Energy & Fuels 2005, 19, 101-110
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Characterization of Polarity-Based Asphaltene Subfractions Piyarat Wattana and H. Scott Fogler Department of Chemical Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, Michigan 48109
Andrew Yen Baker Petrolite Corporation, 12645 West Airport Blvd., Sugar Land, Texas 77478
Marı`a Del Carmen Garcı`a and Lante Carbognani PDVSA-INTEVEP, Caracas, Venezuela Received March 11, 2004. Revised Manuscript Received September 10, 2004
Structural and compositional characterization of asphaltenes that were extracted from unstable crude oils, stable crude oils, and organic solid deposits was performed to elucidate their similarities and differences. A fractionation technique that divided the asphaltenes into different subfractions, based on polarity, was used to characterize these asphaltene samples. The parameters affecting the stability of these asphaltene subfractions were elucidated. The asphaltenes that were extracted from unstable crude oils and from solid deposits contained substantially greater portions of the higher polar fractions and have a higher polarity, compared to the asphaltenes obtained from crude oils with no asphaltene stability problems in the field. The dielectric constant, solubility, and flocculation experiments showed that these higher-polarity fractions have a greater tendency to aggregate and are more difficult to remediate. These results suggested that the presence of a certain type of asphaltenesparticularly, a high-polarity asphalteneshas a key role in the stability of asphaltene in crude oils.
1. Introduction Asphaltenes are complex mixtures of heteroatom-rich polycyclic hydrocarbons, naturally suspended in crude oil.1-3 Asphaltenes, which are the heaviest and the most polar fraction in crude oil, have a major role in the formation of deposits that affect the production of crude oil worldwide.4-6 The deposition of asphaltenic organic scales causes severe problems in all facets of the production, transportation, and refining of crude oils, thereby increasing the cost of oil production.7-9 Despite the significant impact of asphaltene deposition on the economics of oil production, the aggregation and deposi(1) Bestougeff, M. A.; Byramjee, R. J. Chemical Constitution of Asphaltenes. In Asphaltenes and Asphalts; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, 1994; pp 67-94. (2) Yen, T. F. Asphaltenes: Types and Sources. In Structures and Dynamics of Asphaltenes; Sheu, E. Y., Ed.; Plenum Press: New York, 1998; pp 1-20. (3) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999. (4) Kokal, S. L.; Sayegh, S. G. Asphaltenes: The Cholesterol of Petroleum. In 9th SPE Middle East Oil Show Conference, Manama, Bahrain, March 11-14, 1995; Society of Petroleum Engineers: Richardson, TX, 1995; Paper No. SPE-29787, pp 169-181. (5) Duyck, C., et al. Trace Element Determination in Crude Oil and Its Fractions by Inductively Coupled Plasma Mass Spectrometry Using Ultrasonic Nebulization of Toluene Solutions. Spectrochim. Acta, Part B 2002, 57 (12), 1979-1990. (6) Yen, T. F., Chilingarian, G. V., Eds.; Asphaltenes and Asphalts; Developments in Petroleum Science Series 40A; Elsevier Science: Amsterdam, 1994.
tion mechanisms have not been completely elucidated. Crude oil can be considered as a colloidal system whose disperse phase is composed of asphaltenes and resins. The stability of crude oil is thought to be dependent on both the dispersion medium and the colloidal disperse phase.10 Numerous research has focused on prediction of the asphaltene precipitation. deBoers et al. suggested that the tendency for asphaltene precipitation was dependent on the extent to which crude oil is saturated with asphaltenes, its saturation with gas under the downhole condition, and the density of the crude oil under the reservoir condition.11 However, a bottomholepressurized crude oil sample is expensive to obtain. Moreover, an analysis must be performed on a special elevated-pressure and elevated-temperature apparatus. (7) Leontaritis, K. J.; Mansoori, G. A. Asphaltene Flocculation During Oil Production and Processing: A Thermodynamic Colloidal Model. In Proceedings of the SPE International Symposium on Oil Field Chemistry, San Antonio, TX, 1987; Society of Petroleum Engineers: Richardson, TX, 1987; Paper No. SPE-16258. (8) Jiang, T. S.; Kawanaka, S.; Mansoori, G. A. Asphaltene Deposition and its Role in Petroleum Production and Processing. Arabian J. Sci. Eng. 1988, 13 (1), 17-34. (9) Thawer, R.; Nicholl, D. C. A.; Dick, G. Asphaltene Deposition in Production Facilities. SPE Prod. Eng. 1990, 5, 475-480. (10) Laux, H.; Rahimian, I.; Butz, T. Thermodynamics and Mechanism of Stabilization and Precipitation of Petroleum Colloids. Fuel Process. Technol. 1997, 53, 69-79. (11) de Boer, R. B., et al. Screening of Crude Oils for Asphalt PrecipitationsTheory, Practice, and the Selection of Inhibitors. SPE Prod. Facil. 1995, 10 (1), 55-61.
