphaltene aggregation

Metal compounds such as vanadyl and nickel porphyrins present in crude oils are associated to the most polar fraction defined as asphaltenes. The amount and kind of these compounds in the crude oil depend of the variations in origin, maturity, pressure, and temperature. During the asphaltene colloid formation the metal porphyrins could be trapped, occluded, or adsorbed, protecting these compounds from oxidation or chemical degradation. In this work the authors induce the aggregation of asphaltenes by changing the solubility parameter of the media in presence of metal porphyrins. UV-vis absorbance is used to monitored asphaltene aggregation at 350 nm to follow the variations in asphaltene aggregation and 405 nm to follow the porphyrin Soret band. The results showed that more than 50% of the porphyrins were trapped inside the asphaltene aggregate and posterior re-dissolution of the aggregates in toluene showed very low release of the porphyrins, demonstrating the strong interaction between the porphyrin and asphaltenes during the aggregation.     

Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy: II. Concentration dependencies of absorptivities

The properties of molecular aggregation in toluene solutions of a crude oil and solid asphaltenes are determined almost solely by the concentration of asphaltenes, as shown by absorptivity measurements at 315–750 nm. From non-monotonic concentration dependencies of absorptivities, it is concluded that asphaltene monomers are abundant in solutions with asphaltene concentrations below 1–5 mg/l, while molecular aggregates are effectively formed above 20–25 mg/l. The most stable oligomers are a dimer and a dimer pair (Yen's “nanocrystallite” [NC]). Nanocrystallites act as building blocks for more complex aggregates at asphaltene concentrations exceeding 90–100 mg/l. These optical absorption results are supported by studies of Rayleigh scattering in asphaltene solutions.     

AGGREGATION OF ASPHALTENES OBTAINED FROM A BRAZILIAN CRUDE OIL IN AROMATIC SOLVENTS

Surface and interfacial tensions in model systems formed by a Brazilian crude oil n-pentane insolubles (C5I) and n-heptane insolubles (C7I) in three different aromatic solvents: toluene, pyridine and nitrobenzene, were measured at room temperature, employing an automatic tensiometer and using the ring method. The results obtained indicated possible asphaltene aggregation and allowed the determination of critical micelle concentrations (c.m.c) for both C5I and C7I fractions in each of the three solvents considered. In toluene and pyridine solutions, the C5I fraction consistently presented higher c.m.c. values indicating a lower tendency of association for the organic fraction contained in the C5I, and absent in the C7I. In nitrobenzene solutions, this extra organic fraction appears to facilitate asphaltene association, possibly due to the formation of mixed aggregates. Average molecular areas for asphaltenes adsorbed at different interfaces were estimated using measured tensions and found to be in agreement with literature values and suggest a the flatwise surface adsorption of asphaltene molecules. This is consistent with the currently accepted stacking aggregation mechanism of asphaltenes.     

沥青质沉淀点的测定与模型化计算

通过自行设计、组装了一套采用透光率法测定原油体系沥青质沉淀点的实验装置,并采用该仪器测定了6个浓度的沥青质+甲苯体系及4个浓度的原油+甲苯体系的正戊烷和正己烷沥青质沉淀点。模型计算中采用目前具有代表性的Hirschberg沥青质沉淀溶解度模型以拟合实验数据。结果发现由实验数据回归得到的原油溶解度参数明显高于状态方程法的计算结果;原始条件下沥青质的溶解度参数值明显低于被提纯后沥青质的溶解度参数值。     

