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The Influence of the Adsorption of Amphiphiles and Resins in Controlling Asphaltene Flocculation O. Leo´n,*,† E. Contreras,† E. Rogel,† G. Dambakli,‡ J. Espidel,† and S. Acevedo‡ PDVSA-INTEVEP, Apdo. 76343, Caracas-1070A, Venezuela, and Escuela de Quı´mica, Universidad Central de Venezuela, Caracas, Venezuela Received February 9, 2001. Revised Manuscript Received July 5, 2001
Amphiphiles are frequently used to prevent asphaltene precipitation in reservoir rocks and wellbore tubing. It is supposed that these substances can stabilize the asphaltenes forming a steric stabilization layer around them. In this study, adsorption isotherms of two amphiphiles (nonylphenol and nonylphenolic resin) and a native resin on asphaltene particles are obtained in order to analyze the relationship between adsorption and effectiveness of these compounds as asphaltene stabilizers. The comparison of the three isotherms reveals a significant difference between the adsorption behavior of the amphiphiles and the resin fraction in terms of the shape of the isotherm. Nonylphenol and nonylphenolic resin adsorption isotherms show an LS-shape that can be explained using an adsorption mechanism in two steps. The adsorption isotherm of the native resin can be explained by penetration of the micropores of the asphaltene particles by resin molecules. There were found significant differences in the activity of the studied compounds as asphaltene precipitation inhibitors that can be linked to the adsorption behavior of these species. In particular, the native resins seem to show an activity stabilization mechanism different than that found for the other species studied.
Introduction Chemical additives containing amphiphiles as active agents are frequently used to prevent asphaltene precipitation in reservoir rocks and wellbore tubing. Amphiphiles can solubilize the asphaltenes otherwise insoluble under production conditions.1 On the basis of the colloidal nature of the crude oil, it is supposed that amphiphiles act in a way similar to resins,2 the natural peptizing agent of asphaltenes in crude oils. According to the mechanism widely accepted, the resins attach to asphaltene particles, the disperse phase of the crude oil, forming a steric stabilization layer around asphaltenes keeping them in solution.3 The steric stabilization mechanism of asphaltenes by amphiphiles is based on the evidence of strong interactions between amphiphiles and asphaltenes.2,4 In particular, adsorption of amphiphiles on asphaltene particles has been confirmed by the determination of the adsorption isotherms of various amphiphiles from different solvents on asphaltene particles.5,6 Additionally, * Author to whom correspondence should be addressed. Phone: 58212-9087804. Fax: 58-212-9087524. E-mail:
[email protected]. † PDVSA-INTEVEP. ‡ Universidad Central de Venezuela. (1) Groffe, P.; Volle, J. L.; Ziada, A. SPE 30128 presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, 1995, May 15-16. (2) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1749-1757. (3) Leontaritis, K. J.; Mansoori, G. A SPE 16258 presented at the SPE International Symposium on Oil Field Chemistry, San Antonio, Texas, 1987, February 4-6. (4) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10, 1758-1766. (5) Jaoui, M.; Achard, C.; Hasnaoui, N.; Rogalski, M. Rev. Inst. Fr. Pet. 1998, 52, 35-40. (6) Leo´n, O.; Rogel, E.; Urbina, A.; Andu´jar, A.; Lucas, A. Langmuir 1999, 15, 7653-7657.
it was found that asphaltene stability depends on the amount of amphiphile adsorbed and also on the length of the amphiphile’s tail for a series of alkyl-benzenederived amphiphiles.2,6 It is well-known that the asphaltene stability displayed significant changes corresponding to different types of stabilizing agents.7-9 Although the chemical bases of the inhibition of asphaltene precipitation are not well understood at the present moment, it has been found that the effectiveness of an amphiphile as asphaltene stabilizer is controlled by the strength of the asphaltene-amphiphile interactions and the capacity of the amphiphiles to form a stabilization layer around the asphaltene particles.2 Both characteristics are strongly related to the structural nature of the amphiphiles. In particular, for alkyl-benzene-derived amphiphiles, it has been shown that a headgroup with high polarity and a long tail are the main characteristics of a good stabilizing agent.2 In this work, the interest is focused on a comparative study of the behavior of amphiphiles and native resins (resins that come from the same crude oil as asphaltenes) as asphaltene stabilizers. For these substances, the relationship between adsorption behavior and activity as asphaltene stabilizers was evaluated. Adsorption isotherms of two amphiphiles and a native resin on asphaltene particles were obtained in order to analyze the asphaltene-stabilizer interactions as well as the formation of a layer around the asphaltene particles. (7) Chang, C. L.; Fogler, H. S. Fuel Sci. Technol. Int. 1996, 14, 75100. (8) Gonzalez, G.; Middea, A. Colloids Surfaces 1991, 52, 207-217. (9) Bandeira, L. F.; Fernandes, E.; Gonzalez, G. J. Appl. Polym. Sci. 1999, 73, 29-34.
