Simulation of Interactions in Asphaltene Aggregates - American

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Energy & Fuels 2000, 14, 566-574

Simulation of Interactions in Asphaltene Aggregates Estrella Rogel† Departamento de Produccio´ n, PDVSA-INTEVEP, Apdo. 76343, Caracas-1070A, Venezuela Received August 2, 1999. Revised Manuscript Received January 14, 2000

Molecular mechanics and dynamics calculations were carried out for asphaltenes and resins from crude oils of different origins. Average structural models were used in order to study the forces that determine the association process of asphaltenes and resins in crude oils. The stabilization energies obtained for asphaltene and resin associates were due mainly to the van der Waals forces between the molecules. Comparatively, the contribution of the hydrogen bonding to the stabilization energy was low. It was also found that few structural changes occur in the molecules due to aggregation. Therefore, the contribution of the conformation changes to the stabilization energy was also very low. According to the results obtained, the stabilization energy of the associates depends on the structural characteristics of the molecules. In particular, the stabilization energy is more favorable for those molecules with lower hydrogen-to-carbon ratio, higher aromaticity, and higher aromatic condensation degree. When different solvents were used to break the associates, this tendency was confirmed. The breaking of the associates occurred for those molecules with higher hydrogen-to-carbon ratio and lower aromaticity. These results can be linked to the experimental finding of a greater tendency to precipitate for those asphaltenes with low hydrogen-to-carbon ratio and high aromaticity.

Introduction Crude oil production is often affected by asphaltene precipitation that can block the pores of reservoir rocks and can also plug the wellbore tubing and other equipment.1 In fact, asphaltene deposition is a very wellknown problem that generates a large cost increase in the petroleum industry.2 Despite this, there is a lack of information concerning the mechanism of asphaltene deposition and its origins.3,4 In particular, the intermolecular forces that give rise to the association of asphaltenes remain as one of the less understood aspects related to asphaltene precipitation. According to literature, hydrogen bonding,5-8 van der Waals forces,9 and charge-transfer interactions10,11 are involved in asphaltene association. However, there is controversy about the relative importance of each of these forces in the association process.12,13 Besides, the associated structure and association dynamics are still not well understood.13 †

E-mail: [email protected]. (1) Taylor, S. E. Fuel 1992, 71, 1338. (2) Park, S. J.; Mansoori, G. A. Energy Sources 1988, 10, 109. (3) Rassamdana, H.; Dabir, B.; Nematy, M.; Farhani, M.; Sahim, M. AICHE J. 1996, 42, 10. (4) Hammami, A.; Ferworn, K. A.; Nighswander, J. A. Pet. Sci. Technol. 1998, 16, 227. (5) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55, 187. (6) Ignasiak, T. M.; Strausz, O. P. Fuel 1978, 57, 617. (7) Tewari, K. C.; Galya, L. G.; Egan, K. M.; Li, N. C. Fuel 1978, 57, 245. (8) Acevedo, S.; Leon, O.; Rivas, H.; Marquez, H.; Escobar, G.; Gutierrez, L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1987, 32, 426. (9) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (10) Wright, J. R.; Minesinger, R. R. J. Colloid Interface Science 1963, 18, 223. (11) Siffert, B.; Kuczinski, J.; Papirer, E. J. Colloid Interface Science 1990, 135, 107. (12) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 68. (13) Schabron, J. F.; Speight, J. G. Pet. Sci. Technol. 1998, 16, 361.

Table 1. Structural Parameters of the Fractions Studied14 molecular aromatic number of weight MW H/C aromati- condensation aromatic (g/mol) ratio city (Fa) degree (CI/C1) rings (Ar) resin LM1 asphaltene LM1 resin LM2 asphaltene LM2 resin NM1 asphaltene NM1 resin NM2 asphaltene NM2