AGGREGATION OF ASPHALTENES OBTAINED FROM A BRAZILIAN CRUDE OIL IN AROMATIC SOLVENTS

Abstract

Surface and interfacial tensions in model systems formed by a Brazilian crude oil n-pentane insolubles (C5I) and n-heptane insolubles (C7I) in three different aromatic solvents: toluene, pyridine and nitrobenzene, were measured at room temperature, employing an automatic tensiometer and using the ring method. The results obtained indicated possible asphaltene aggregation and allowed the determination of critical micelle concentrations (c.m.c) for both C5I and C7I fractions in each of the three solvents considered. In toluene and pyridine solutions, the C5I fraction consistently presented higher c.m.c. values indicating a lower tendency of association for the organic fraction contained in the C5I, and absent in the C7I. In nitrobenzene solutions, this extra organic fraction appears to facilitate asphaltene association, possibly due to the formation of mixed aggregates. Average molecular areas for asphaltenes adsorbed at different interfaces were estimated using measured tensions and found to be in agreement with literature values and suggest a the flatwise surface adsorption of asphaltene molecules. This is consistent with the currently accepted stacking aggregation mechanism of asphaltenes.     

ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

Effect of pressure,temperature, and mass fraction of CO2 on the stability of the asphaltene constituents in crude oil

This paper focus on the main influence factors (temperature, pressure, and mass fraction of CO₂) on the state of asphaltene in the crude oil during CO₂ flooding by using high temperature and high pressure microanalysis system of solid precipitation. For the simulated oil sample – CO₂ system, the state of asphaltene is not affected by temperature within this range of 50°C to 100°C, the particle size of the asphaltene has an increase with the increase of the pressure from 8MPa to 40 MPa. When the mass fraction of CO₂ is less than 35%, the state of the asphaltene has not changed and the asphaltene particles are in a suspension state. When the mass fraction of CO₂ increases to 40%, the aggregation of the asphaltenes occurs and then form precipitation. With the further increase of the mass fraction of CO₂, the particle of the asphaltene aggregates has a significant increase. For the field development project design of CO₂ flooding, the influence of the temperature can be ignored, the appropriate mass fraction of CO₂ is below 35% and the gas injection pressure should maintain a relatively low value. The results can provide a theoretical basis to avoid the asphaltene precipitation during CO₂ flooding.     

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

Abstract

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.     

高凝原油降凝剂的制备及其降凝机理研究

 由丙烯酸十八酯、马来酸酐和醋酸乙烯酯三元共聚物的胺解改性物（MAVA）和乙烯-醋酸乙烯酯的共聚物（EVA）为原料,通过复配制备了一种高凝原油降凝剂(PPD),并对其降凝效果和降凝机理进行了研究。结果表明,所制备的高凝原油降凝剂对凝点为43℃的胜利原油具有明显的降凝效果,在加剂量为400?g/g时,原油的凝点降低了11℃;由原油中的沥青质和胶质所形成的聚集体结构是原油的蜡晶成核剂,降凝剂可以同原油中的沥青质和胶质结合形成新的沥青质-降凝剂-胶质聚集体结构,该聚集体结构作为加剂原油的蜡晶成核剂,可以改变蜡的结晶方式,延缓蜡晶的析出速率,大幅地降低了原油的凝点。     

Study of asphaltene deposition from Tahe crude oil

Borehole blockage caused by asphaltene deposition is a problem in crude oil production in the Tahe Oilfield, Xinjiang, China. This study has investigated the influences of crude oil compositions, temperature and pressure on asphaltene deposition. The asphaltene deposition trend of crude oil was studied by saturates, aromatics, resins and asphaltenes (SARA) method, and the turbidity method was applied for the first time to determine the onset of asphaltene flocculation. The results showed that the asphaltene deposition trend of crude oil by the turbidity method was in accordance with that by the SARA method. The asphaltene solubility in crude oil decreased with decreasing temperature and the amount of asphaltene deposits of T739 crude oil (from well T739, Tahe Oilfield) had a maximum value at 60 o C. From the PVT results, the bubble point pressure of TH10403CX crude oil (from well TH10403CX, Tahe Oilfield) at different temperatures can be obtained and the depth at which the maximum asphaltene flocculation would occur in boreholes can be calculated. The crude oil PVT results showed that at 50 , 90 and 130 o C, the bubble point pressure of TH10403CX crude oil was 25.2, 26.4 and 27.0 MPa, respectively. The depth of injecting asphaltene deposition inhibitors for TH10403CX was determined to be 2,700 m.     

ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

ABSTRACT

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

EFFECT OF ASPHALTENE AND RESINS ON THE STABILITY OF WATER-IN-WAXY OIL EMULSIONS

Asphaltene, resins and paraffin waxes, their mutual interactions and their influence on the stability of water-in-oil emulsions have been studied. 20 wt % paraffin wax dissolved in decalin was used to model the waxy crude oil. Asphaltene and resins separated from a crude oil were used to stabilize the water-in-oil emulsions. Synthetic formation water was utilized as the aqueous phase of the emulsion. The emulsion stability increased with increasing the concentration of asphaltene with a subsequent decrease in the average particle size distribution of the emulsion. Resins alone are not capable of stabilizing the emulsion, however, in the presence of asphaltene they form very stable emulsions. Dynamic viscosity and pour point measurements provided evidence for resins-paraffin waxes interactions. Asphaltene in the form of solid aggregates form suitable nuclei for the wax crystallites to build over with a mechanism similar to that of paraffin wax crystal-modifiers. As asphaltene are polar in nature they are derived at the oil/water interface which was proved by the ability of asphaltene to reduce oil/water interfacial tension. Consequently, nucleation of the wax crystallites by asphaltene and resins at the interface will add to the thickness of the oil-water interfacial film and hence increase the stability of the emulsion.     

PEPTIZATION AND COAGULATION OF ASPHALTENES IN APOLAR MEDIA USING OIL-SOLUBLE POLYMERS

The asphaltene fraction of crude oil contains a variety of acidic and basic functional groups. During oil production and transportation, changes in temperature, pressure or oil composition can cause asphaltenes to precipitate out crude oil through the flocculation among these polar functional groups. In this study, two types of oil-soluble polymers, dodecylphenolic resin and poly (octadecene maleic anhydride), were synthesized and used to prevent asphaltenes from flocculating in heptane media through the acid-base interactions with asphaltenes. The experimental results indicate that these polymers can associate with asphaltenes to either inhibit or delay the growth of asphaltene aggregates in alkane media. However, multiple polar groups on a polymer molecule make it possible to associate with more than one asphaltene molecule, resulting in the hetero-coagulation between asphaltenes and polymers. It was found that the size of the asphaltene-polymer aggregates was strongly affected by the polymer-to-asphaltene weight (or number) ratio. At low polymer-to-asphaltene weight ratios, asphaltenes keep flocculating with themselves and with polymers until the floes precipitate out of solution. On the other hand, at high polymer-to-asphaltene weight ratios, asphaltene-polymer aggregates peptized by the extra polymer molecules can remain fairly stable in the solution.     

The Solubility and Three-Dimensional Structure of Asphaltenes

The tendency of the asphaltenes to form aggregates in hydrocarbon solution is one of their most characteristic features and has tended to complicate the determination of the structure of petroleum In addition, if the composition and properties of the precipitated asphaltenes reflect those of the micelles in solution, the latter should be considered as mixed micelles. This is a reasonable assumption in view of the large quantities of soluble resins found in the precipitated solid

Empirical observations indicate that the resins play an important role in stabilizing asphaltenes in crude oil and under unfavorable solvent conditions the asphaltene species are prone to further aggregation into clusters that are unstable and precipitate from the crude oil. It is also suggested that the resins and the asphaltenes from a particular crude oil have points of structural similarity relative to the asphaltenes and resins from another crude oil. On a more localized scale, i.e. in one particular crude oil there are also structural differences within the constituents of asphaltenes and structural differences within the constituents of the resins are also anticipated