10.1021/ef010032n CCC: $20.00 © 2001 American Chemical Society Published on Web 08/24/2001
Amphiphiles and Resins in Controlling Asphaltene Flocculation
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Table 1. Chemical and Physical Characteristics of Crude Oil OL1 and Its Asphaltenes Crude Oil OL1 °API gravity
density (g/cm3)
saturates (wt %)
aromatics (wt %)
resins (wt %)
asphaltenes (wt %)
22.4
0.92
36.9
37.9
19.4
5.8
Asphaltenes OL1
Figure 1. Average molecule of native resins from OL1 crude oil built using elemental composition and H NMR spectra.
surface area (m2/g)
%C
%H
%S
%N
42
84.40
6.75
3.50
1.31
On the other hand, the activity of these substances as asphaltene stabilizers was tested by flocculation onset measurements. The main objective of this work is to explore the mechanism of asphaltene stabilization using substances of different nature. A better understanding of the differences observed can help to unveil the relationship between activity and structure of the stabilizers, providing useful information for the selection and design of new asphaltene inhibitors. Methods Materials. The asphaltene used as adsorbent throughout this study was extracted from a Venezuelan Crude Oil (OL1) with a 30:1 volume ratio of n-heptane to crude oil according to the preparation method described in IP-143/90. Chemical and physical characteristics of crude oil OL1 and its asphaltene are shown in Table 1. The native resins were obtained from crude oil OL1 using a modification of the method described in ASTM D-2007. The maltenes were separated into saturates, aromatics, and resins using a chromatographic column packed with attapulgus clay. These fractions were sequentially eluted and collected using different solvents: saturates and aromatics were eluted using toluene-n-heptane mixtures, while resins were obtained using a mixture of methanol, acetone, and chloroform. For comparison purposes two amphiphiles were selected in this study: nonylphenol (NP) and a nonylphenolic resin (NPR). Both substances have shown activity as asphaltene precipitation inhibitors.2,7,9 Characteristics of these compounds are shown in Table 2. Characterization of Native Resins. Elemental compositions were determined on a LECO CHNS 244 elemental analyzer model, and the average number molecular weight on a Knauer vapor pressure osmometer in CH2Cl2 at 25 °C. Native resin NMR spectra were obtained on a Bruker ACP400 spectrometer, at a resonance frequency of 400 MHz for protons. A flip angle of 45° was used, with a repetition rate of 3 s, spectral width of 12 ppm, and the chemical shift was referenced relative to TMS. Samples in the amount of 25 mg were dissolved in 1 mL of dichloromethane, and 5 wt % hexamethyl cyclosiloxane was added as internal standard. Average molecular parameters (AMP) and an average molecular model for resins were calculated according to a method used previously.10,11 Table 3 shows the main characteristics of the resins and Figure 1 shows the average molecular model built using the AMP calculated. Adsorption Isotherms. Solutions of a known concentration of each amphiphile were prepared in n-heptane. A 10 cm3 aliquot was added to 50 mg of asphaltenes in a flask. The flask was covered and the suspension was stirred in a thermostatic bath at 25 °C during 16 h. After reaching equilibrium, the supernatant liquid was separated by centrifugation and its (10) Leon, V. Fuel 1987, 66, 145-146. (11) Carbognani, L.; Espidel, J. Personal communication, 1993.