800 1308 636 1158 975 1621 615 1212

1.40 1.22 1.46 1.11 1.29 0.96 1.36 0.99

0.38 0.46 0.35 0.53 0.43 0.60 0.41 0.60

0.79 1.48 0.54 1.52 1.10 2.09 0.66 1.72

6 14 4 14 9 23 5 17

For these reasons, the main purpose of this work is to gain insight into the interactions of asphaltenes, resins, and associates with different solvents. Gaining an understanding of the intermolecular interactions in these systems is fundamental to unveil the mechanism of asphaltene precipitation and also to the development of new tools for the prevention of asphaltene deposition. In the present paper, molecular dynamics calculations of resins, asphaltenes, and aggregates in the vacuum and in different solvents were carried out in order to study the intermolecular forces that determine the stability of the aggregates. Four different sets of molecular average structures of asphaltenes and resins were used to accomplish this objective. These average structural models were obtained from analytical techniques and have been reported earlier14 (Table 1). Although the use of average structural models is controversial because of its inherent ambiguity, in this work this limitation is overcome by the selection of different fractions representative of a wide range of (14) Rogel, E.; Leon, O.; Espidel, J.; Gonza´lez, J. 1999 SPE Latin American and Caribbean Petroleum Engineering Conference, SPE53998, Venezuela, 1999.

10.1021/ef990166p CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000

Simulation of Interactions in Asphaltene Aggregates

Energy & Fuels, Vol. 14, No. 3, 2000 567

Figure 1. Average structural models for asphaltenes and resins.

asphaltene stability behaviors; two of the sets of fractions are from crude oils (LM1 and LM2) classified as stable materials, and the other two come from unstable crude oils (NM1 and NM2) presenting asphaltene deposition problems. The fractions selected are also different in structure and molecular weight as can be seen in Figure 1. As a consequence, this paper describes relationships between interaction energies and structural characteristics of the fractions and also provides some insight into the nature and relative importance of the different interactions involved in asphaltene association.

Method The molecular mechanics and molecular dynamics methods used are those included in the commercial programs InsightII, Discover 2.9, and Amorphous Cell of Molecular Simulation Inc.15 The force field cvff15 (consistent valence force field) was used in all the calculations. This force field was originally developed for amino acids, water, and a variety of organic molecules. In a preliminary study, this force field correctly described the formation of dimers for a series of aromatic compounds from benzene to coronene. The charges (15) MSI, Simulation Tools InsightII Version 4.0.0, San Diego, 1996.

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of the atoms are assigned according to the database cvff and remain constant during the calculations. Average Model Structures. The models corresponding to the asphaltenes and resins studied are shown in Figure 1. The structural parameters of these fractions are given in Table 1 and were obtained in a previous work.14 The asphaltenes were extracted from crude oils according to the method described in IP-143/ 90. Elemental composition was determined on a LECO CHNS 244 elemental analyzer model. Asphaltene and resin NMR spectra were obtained on a Bruker ACP-400 spectrometer, at a resonance frequency of 400 MHz for protons. Average molecular parameters (AMP) and molecular weights (MW) were calculated according to the method developed16 by V. Leo´n. The average molecular models were constructed using a method developed by L. Carbognani et al.17 The information about heteroatom content was available only for asphaltene fractions. For this reason, only for asphaltenes was it possible to build structures with heteroatoms. The optimized structures for asphaltenes and resins were obtained using the following procedure. The potential energy of the constructed model was minimized, followed by the calculation of a molecular dynamics for 100 ps. The five conformations of lowest energy from the molecular dynamics calculation were selected, and then, the potential energy of each one was minimized again in order to lower the energy even more. Finally, the conformation with the lowest energy was selected as the most stable structure and the rest of the calculations were carried out using it. Construction of Aggregates. In the building of the aggregates, the procedure was somewhat different. By using a docking process, the best approximate spatial conformation between a pair of molecules was obtained by minimizing the interaction energy between them. In the docking process, the spatial disposition of the two molecules involved in the aggregate is changed (with no conformational changes in the molecules) and after each change, the interaction energy is calculated. During this process, different spatial conformations are tested and the conformation with the minimum interaction energy is selected. After the best spatial disposition of the molecules is obtained, a process that alternates potential energy minimization and molecular dynamics (10 ps) was carried out for the aggregate and repeated two times. Then, a minimization followed by a molecular dynamics for 100 ps was made. The five conformations of lowest energy from the molecular dynamics calculation were selected, their potential energies were minimized, and the one with the lowest energy was selected to continue the calculations. Following this procedure, dimers for all the fractions studied were built, including asphaltene-resin dimers. Molecules and Aggregates in Solvents. In these simulations, the objective was to study the interactions between aggregates or monomers and the nearest solvent molecules around them. It is important to point out that these simulations are not carried out in the liquid phase and, as a consequence, they do not represent the solubility behavior of asphaltenes and resins