Therefore, the structure of the micelles within any one crude oil must be expected to be varied and non-homogenous. From the evidence cited herein, it follows that the potential for graphite-type stacking by the asphaltene molecules in the center of a micelle might not be as great as the potential for the micelles forming by asphaltene-resin interactions rather than by asphaltene-asphaltene interactions     

PEPTIZATION AND COAGULATION OF ASPHALTENES IN APOLAR MEDIA USING OIL-SOLUBLE POLYMERS

ABSTRACT

The asphaltene fraction of crude oil contains a variety of acidic and basic functional groups. During oil production and transportation, changes in temperature, pressure or oil composition can cause asphaltenes to precipitate out crude oil through the flocculation among these polar functional groups. In this study, two types of oil-soluble polymers, dodecylphenolic resin and poly (octadecene maleic anhydride), were synthesized and used to prevent asphaltenes from flocculating in heptane media through the acid-base interactions with asphaltenes. The experimental results indicate that these polymers can associate with asphaltenes to either inhibit or delay the growth of asphaltene aggregates in alkane media. However, multiple polar groups on a polymer molecule make it possible to associate with more than one asphaltene molecule, resulting in the hetero-coagulation between asphaltenes and polymers. It was found that the size of the asphaltene-polymer aggregates was strongly affected by the polymer-to-asphaltene weight (or number) ratio. At low polymer-to-asphaltene weight ratios, asphaltenes keep flocculating with themselves and with polymers until the floes precipitate out of solution. On the other hand, at high polymer-to-asphaltene weight ratios, asphaltene-polymer aggregates peptized by the extra polymer molecules can remain fairly stable in the solution.     

EFFECT OF ASPHALTENE AND RESINS ON THE STABILITY OF WATER-IN-WAXY OIL EMULSIONS

ABSTRACT

Asphaltene, resins and paraffin waxes, their mutual interactions and their influence on the stability of water-in-oil emulsions have been studied. 20 wt % paraffin wax dissolved in decalin was used to model the waxy crude oil. Asphaltene and resins separated from a crude oil were used to stabilize the water-in-oil emulsions. Synthetic formation water was utilized as the aqueous phase of the emulsion. The emulsion stability increased with increasing the concentration of asphaltene with a subsequent decrease in the average particle size distribution of the emulsion. Resins alone are not capable of stabilizing the emulsion, however, in the presence of asphaltene they form very stable emulsions. Dynamic viscosity and pour point measurements provided evidence for resins-paraffin waxes interactions. Asphaltene in the form of solid aggregates form suitable nuclei for the wax crystallites to build over with a mechanism similar to that of paraffin wax crystal-modifiers. As asphaltene are polar in nature they are derived at the oil/water interface which was proved by the ability of asphaltene to reduce oil/water interfacial tension. Consequently, nucleation of the wax crystallites by asphaltene and resins at the interface will add to the thickness of the oil-water interfacial film and hence increase the stability of the emulsion.     

The Role of Asphaltene Solubility and Chemical Composition on Asphaltene Aggregation

Asphaltenes from four different crude oils (Arab Heavy, B6, Canadon Seco, and Hondo) were fractionated in mixtures of heptane and toluene and analyzed by small angle neutron scattering (SANS). Fractionation appeared to concentrate the most polar species into the least soluble sub-fraction as indicated by elemental analysis. SANS results indicated a wide spectrum of asphaltene aggregate sizes and molecular weights; however, the less soluble (more polar) fraction contributed the majority of the species responsible for asphaltene aggregation in solution. This more polar, less soluble fraction is likely the major cause for many petroleum production problems such as deposition and water-in-oil emulsion stabilization. A comparison of molecular weight and aggregate size indicated that asphaltenes formed fractal aggregates in solution with dimensions between 1.7 and 2.1. This was consistent with the “archipelago” model of asphaltene structure. Resins were shown to effectively solvate asphaltene aggregates as observed by an increase in asphaltene solubility, reduction in aggregate size and molecular weight, and an increase in the fractal dimension to ~ 3.