10.1021/ef0499372 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/23/2004
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Figure 1. Physical appearance of asphaltene separated from (a) West Texas solid deposit and (b) West Texas crude oil. Table 1. Properties of Crude Oil Samples crude oil sample
saturates
Content (%) aromatics resins
asphaltenes
NM1 FR1
36.9 39.9
37.9 36.5
Unstable Crude Oils 19.4 5.8 17.1 6.5
NM5 FRE2
43.5 42.2
43.2 35.2
11.3 18.3
Louisiana West Texas
63.5 52.4
Stable Crude Oils 2.0 4.3
asphaltene/resin
(asphaltene + saturate)/ (aromatic + resin)
0.30 0.38
0.75 0.87
0.18 0.23
0.83 0.87
Crude Oils from Fields from which Solid Deposits are Obtained 28.2 7.2 1.1 0.15 42.5 4.8 0.3 0.06
A less-expensive and approximate method to predict the tendency of asphaltene precipitation is based on the properties of stock tank oil. Amount of saturates, aromatics, resin, and asphaltenes fractions in crude oils can be obtained from SARA analysis.3 The ratio of the amount of resins to the amount of asphaltenes in the crude oil has been used as a stability index.3 Recently, the ratio the amount of saturates and asphaltenes to the amount of aromatics and resins has been considered as a colloidal instability index (CII) for petroleum fluids.12 It has been proposed that the amount of aromatics and resins reflect the dispersive power, whereas the amount of saturates and asphaltenes reflects the tendency of the asphaltenes to flocculate. Analysis over the large SARA database has shown that the accuracy of the CII, in regard to predicting crude oil stability, is better than the asphaltenes-to-resins ratio.13 It has been suggested that the nature of the asphaltenes is one of the main factors affecting the stability of crude oil.14 Our research seeks a better understanding of the fundamental properties of asphaltenes. A characterization of asphaltene samples from unstable, stable, and field deposits was conducted to explore the major characteristics that differentiate these samples and the cause of asphaltene deposition. This information could lead to better prevention and remediation techniques. 2. Methods and Materials 2.1. Materials. Asphaltene samples were extracted from unstable and stable crude oils. The unstable crude oils, which cause deposition problems in production, are Col-2, NM1, and (12) Loeber, L., et al. Bitumen in Colloid Science: A Chemical, Structural and Rheological Approach. Fuel 1998, 77 (13), 1443-1450. (13) Asomaning, S. Test Methods for Determining Asphaltene Stability in Crude Oils. Pet. Sci. Technol. 2003, 21 (3-4), 581-590. (14) Leo´n, O.; Rogel, E.; Espidel, J.; Torres, G. Asphaltenes: Structural Characterization, Self-Association, and Stability Behavior. Energy Fuels 2000, 14, 6-10.