in different solvents. The simulation of the interactions of molecules (monomers) and aggregates (dimers) with different solvents was carried out as follows. The solvent molecules were distributed around the minimum energy molecules or aggregates forming a layer of 10 Å. As a first step, a minimization of the potential energy of the whole system was made and then, a molecular dynamic simulation for 100 ps was carried out. The solvents used were toluene, pyridine, tetrahydrofuran, and naphthalene. The number of solvent molecules around monomers and aggregates depends on size of the layer, which is proportional to the size of the aggregate or molecules and the density of the solvent. As a consequence, the number of solvent molecules used to solvate varies. Surfaces in Contact with Solvents. As a first step toward the study of the solubility of asphaltenes and resins in different solvents, some periodical systems were built to try to simulate the dissolution process of a surface composed of resin molecules. Two resin surfaces were built using the average structure of resin LM2 and resin NM1. First, dimer and trimer structures of the resins were built using a procedure reported earlier.18 The optimized aggregates were placed in a 2D periodic unit cell, simulating an infinite surface using periodic boundary conditions in the xy plane. Consecutive steps of minimization and molecular dynamics (50 ps) were made. After each step, new resin molecules were introduced in the system to avoid the formation of holes in the generated resin layer. For the final system, a molecular dynamics simulation of 100 ps was made and the last configuration was minimized and taken as the model for the resin surface. The two surface models consisted of a layer composed of 10 resin molecules (LM2) and seven resin molecules (NM1), respectively, and covered a 30 × 30 Å2 area in the xy plane. Finally, different solvents were added: toluene, naphthalene, and n-heptane. The final size of the systems was 30 × 30 × 60 Å3. The calculation procedure for these systems was the following. A minimization followed by a molecular dynamics for 100 ps was carried out. The first 30 ps were used in an equilibration process. This procedure has the objective to relax the system to equilibrium at a new state point. This means that it is necessary to run for a period so that the system can come to an equilibrium.19 The equilibration period should be extended at least until potential energy and pressure have started to oscillate about steady mean values. At the end of the equilibration period, all memory of the initial configuration should have been lost. This implicates that the final results will not depend on the initial configuration. Following the equilibration period (the last 70 ps), a standard molecular dynamics algorithm is used to solve the classical equations of motion. Each 1000 time steps of calculation corresponds to 1 ps. 2D periodic boundary conditions were used to simulate an infinite resin surface in the xy plane. Figure 2 shows an example of these systems: a layer composed by resin molecules (stick and ball representation) which represented the surface of a deposit in contact with solvent molecules (in this case n-heptane molecules represented by sticks). The replication (boundary conditions) of the cell shown in Figure 2

(16) Leon, V. Fuel 1987, 66, 145. (17) Carbognani, L.; Espidel, J. Personal communication, 1993.

(18) Rogel, E. Colloids Surfaces A. 1995, 104, 85-93. (19) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford Press: New York, 1993.



Evdw )

∑ij[(r*ij/rij)12 - 2(r*ij/rij)6]

(2)

where ij is the potential well depth in kcal mol-1, r*ij is the interatomic distance in Å at which the minimum occurs, and rij is the distance between atoms i and j in Å.

Ee )

∑qiqj/ijrij

(3)

where qi and qj are the charges on atoms i and j, respectively. The stabilization energy is defined as:

Est ) Eass - 2Emon

Figure 2. Periodical system resin surface (LM2)-n-heptane.

in x and y axis (z axis is considered perpendicular to the resin layer) produces an infinite surface of resins in contact with n-heptane, which helps to eliminate the border effects produced if only one cell is used in the calculations. Results and Discussion Formation of Dimers. Figure 3 shows the conformations in the minimum-energy state for dimers of asphaltenes. As can be seen, there is a tendency to the association through the interaction between the aromatic sheets. It is also observed that in the asphaltene dimers NM1 and NM2, hydrogen bonds are formed. These are the fractions with the highest heteroatom content. The stabilization energies for the associates were calculated from the difference between the energy of the dimer and the sum of the energies of the molecules alone. The total energy of associates and monomers can be split in two parts:

ET ) Ebond + Enon-bond

(1)