Figure 2. Adsorption isotherms of nonylphenol (NP) and nonylphenolic resin (NPR) in n-heptane onto asphaltenes OL1 at 25 °C. absorbance was measured on a Perkin-Elmer UV-Vis spectrophotometer using 1 cm path length cells. The absorbance of the supernatant was measured at 283, 292, and 400 nm for nonylphenol, nonylphenolic resin, and native resin, respectively. The absorbance allowed the calculation of the equilibrium concentration and, as a consequence, the total amount of adsorbed material on the asphaltene surface. Flocculation Onset. The activity of asphaltene stabilizers was evaluated by means of flocculation measurements. In this work, the flocculation points were determined by a titration method: n-heptane is added at a constant rate (1 cm3/min) to a solution of asphaltene and stabilizer in toluene under intensive stirring. The titration is monitored by means of a Guided Wave NIR spectrophotometer at the wavelength of 768 nm. The flocculation point is defined as the amount of n-heptane needed to obtain the maximum of the light intensity, which corresponds to the beginning of aggregation and coagulation of the asphaltenes. The activity of the stabilizers was determined by titration of 30 mg of asphaltene in 10 cm3 of toluene. The stabilizer-asphaltene weight ratio range from 0.1 to 6.6. Temperature, titration rate, stirring speed, time of sample preparation were the same in all titrations. Solutions without stabilizers were used as references. The activity of the stabilizers was calculated as the difference in the flocculation onset for crude oil solutions with and without the stabilizers.
Results and Discussion Adsorption Isotherms. Figure 2 shows the adsorption isotherms onto asphaltenes OL1 from n-heptane determined at 25 °C for NP and NPR, respectively. The isotherms obtained could be classified as two-plateaustype (LS) and are similar to those reported for other amphiphiles on asphaltenes OL1.6 The two-plateaustype isotherms can be related to a two-step adsorption mechanism where the amphiphile molecules are initially adsorbed through interactions with the asphaltene surface and then, in the second step, through interaction
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Table 2. Alkyl-Benzene-Derived Amphiphiles Used in the Study
a
Average molecular weight. Table 3. Characteristics of the Native Resins
MWa (g/mol)
C (wt %)
H (wt %)
N (wt %)
S (wt %)
fab
Ci/C1c
Ard
683
84.60
10.37
1.15
0.31
0.40
0.52
4
a
b
Determined by VPO in CH2Cl2 at 25 °C. Aromaticity (aromatic carbons to total carbons ratio). c Aromatic condensation index (internal aromatic carbons to nonbridging aromatic carbons ratio). d Number of aromatic rings.
with the amphiphiles adsorbed. After the first step, a sharply increase in the adsorption is observed which corresponds to the formation of aggregates of amphiphiles on the solid/liquid interface.12 Other authors also reported an adsorption mechanism in two stages for amphiphiles from water onto asphaltenes.5 The adsorption isotherms were fitted using the following adsorption equation:12
Γ ) Γ∞k1C(n-1 + k2Cn-1)/(1 + k1C(1 + k2Cn-1)) (1) where n is the mean aggregation number of the adsorbed hemimicelles, Γ∞ is the maximum coverage of the surface, k1 and k2 are the equilibrium constants of the first and second adsorption step, and C is the equilibrium concentration of monomer in solution. As can be seen in Figure 2, this equation fit adequately the experimental data. The extension of the NP and NPR adsorption to OL1 asphaltenes in n-heptane is quite similar. The maximum surface excess concentrations (Γ∞) are 0.0032 mol/g asphaltene and 0.0030 mol/g asphaltene for NP and NPR, respectively. However, the formation of aggregates of amphiphiles on the solid/liquid interface begins at lower concentration for NPR (0.00015 mol/L) than that for NP (0.0012 mol/L). This can be interpreted as a higher affinity of NPR for the asphaltene surface. In fact, if the extension of the adsorption for both compounds is compared per mole of nonylphenol, the NPR shows a significantly higher adsorption than NP as can be seen in Figure 3. It has been found that phenol-derived amphiphiles adsorbed on asphaltene surfaces lie parallel to the surface at the first plateau.6 As it can be expected, at the first plateau, NP occupies approximately 128 Å2 on the asphaltene surface in accordance with a planar configuration. Other authors have found the same behavior for similar systems. They reported the forma(12) Zhu, B. Y.; Gu, T. Adv. Colloid Interface Sci. 1991, 37, 1-32.