1.82 1.11
FR1 crude oils. NM5 and FRE2 crude oils are classified as stable crude oils, according to their good stability in the field. In addition to asphaltene samples from unstable and stable crude oils, asphaltene samples that were extracted from solid deposits from two oil fields and asphaltene samples precipitated out from the matching dead crude oils were also studied. The first solid deposit and matching dead crude samples came from a West Texas oil field. As this oil field aged, the producer has used CO2 flooding to extract oil after water flooding. This CO2 injection resulted in asphaltene deposition that caused the failure of electrical submersible pumps, and deposition in the well tubing severely restricted the production flow. One well had to be shut down as a result of the asphaltene deposits in the lower 1800 ft of tubing, which had restricted the 2 7/8 in. inner diameter (ID) tubing to ∼1/2 in.15 The second samples in this study were obtained from a Louisiana oil field. The producer in the Louisiana field had experienced a decline in oil production and found a solid mixture of sand and asphaltene deposits in the wellbore. The asphaltene samples used in this study have been classified into three groups: unstable asphaltenes, stable asphaltenes, and solid deposit asphaltenes. These classifications were based on their stability in the field from which they were obtained. The asphaltene samples from the solid deposit were categorized as the most unstable asphaltenes, because of the deposition problem they created. On the other hand, the asphaltene samples from the matching crude oils are considered to be more-stable asphaltenes, because of their ability to be stabilized and remain in the crude oils after the precipitation of asphaltenes occurred in the fields. the crude oils used in the study are stock tank oil at atmospheric pressure. All crude oils, solid field deposits, and asphaltene samples were stored in a sealed container at 4 °C. The weight percentages of saturates, aromatics, resin, and asphaltenes fractions (SARA analysis) of crude oils used in this study are shown in Table 1. The West Texas and Louisiana crude oil are the crude oils obtained from the field where the solid deposit has been (15) Yin, Y. R.; Yen, A. T.; Asomaning, S. Asphaltene Inhibitor Evaluation in CO2 Floods: Laboratory Study and Field Testing. In SPE Permian Basin Oil and Gas Recovery Conference, Midland, TX, 2000: Society of Petroleum Engineers.
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Table 2. Summaries of Fractionation Results wt % of each fraction as at volume ratio of CH2Cl2:pentane asphaltene source Col2 crude NM1 crude FR1 crude NM5 crude FRE2 crude Louisiana deposit Louisiana crude West Texas deposit West Texas crude
F40/60
F30/70
F20/80
F10/90
supernatant
14 6 21
Unstable Crude Oils 49 11 59 6 61.7 3.8
5 22 0.3
21 7 13.2
Stable Crude Oils 48 18 63.5 18
23 0.6
11 16.7
0.9 6.6 0.3 14.2
19.6 18.8 15.8 16.7
0 1.2
Solid Field Deposit and Its Matching Dead Crude Oils 34.1 36.1 9.3 0 56 18.6 43.3 32.1 8.5 0 17 52.1
reported. The weight percentage of the asphaltenes fraction of the West Texas and Louisiana crude oils is in a low range (0.3 and 1.1, respectively), because the SARA analysis of the West Texas and Louisiana crude oils reflected the composition of crude oil, in which some asphaltenes has already been precipitated out and deposited in the field. 2.2. Sample Preparation. Asphaltenes were precipitated from crude oils with a 40:1 volume ratio of n-heptane to crude oil, following the IP-143/90 standard procedure. The precipitated asphaltene samples were then washed with heptane until the washing solvents were colorless. For the solid field deposits, asphaltene samples were extracted from solid field deposits from Louisiana and West Texas fields. First, solid deposits were washed in decane, to remove paraffinic materials from the deposit. The solid deposits were then washed in heptane until the washing solvents were colorless. Next, the treated solid deposits were dissolved in toluene. The insoluble inorganic material, such as sand or clay, was separated out and the soluble portion was dried in a vacuum desiccator until all of the solvent evaporated. The dried solids were collected and used as the asphaltene samples from the solid deposit. Asphaltene samples were also extracted from the matching Louisiana and West Texas crude oils, following the same procedure that was described previously. Figure 1 shows the physical appearance of asphaltene that has been extracted from the solid deposit and from the crude oil. The asphaltene sample from the solid deposit was dense and shiny, whereas the asphaltene sample from the crude oil was powdery and dull. 2.3. Fractionation Procedure. The asphaltene samples were fractionated into different polar fractions, using the fractionation technique developed earlier in our research group.16 A binary mixture of polar (CH2Cl2) and nonpolar (npentane) solvents was utilized. The unfractionated asphaltene was first completely dissolved with CH2Cl2 at ratio of 1:10 by weight. Pentane was then added in discrete increments of 10 vol % until the first fraction precipitated out. The first asphaltenes fraction that precipitated out is the most polar fraction, followed by precipitation of lesser polar fractions when more n-pentane is added. The fraction precipitated was separated by centrifugation at 3500 rpm for 30 min in a Beckman TJ-6 centrifuge (relative centrifugal field of 1300 g), after which pentane was then added to the supernatant to obtain the next fraction. The process repeated until no more polar fractions precipitated. The precipitated polar fractions were dried in vacuum desiccators until there was no change in mass. The mass of each polar fraction precipitated was then measured. 2.4. Chemical Composition and Structure Analysis. The elemental composition of carbon, hydrogen, nitrogen, oxygen, and sulfur was analyzed using a elemental analyzer (16) Nalwaya, V.; Tantayakom, V.; Piumsomboon, P.; Fogler, S. Studies on Asphaltenes through Analysis of Polar Fractions. Ind. Eng. Chem. Res. 1999, 38 (3), 964-972.