The first term, Ebond corresponds to the bonded terms. These terms take into account bond-length torsion, dihedral angle torsion, bond-angle torsion and, in general, the terms related to the deformation of angles and distances between the atoms that composed each molecule. In relation to the second term Enon-bond, this is split in two terms in the force field cvff: van der Waals interaction energy (EvdW) represented by a Lennard-Jones function and the Coulombic representation of the electrostatic interaction energy (Ee):

(4)

where Eass is the total energy of the associate and Emon is the total energy of the monomer. The stabilization energy as it is calculated according to this equation represents the energy gained when two noninteracting monomers (at infinite separation) are come together to form an aggregate. On the other hand, the interaction energy, which represents the sum of the van der Waals interaction energy plus the electrostatic interaction energy, originates from the interaction between the atoms of the two monomers. The comparison between the interaction energy and the stabilization energy shown in Table 2 indicates that in almost all the cases the values are very similar. This means that the stabilization energy is almost completely due to the interaction energy between the monomers. Even more, the comparison of the bond energy of any aggregate to the sum of the bond energies of the corresponding monomers indicates that both values are very similar. This means that very few structural changes occurred upon aggregation, because there are not significant changes in the bond energy. On the other hand, and as it can be expected, the comparison of the nonbond energy of the aggregate to two times the nonbond energy of the corresponding monomer indicates a difference which is similar to the interaction energy. These results indicate that the stabilization energies obtained are almost completely due to the interaction energy between the molecules in the aggregates and are not due to structural changes of the molecules upon aggregation. A similar result was reported by Brandt et al.20 They studied the stacking of asphaltenes using molecular dynamics and found that very few structural changes occurred in the molecules upon stacking in the aggregates. However, an example of structural changes upon association in molecules similar to asphaltenes and resins can be found in the study of the association of molecules of bituminous coal by Takanohashi et al.21 These contrasting results can be attributed to the different kinds of molecules used in the calculations. To study the influence of the heteroatoms in the stabilization energy of the dimers, aggregates were built using asphaltenes without heteroatoms. In these asphaltenes, the heteroatoms were changed by carbon atoms. Figure 4 shows the comparison between the stabilization energies for both types of systems as a (20) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. J. Phys. Chem. 1995, 99, 10430. (21) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395.

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Figure 3. Conformations in the minimun-energy state for dimers of asphaltenes. Table 2. Stabilization Energies for the Associates and Total Energies for Monomers and Associates associates

monomers

internal energy

nonbond energy

total energy

LM1 LM2 NM1 NM2

518.2 499.5 684.9 463.0

613.1 560.0 891.2 663.6

1131.3 1059.5 1576.1 1126.6

LM1 LM2 NM1 NM2

517.6 494.2 679.3 477.3

611.5 585.1 912.8 732.3

1129.1 1079.3 1592.1 1209.6

LM1 LM2 NM1 NM2

176.7 129.0 278.0 215.9

324.9 224.0 484.1 309.5

501.6 353.0 762.1 525.4

internal energy

stabilization energy(kcal/mol)

interaction energy (kcal/mol)

Asphaltenes (with heteroatoms) 261.9 351.0 612.9 249.9 330.0 579.9 343.0 533.0 876.0 229.9 395.3 625.2

-94.5 -100.4 -172.9 -123.8

-102.4 -114.3 -195.9 -136.0

Asphaltenes (without heteroatoms) 259.1 356.2 615.3 245.0 349.0 594.0 338.7 543.9 882.6 241.6 424.7 666.3

-101.4 -108.8 -173.2 -123.0

-112.1 -116.2 -173.5 -123.9

-61.1 -41. 6 -80.6 -42.7

-63.0 -41.3 -82.0 -49.9

88.9 68.7 140.2 107.1

nonbond energy

Resins 192.4 128.5 281.2 177.0

function of the hydrogen-to-carbon ratio (H/C). As it can be seen, at least for these structures, the presence of heteroatoms does not affect significantly the stabilization energy. Even more, according to Figure 4, for asphaltenes NM1 and NM2, the formation of hydrogen bonds between the molecules does not contribute in a significant way to the stabilization energy of the aggregates. It is important to indicate that in the cvff force field, hydrogen bonds are a natural consequence of the standard van der Waals and electrostatic parameters.15 According to these results, the stabilization energy is mainly due to the van der Waals interactions between the molecules, since the contribution of electrostatic interactions to the interaction energy is less than 5%

total energy

281.3 197.3 421.4 284.1

in all the cases studied in the present work. Recently, similar results were reported by Takanohashi et al.21,22 and Murgich et al.23 They reported the preponderance of the van der Waals forces in the association of asphaltenes and other related molecules. Considering that these previous studies used molecular models that are very different in structural characteristics to the models used in the present work, it is possible to suppose that the preponderance of the van der Waals forces in the association of asphaltenes occurs for a wide spectra of different molecules. (22) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1998, 12, 1168. (23) Ignasiak, T.; Strausz, O. P.; Montgomery, D. S. Fuel 1977, 56, 359.