tion of closely packed monolayers with molecules lying with their long axis parallel to the surface.13-15 These reports included alcohols and carboxylic acids adsorbed from organic solutions onto graphite, as well as large alkanes adsorbed from organic solutions onto graphitized carbon.16 Lamellar structures have been identified for fatty acids with extended carbon chains oriented parallel to a lattice axis in the graphite basal plane.13 For NPR, the calculation of the area occupied on the surface indicates that only a small section of this polymer is in direct contact with the asphaltene surface (approximately 69 Å2). In comparison, it has been shown that the adsorption of phenol in a planar configuration results in an area occupied per molecule of phenol of 52.2 Å2.17 The low area occupied by NPR at the first plateau could indicate that this polymer adsorbs in a globular configuration. Studies of the layers formed by phenolic resins on particles of technical carbon, using electron microscopy technique, indicate the presence of globular structures.18 Molecular dynamics simulations of the adsorption of phenolic resins on asphaltene surface indicate that these polymers adsorb in a globular configuration, similar to the one shown by them in the solvent (n-heptane).19 This configuration can be attributed to the formation of intramolecular hydrogen bonds that are not broken by the adsorption at low concentration. Figure 4 shows the adsorption isotherm of the native resins (NR) on the asphaltene surface. As can be seen, the comparison of the three isotherms reveals a significant difference between the adsorption behavior of the amphiphiles and the native resin in terms of the shape of the isotherm. For the native resins, the isotherm is characterized by the continuous increase in the amount of adsorbed resins. There is no indication of the presence of a plateau similar to the ones shown by the other compounds studied. According to its particular characteristics, the adsorption isotherm of native resins can be classified as C-class. This type of isotherms can be explained by the penetration of substrate micropores by (13) Rabe, J. P.; Bucholz, S. Science 1991, 253, 424. (14) Groszek, A. J. Proc. R. Soc. London 1970, A314, 473. (15) Castro, M. A.; Clarke, S. M.; Inaba, A.; Dong, C. C.; Thomas, R. K. J. Phys. Chem. B 1998, 102, 777-781. (16) Findenegg, G. H.; Liphard, M. Carbon 1987, 25, 119-128. (17) Dargaville, T. R.; Guerzoni, F. N.; Loney, M. G.; Solomon, D. H. J. Colloid Interface Sci. 1996, 182, 17-25. (18) Varlakov, V. P.; Smirnov, B. N.; Agafonov, M. V.; Fialkov, A. S. Colloid J. USSR 1989, 51, 1023-1026. (19) Rogel, E. Unpublished results.
Amphiphiles and Resins in Controlling Asphaltene Flocculation
Figure 3. Comparison of the adsorption isotherms of nonylphenol (NP) and nonylphenolic resin (NPR) in n-heptane onto asphaltenes OL1 at 25 °C. Adsorbed amount and equilibrium concentration are plotted per mole of NP.
Figure 4. Adsorption isotherm of native resins (NR) in n-heptane onto asphaltenes OL1 at 25 °C.
solute with or without solvent.20 It is also supposed that the penetration of the solute in the microporous structure of the substrate can lead to the partial breakdown of the structure, making available new internal surface for additional adsorption of the solute. In fact, the dissolution of asphaltenes during the adsorption experiments was observed and prevented to obtain adsorption data at higher concentrations. For the other stabilizers studied, asphaltene dissolution was observed at considerably higher concentrations, after reaching the second plateau. The reasons for the differences in dissolution behavior are not clear. However, it is possible to assume that the broad distribution of different molecules that compose the native resins included aromatic compounds with a high affinity for the asphaltenes. It has been shown that polynuclear aromatic compounds have a special capacity to dissolve asphaltene deposits. In particular, the asphaltenes are more efficiently dissolved by condensed aromatic hydrocarbons than by alkylbenzenes, which are generally used to dissolve deposits.21 A previous study of the adsorption of resins on asphaltene particles from different sources reported the same adsorption behavior and attributed it to multilayer formation at lower concentrations. At higher concentra-
Energy & Fuels, Vol. 15, No. 5, 2001 1031
Figure 5. Activity of the stabilizers as a function of their concentrations. Activities are calculated as the difference in the flocculation onset between asphaltene-stabilizer solution and asphaltene solution.