(LECO). Trace metal contents (vanadium, nickel, and iron) were measured using inductively coupled plasma (ICP) spectrometry (Quantitative Technologies, Inc.). The Fourier transform infrared (FTIR) spectra of the asphaltenes were obtained (Mattson Instruments, Galaxy Series 3000). The spectrophotometer was used at a setting of 128 scans/min. The spectra were obtained for a solid sample consisting of 2 wt % of asphaltene powder in potassium bromide (FTIR grade). Number-average molecular weights of asphaltenes were determined in nitrobenzene at 120 °C, using vapor pressure osmometry (VPO) (Corona Wescan, by D&H Mahlow, Ltd.). 2.5. Asphaltene Dielectric Constant Measurements. Dielectric constant measurements were performed at the Vanton Research Laboratory in Lafayette, CA. Dielectric constants of asphaltene-in-toluene solutions were measured using a dielectric cell that was connected to an impedance analyzer (Hewlett-Packard, model HP 4192A). The cell was first calibrated with toluene. All measurements were performed at a frequency of 100 kHz and a temperature of 25 °C. 2.6. Asphaltene Refractive Index Measurements. The refractive index of the asphaltene-in-toluene solution was measured using a Schmidt and Haensch DUR-HT refractometer. The temperature of the samples and the measuring chamber were kept constant, using a temperature-controlled bath. All measurements were performed at 25 °C. 2.7. Determination of Asphaltene Dipole Moment. The dipole moment of asphaltenes can be determined from dielectric constant and refractive index measurements.17,18 The relation between the dielectric constant of the dilute solution of polar molecules (s) in a nonpolar solvent (m) and dipole moment was described by Onsager’s equation:19
[
]
3n2m n2m(n2s + 2) 2 Nsµ 2 2 (1) ) n2m + φs 2 (n n ) + s m 2nm + n2s 2n2m + n2s 30kBT where is the dielectric constant of solution, n the refractive index, φs the volume fraction of the polar solute, Ns the number of solute molecules in a unit volume, 0 the permittivity of free space (8.85 × 10-12 J-1 C2 m-1), kB the Boltzmann’s constant (1.38 × 10-23 J/K), T the temperature (in Kelvin), and µ the dipole moment of the polar solute (given in units of Debye). 2.8. Asphaltenes Solubility Measurements. The solubility of asphaltene samples in a mixture of alkane/aromatic solvents was determined. A stock solution of saturated asphaltene in a binary solvent mixture of 60 vol % toluene and 40 vol % heptane was first prepared. This stock solution was (17) Goual, L.; Firoozabadi, A. Measuring Asphaltenes and Resins, and Dipole Moment in Petroleum Fluids. AIChE J. 2002, 48 (11), 2646-2663. (18) Maruska, H. P.; Bhaskara, M. L. R. The Role of Polar Species in the Aggregation of Asphaltenes. Fuel Sci. Technol. Int. 1987, 5 (2), 119-168. (19) Onsager, L. Electric Moments of Molecules in Liquids. J. Am. Chem. Soc. 1936, 58 (8), 1486-1493.
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3. Results and Discussion
Figure 2. Distribution of asphaltene polar fractions of the asphaltene samples separated from the West Texas solid field deposit and its matching dead crude oil. distributed in vials, and heptane was added to each vial to achieve the necessary volume percentage of toluene in solvent mixtures in the range of 10%-50%. The samples were then placed in the sonicator bath for 30 min, after which time they were left outside the sonicator bath at room temperature until equilibrium was reached. The solid asphaltenes precipitated was then separated from the solution by centrifugation at 3000 rpm for 45 min. The concentration of asphaltenes in the supernatant was then analyzed using ultraviolet-visible (UVVis) spectroscopy at a wavelength of 450 nm. 2.9. Determination of the Stability of an Asphaltene Subfraction. The relative stability of each asphaltene subfraction was examined by measuring the onset of flocculation of the asphaltene-in-toluene solution. A solution of 0.2 wt % asphaltenes in toluene was prepared by completely dissolving 20 mg of asphaltene sample in 10 mL of toluene. The solution was then titrated with n-heptane at a constant rate of 1 cm3/ min under stirring. The titration was monitored by means of a guided wave spectrophotometer at a wavelength of 768 nm. All experiments were conducted at room temperature. The intensity of light through the solution mixture was recorded during the course of the titration. The flocculation onset point is defined as the point at which the first inflection point of light intensity occurred. In addition, the effect of the lower-polarity fraction on the stability of the higher-polarity fractions was investigated. A flocculation experiment was performed with a mixture of a high-polarity fraction (F40/60) and a low-polarity fraction (F10/90 or F20/80). Ten milligrams of a low-polarity fraction (F10/90 or F20/80) was added to 10 mL of the 0.2 wt % F40/60 fraction/toluene solution. The volume fractions of heptane at the onset of flocculation of the systems with and without the addition of a low-polarity fraction were then compared.