Figure 4. Stabilization energies as a function of the H/C ratio for dimers of asphaltenes with and without heteroatoms.

Experimentally, although the presence of hydrogen bonding in asphaltenes has been proved by many authors5-7,23,24 during the last twenty years using different techniques, the relative importance of hydrogen bonding in the association process of asphaltenes is still not clear. The decrease in molecular weight of the alkylated asphaltene molecules8,23 has been used as a proof of the role of hydrogen bonding in the aggregation of asphaltenes. It is supposed that the reductive alkylation destroys the hydrogen bonding in asphaltenes and similar molecules.24 However, other authors25,26 have shown that the chemistry of reductive alkylation also destroys the aromatic stacking interactions in petroleum residue. In other words, the decrease in molecular weight cannot be completely attributed to the breaking of the hydrogen bonding interactions. On the other hand, studies of the solubility of asphaltenes and residue fractions in different solvents9,27,28 have shown the major role played by the van der Waals forces in the solubilization of asphaltenes. In fact, it has been shown that the solubility parameter of the asphaltenes depends mainly on the hydrogen and carbon contents9 and not on the heteroatom content, which is supposed to be strongly related to the number of hydrogen bonds. Even more, the measurements of refractive index made by Buckley et al.27,28 indicate that London dispersion forces dominate aggregation and precipitation of asphaltenes. This finding also indicates that the most important role in aggregation of asphaltenes is played by van der Waals forces that include London dispersion forces. Stabilization Energies and Molecular Parameters. The stabilization energies of the dimers as a function of the main structural parameters of asphaltenes and resins are shown in Figure 5. These parameters are hydrogen-to-carbon ratio (H/C), aromaticity (Fa: number of aromatic carbon to total number of carbon ratio), aromatic condensation degree (CI/C1:number of bridging aromatic carbon to number (24) Taylor, S. R.; Li, N. C. Fuel 1978, 57, 117. (25) Speight, J. G.; Moschopedis, S. E. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1981, 26, 907. (26) Monin, J. C.; Vignat, A. Rev. Inst. Fr. Pe´ t. 1984, 39, 821. (27) Buckley, J. S. Fuel Sci. Technol. Int. 1996, 14, 55. (28) Buckley, J. S.; Hirakasi, G. J.; Liu, Y.; Von Drasek, S.; Wang, J.-X. Fuel Sci. Technol. Int. 1998, 16, 251.


Figure 5. Stabilization energies of the dimers as a function of the structural parameters of the molecules.

Figure 6. Comparison between the stabilization energies and the values calculated according to eq 1.

of peripheric nonbridging carbon ratio) and molecular weight (MW). For all these variables, there are clear tendencies that show more favorable stabilization energies for those molecules with lower hydrogen-to-carbon ratio, higher aromaticity, higher aromatic condensation degree, and higher molecular weight. Recently, Wiehe and Liang9 reported a direct relationship between solubility parameter and weight percent hydrogen together with the molecular weight. They found that the relative interaction energy measured for all fractions from saturates to coke correlated inversely with the H/C ratio and directly with MW. Based on this finding, a correlation between the stabilization energy of the dimers (SE) and the two variables H/C and MW was calculated:

SE ) 71.9 - 63.1 × (H/C)-1 - 0.11 × MW

(5)

A comparison between the stabilization energies and the values calculated from eq 5 is shown in Figure 6. This result supports the findings of other authors9,12,27,28 that suggested a major role for the dispersion forces in the aggregation and solubility behavior of asphaltenes and other fractions. On the other hand, it is important to point out that the dimers of asphaltenes from unstable crude oils show

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Figure 7. Stability of the crude oils (flocculation onset and unstability index) as a function of the stabilization energies of the asphaltene dimers.