tions, the continuous rise observed in the adsorption of resins is explained by the penetration of resins on asphaltene pores.22 Flocculation Onset. Figure 5 shows the activities of the stabilizers at different concentrations. At each concentration, the activity was calculated as the difference in the flocculation onset between asphaltenestabilizer solution and the reference solution (asphaltenes alone). Figure 5 shows that the activity increases in the order: NP < NR < NPR. Previous experimental evidence indicated that the activity of alkyl-benzenederived amphiphiles as asphaltene stabilizers is related to the maximum amount of amphiphile adsorbed on the asphaltene surface.6 The comparison of the extension of adsorption and the activities for NP and NPR indicates a higher activity for the latter compound, even though if NPR polymerization degree is considered (7 mol of NP per molecule of NPR). In other words, the activity per mole of NP is higher for the copolymer NPR than for the monomer NP as it is shown by Figure 6. Then, the polymerization favors the activity as asphaltene stabilizer of NPR. In principle, this effect can be attributed to the higher adsorption per mole of NP observed for NPR. The activity of NR is higher than the value expected according to its adsorption isotherm. In fact, at the same equilibrium concentrations, the native resins adsorb in a lower extension than the other compounds studied. Then, it can be expected from previous results found for alkyl-benzene-derived amphiphiles6 that the NR activity would be lower than the activity found for the other stabilizers. On the contrary, NR shows higher activity than NP that adsorbs in a larger extension. This behavior could indicate that the mechanism of asphaltene precipitation inhibition for amphiphiles and native resins is different. (20) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47, 755-765. (21) Del Bianco, A.; Stroppa, F. SPE 28992 presented at the SPE International Symposium on Oilfield Chemistry, San Antonio, Texas, 1995, February 14-17. (22) Acevedo, S.; Ranaudo, M. A.; Escobar, G.; Gutie´rrez, L.; Ortega, P. Fuel 1995, 74, 595-598.
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aggregates. On the contrary, resins seem to be able to decrease the aggregation number of asphaltene aggregates. These previous results support the differences found in the adsorption mechanisms of NP and NR on asphaltene surfaces. Clearly, there are significant differences in the activity of the stabilizers studied, strongly influenced by structure and molecular size. Currently, an experimental effort is carried out to identify different activity mechanisms depending on these two variables. In particular, a thoughtful evaluation of other asphaltene and resin samples from different origins is needed. Conclusions
Figure 6. Comparison of the activity of nonylphenol (NP) and nonylphenolic resin (NPR). The concentration of the stabilizers is plotted per mole of NP.
Taking into account the adsorption isotherm obtained for NR and its asphaltene dissolution capacity at low concentrations, the high activity found for NR as asphaltene stabilizer can be attributed not only to the formation of a stabilization layer, but also to other factors. It is possible to suppose that one of these factors is the capacity of NR to break down the asphaltene macrostructure and dissolve asphaltenes at low concentrations. Vapor pressure osmometry (VPO) measurements of the asphaltene molar mass have shown that the presence of small amounts of low molar mass impurities, mainly resins, can significantly decrease the number-average-molar mass of asphaltenes.23 A similar result has been found using small-angle X-ray scattering.24 These findings have been related to the antiflocculant action of resins. In contrast, small-angle X-ray scattering measurements indicate that NP-asphaltene colloids are slightly larger than asphaltene colloids.4 This suggests that the addition of NP to asphaltene solutions does not decrease the size of the asphaltene (23) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (24) Bardon, Ch.; Barre´, L.; Espinat, D.; Guille, V.; Li, M. H.; Lambard, J.; Ravey, J. C.; Rosenberg, E.; Zemb, T. Fuel Sci. Technol. Int. 1996, 14, 203-242.
The comparison of adsorption isotherms of nonylphenol, nonylphenolic resin, and native resin on asphaltene particles reveals a significant difference in the adsorption behaviors in terms of the shape of the isotherm. Nonylphenol and nonylphenolic resin adsorption isotherms show an LS-shape that can be explained using an adsorption mechanism in two steps. In the first step, the amphiphiles are adsorbed individually on the asphaltene surface; in the second step, the interactions between adsorbed amphiphiles become predominant and the formation of amphiphile aggregates in the surface begins. The adsorption isotherm of the native resin can be explained by penetration of the micropores of the asphaltene particles by resin molecules. The penetration of the resins in the microporous structure of asphaltenes can lead to the partial breakdown of asphaltene macrostructure. The activity as asphaltene precipitation inhibitors increases in the order: nonylphenol < native resin < nonylphenol resin and can be linked to the adsorption behavior of these species. In particular, the native resins seem to show an activity stabilization mechanism different than that found for the other species studied. Acknowledgment. The authors thank PDVSAINTEVEP for financial support and for permission to publish this work. EF010032N