3.1. Fractionation. A summary of the fractionation results is presented in Table 2. F40/60, which denotes the fraction precipitated at 40 vol % CH2Cl2 and 60 vol % pentane, is the most polar fraction for all unstable asphaltenes from the unstable crude oils and from the field deposits. Each of the unstable asphaltenes contains a significant amount of this fraction. On the other hand, the stable FRE2 asphaltenes contain merely 1% of the F40/60 fraction and the stable NM5 asphaltenes contain none of this fraction. The distribution of asphaltene polar fractions extracted from the West Texas solid deposit and its dead crude are shown in Figure 2. The distribution indicates that the asphaltenes from the solid deposit consist mainly of the higher polar fractions, whereas the asphaltene from the matching crude oil from the same field from which the solid deposit was removed consist mainly of the lower-polarity fractions. This result suggests that the asphaltenes from the solid deposit are more polar, when compared to the asphaltenes from the matching crude oil from the same field. Previous work has shown that crude oils with greater amounts of the higher-polarity fractions have a tendency to be unstable.20 These results suggest that the higherpolarity fractions have a greater tendency to precipitate and form deposits in the field. 3.2. Chemical Composition and Structure Analysis. The chemical composition and structure of the asphaltene samples were examined to investigate their relationship to the stability of the asphaltenes. For this purpose, the asphaltene samples extracted from solid field deposits and their matching crude oils, which represent the unstable and stable asphaltene samples obtained from the same oil field, were chosen for further study. Table 3 compares the chemical compositions of asphaltenes extracted from solid deposits with the chemical composition of asphaltenes obtained from their matching crude oils from the Louisiana and West Texas oil fields. The elemental composition of carbon, hydrogen, nitrogen, oxygen, and sulfur was analyzed using the LECO elemental analyzer. The compositions of the metals (vanadium, nickel, and iron) in the asphaltenes shown in the table were determined using the ICP spectrometry technique. There is no significant difference in the elemental composition of asphaltene samples between the solid deposits and those obtained from the crude oils. However, one does observe that the asphaltene samples from solid deposits contained a greater
Table 3. Chemical Compositions of West Texas and Louisiana Asphaltene Samples West Texas Asphaltene parameter elemental composition (wt %) carbon hydrogen nitrogen sulfur oxygen total heteroatoms atomic H/C ratio metal content, via ICP vanadium (ppm) nickel (ppm) iron (ppm) VPO-measured molar mass (g/mol)
deposit
crude
Louisiana Asphaltene deposit
crude
84.71 7.15 1.24 3.01 1.3 5.55 0.97
85.78 7.16 1.19 2.71 1.34 5.24 0.94
85.66 6.95 0.89 0.53 2.64 4.06 1.01
86.24 6.78 1.23 0.65 3.19 5.07 1.00
640 355 1200 3759
190 266 178 3259
15 72 1200 2699
13 63 526 1886
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Figure 3. Fourier transform infrared (FTIR) spectra of asphaltenes from (- - -) West Texas deposit and (s) West Texas crude oil.