the higher stabilization energies. This means that their aggregates are more stable, and as a consequence, more difficult to dissociate. Figure 7 shows two different measurements of the stability of the corresponding crude oils (flocculation onset and unstability index)14 as a function of the stabilization energies of asphaltene dimers. As it can be seen, there is an increase in the flocculation onset (n-heptane volume needed to begin the flocculation of the asphaltenes) as the stabilization energy of the dimer decreases. On the other hand, the unstability index decreases. These results indicate that the asphaltene precipitation is strongly related to the stability of the aggregates. A greater stability of the aggregates produces a greater tendency toward precipitation. On the other hand, as the stability of the aggregates increases with lower hydrogen-to-carbon ratio, higher aromaticity, higher aromatic condensation degree, and higher molecular weight, this result can be linked to the fact that asphaltene deposits extracted from tubing are usually enriched with high molecular weight-low H/C-high aromaticity molecules.29 Stabilization Energies of the Asphaltene-Resin Aggregates. The stabilization energies calculated for the asphaltene-resin dimers were -78.8 kcal/mol (LM1), -69.1 kcal/mol (LM2), -115.8 kcal/mol (NM1) y -70.44 kcal/mol (NM2). These values are very similar to the interaction energies between the molecules for each case. In a similar way to the other aggregates studied, there was not a significant change in the internal energies of the molecules after aggregation. For this reason, it is possible to suppose that in the interaction between asphaltenes and resins, very few structural changes occur during aggregation. The other aspect that was studied in this work corresponds to the preferential aggregation of asphaltenes and resins. The energy of the process in which an asphaltene in the dimer is exchanged by a resin was calculated:

asphaltene-asphaltene dimer + resin f asphaltene-resin dimer + asphaltene The results indicate that, in all the cases, this process was unfavorable: 16.4 kcal/mol (LM1), 31.3 kcal/mol (29) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 531.

Rogel

Figure 8. Interaction energies between molecules and different solvents.

(LM2), 60.1 kcal/mol (NM1) y 53.4 kcal/mol (NM2). However, the energy values for the molecules from unstable crude oils almost duplicate the energy values for the molecules from stable crude oils. These results mean that for unstable crude oils, this process can be less favorable. However, these results are not conclusive because the solvent medium also plays an important role in the adsorption of the resins on the asphaltene as it was pointed out earlier by other authors.2 Interaction between Molecules and Solvents. Figure 8 shows the interaction energies between molecules and different solvents as a function of the molecular weight (MW). The interaction energies shown in this figure were calculated as the temporal average during the last 40 ps of the molecular dynamics simulations. As it can be seen, there is an almost lineal relationship between the interaction energies and the molecular weight, which indicates the preponderance of the van der Waals forces in the interaction between the asphaltenes and resins with the different solvents, including those solvents able to form hydrogen bonds. In relation to the formation of hydrogen bonds between the molecules studied and the solvents, very few hydrogen bonds were observed for asphaltenes LM1 and LM2, while this number duplicates for asphaltenes NM1 and NM2. However, according to the observed in Figure 8, hydrogen bonding does not affect significantly the interaction energy between these molecules and the solvents. Effect of Solvents on the Interaction Energies in Aggregates. In this section, the evolution of the interaction energies between the molecules that compose dimers in different solvents was studied. In these studies, two distinct behaviors were observed: (a) decrease of the interaction (the interaction energy is less negative) (Figure 9a); (b) the interaction energy remains almost constant during the calculation (Figure 9b). This last behavior was observed for all the asphaltene dimers. Figure 10 shows the effects of the solvents studied on the interaction energies in the aggregates as a function of the molecular weight. These energies were calculated as temporal averages during the last 40 ps of the molecular simulations made. As a reference, Figure 10 includes the interaction energies calculated in the vacuum under similar conditions.


Figure 9. (a) Interaction energy as a function of time for the system resin dimer LM2-toluene. (b) Interaction energy as a function of time for the system resin dimer NM2-toluene.

Figure 10. Effect of the solvent on the interaction energy of the aggregates as a function of the molecular weight of the monomer.