amount of metals (vanadium, nickel, and iron) than the asphaltene samples from crude oils did. The molar mass of asphaltene samples from the solid deposits and those from the crude oil were determined via VPO, using nitrobenzene as a solvent at 120 °C. The average molar masses of the samples are also shown in Table 3. The measurement of asphaltene molecular weight using the VPO method varies over a range of 1000-7000 g/mol, depending on the solute concentration, the nature of the solvents, and the temperature. It has been reported that the unfractionated asphaltene has a molar mass of ∼3000 g/mol.21 The results suggest that average molar masses of the asphaltene from the solid deposit are higher than the asphaltenes from the crude oil. Note that the molar mass determined via VPO might not give a true molecular weight, because of the aggregation of asphaltenes. The FTIR spectra of the asphaltene samples obtained from the West Texas solid deposit and the corresponding dead crude oil are shown in Figure 3. The spectra show no noticeable differences, indicating that the functional groups of both asphaltene samples from the solid deposits and the matching crude oils are similar. 3.3. Dielectric Behavior and Dipole Moment of Asphaltenes. The fractionation results showed that the asphaltene samples extracted from the solid deposits consist mainly of the higher polar fractions, whereas the asphaltene samples that separated from the matching crude oil from the same field consist mainly of lower polar fractions. It has been shown that, among the four SARA fractions of crude oils, asphaltenes contain the highest amount of heteroatoms (mainly N, O, and S) and have the greatest metal content.5 The presence of such heteroatoms and metallic elements leads to charge imbalances, on an atomic level, and should produce permanent electrical dipoles in the asphaltene molecule. The magnitude of polarity of a molecule is expressed in (20) Wattana, P.; Wojciechowski, D. J.; Bolan˜os, G.; Fogler, H. S. Study of Asphaltene Precipitation Using Refractive Index Measurement. Pet. Sci. Technol. 2003, 21 (3&4), 591-613. (21) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Investigation of Asphaltene Association with Vapor Pressure Osmometry and Interfacial Tension Measurements. Ind. Eng. Chem. Res. 2000, 39 (8), 2916-2924.
Figure 4. Dielectric constant as a function of weight percentage of (a) the Louisiana crude oil asphaltenes-in-toluene solution and (b) the Louisiana solid deposit asphaltenes-intoluene solution.
term of dipole moment22 and can be determined from dielectric constant and refractive index measurements.19 The dielectric constants of the solutions of asphaltenes are shown as a function of weight fraction of the asphaltenes in toluene in Figure 4. Figure 4a shows the dielectric constants of the solution of the asphaltenes obtained from the Louisiana crude oil, and Figure 4b shows the dielectric constants of the solution of the asphaltenes obtained from the Louisiana solid deposit. The dielectric constant of the solution of the asphaltenes obtained from the Louisiana crude oil exhibited an S-shaped curve. In the concentration range of 0-0.25 wt %, the dielectric constant of the solution increases linearly as the weight percentage of asphaltenes in the toluene solution increases. At a concentration of 0.25 wt %, the dielectric constant of the solution started to deviate from the straight line. It has been observed, from the examination of fifty compounds of widely different nature, that the dielectric constant of a dilute solution is a linear function of the weight percentage of solute when there is no intermolecular interaction or (22) Exner, O. Dipole Moments in Organic Chemistry; Georg Thieme Publishers: Stuttgart, Germany, 1975.
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association among solute molecules.23,24 However, intermolecular interactions or associations of solute molecules result in a deviation from the linear relationship. The S-shaped curve of the dielectric constant of the Louisiana crude oil asphaltene solution indicated that the aggregation of the asphaltenes occurs at an asphaltenes concentration of >0.25 wt %. The dielectric constant of the solution of the asphaltenes obtained from the Louisiana solid deposit exhibited a sharp jump from the dielectric constant of the toluene solvent to the dielectric constant of the 0.0042-wt %-asphaltenes-intoluene solution and then showed a linear increase in the dielectric constant of the asphaltene solution with an increase in weight percentage of the asphaltenes in toluene over the concentration range of 0.0042-1 wt %. The sharp increase in dielectric constant of the Louisiana solid deposit solution at a concentration of 0.0042 wt % indicates that the aggregation of the asphaltenes occurs even in a very dilute solution. These results show that the asphaltenes obtained from the solid deposit have a very strong tendency to aggregate at a concentration as low as 0.0042 wt %. The asphaltenes obtained from crude oil also showed an aggregation tendency but to a lesser extent, compared to the asphaltenes obtained from the solid deposit. One objective of this work is to determine the dipole moment of asphaltenes. Ideally, the dipole moment of a molecule should be determined from the dielectric constant data of a dilute solution, where interaction between individual molecules is at a minimum. However, because of the sensitivity limitations of the dielectric constant instrument, the dielectric constant measurement at a lower range of asphaltene concentration (e.g.,