As it can be seen in Figure 10, the solvents toluene, pyridine, and naphthalene affect the interaction energies. These solvents are able to decrease the interaction between the molecules, but only for molecules with lower MW, Fa, CI/C1, and higher H/C. This indicates that the increase in MW, Fa, and CI/C1 in the molecules (and, as a consequence, a decrease in H/C) increases the


stability of the aggregates and makes them more difficult to dissolve. In fact, as it was pointed out, a decrease was not observed in the interaction energy for the asphaltene molecules studied. These findings are also in agreement with the experimental evidence that indicates that asphaltenes from deposits show high MW, Fa, CI/C1, and low H/C.29 On the other hand, Takanohashi et al.22 using molecular dynamics calculations also found that coal associates were broken by their interactions with pyridine molecules. However, when they used methanol or benzene, the breaking of the coal associates did not take place, although these molecules interacted with the coal molecules. They concluded that only some of the solvents studied are able to break the aromatic-aromatic interactions and, as a consequence capable to dissociate the coal associates. In particular, they found that although methanol could break the hydrogen bonds in the associates, these breakings were not enough to dissociate the coal aggregates. Furthermore, they also found that the heavier the coal fractions were, the more stable the association structure were that they formed. These previous results are in close agreement with the findings of the present work and indicate that a wide spectra of different heavy fractions from coals and crude oils behave in a similar way. Effect of Solvents on a Resin Surface. One of the methods used to remove asphaltene deposits from the near-wellbore region is by adding aromatic solvents alone or in combination with various dispersants.30,31 In the present article, the process of removal was simulated by putting a surface composed of resin molecules in contact with different solvents. In this section, three solvents were used: toluene, naphthalene, and n-heptane. This last solvent was selected in order to compare the behavior of a nonsolvent with the behavior of aromatic solvents, often used in the removal of asphaltene deposits.30,31 In the systems studied, the evolution of the distribution of atoms of the surface along the axis z (perpendicular to the surface) was calculated during the molecular dynamic simulations. The distribution of carbon atoms of the surface along the axis z gives a measure of the transfer of the resin molecules that compose the resin layer to the bulk of the solvent. The translation of the molecules must be influenced by the dissolving power of the solvent and can be a way to measure the relative dissolving power of the different solvents. In Figure 11, parts a and b, the comparisons between the final atomic distributions are shown for the solvents and surfaces used. The initial distribution is the same for all the simulations and is independent of the solvent. The final distributions corresponding to the simulations in the vacuum were added as references. In both figures, the distributions show significant displacements for solvents in relation to the displacement in the vacuum. The order in such displacements is naphthalene > toluene > n-heptane > vacuum. According to this, the solvent with the higher dissolving power is naphthalene. This result is in agreement with the experimental (30) Samuelson, M. L. SPE International Symposium on Formation Damage Control, SPE 23816, USA, 1992. (31) Del Bianco, A.; Stroppa, F. SPE Production & Facilities 1997, 80.

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Conclusions

Figure 11. (a) Final atomic distributions for the surface LM2 in different solvents. (b) Final atomic distributions for the surface NM1 in different solvents.

evidence found by Bianco et al.31 that indicated that tetralins and naphthalenes were better solvents than alkylbenzenes for asphaltene removal.

The stabilization energies of the asphaltene and resin aggregates are due mainly to the van der Waals interactions between the molecules in the aggregates. The interaction energy due to hydrogen bonding is considerably lower. No significant changes in the intramolecular energy upon aggregation were observed which means that very few structural changes occur in the molecules studied due to aggregation. The stabilization energies of the dimers are related to the structural characteristics of the molecules. The stabilization energy is more favorable for those molecules with a lower hydrogen-to-carbon ratio, a higher aromaticity, and a higher aromatic condensation degree. A higher stabilization energy produces a greater tendency toward the precipitation of asphaltenes in the crude oil. The aggregates can be dissociated by the interaction with different solvents. However, the dissociation of the associates will take place depending not only on the solvent but also on the asphaltene and resin characteristics, which determine the strength of the van der Waals interactions. The breaking of the aggregates occurs for molecules with lower MW, Fa, CI/C1, and higher H/C. This finding can be linked to the fact that asphaltene deposits extracted from tubing are usually enriched with high molecular weight-low H/C-high aromaticity molecules. Acknowledgment. I thank O. Leo´n and Y. Espidel for their help in the interpretation of the structural parameters. PDVSA-INTEVEP is acknowledged for permission to publish this paper. EF990166P

Simulation of Interactions in Asphaltene Aggregates - American

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