Asphaltene−Paraffin Structural Interactions. Effect on Crude Oil Stability

Energy & Fuels 2001, 15, 1021-1027

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Asphaltene-Paraffin Structural Interactions. Effect on Crude Oil Stability Marı´a del Carmen Garcı´a* and Lante Carbognani PDVSA - Intevep, PO. Box 76343, Caracas 1070A, Venezuela Received February 9, 2001. Revised Manuscript Received May 2, 2001

The effect of asphaltene-paraffin complexes on crude oil stability, in terms of wax crystallization and asphaltene deposits solubility, was studied. The influence of flocculated asphaltenes on the wax crystallization tendency of a paraffinic crude oil and on the effectiveness of a maleic anhydride copolymer derivative paraffin inhibitor was evaluated by means of polarized light microscopy. It was demonstrated that flocculated asphaltenes provide wax crystallization sites whose presence interferes with the inhibitor crystal modification mechanism. The existence of asphaltene-paraffin complexes was assessed by HPLC kinetic dissolution studies of composite materials formed by combinations of such hydrocarbon fractions. Commercial macrocrystalline and microcrystalline waxes were used in these experiments. Different types of asphaltenes were probed as well, covering varying ranges of aromaticities and stability. Asphaltene properties were observed to be important parameters influencing composites dissolution. A selective partition of paraffins and asphaltenes in solubilized fractions was determined by high-temperature thin-layer chromatography.

Introduction Organic solid deposition is one of the most serious problems faced in oil production operations. This problem can occur through all the production, transportation, and storage steps, depending on the fluid nature and surrounding conditions, affecting the flow behavior of the reservoir, oil wells, and tubing.1 Wax deposition is often observed when paraffinic oils are produced, or when very low temperatures are involved, which is the case of offshore operations (crude oil sub-sea production and transportation). The most significant causes for the separation of paraffins from the crude oil include heat loss from oil and gas to the surroundings, cooling produced by gas expansion through a restriction, and intrusion of water and evaporation of lighter constituents.2-5 This phenomenon can decrease the system temperature below the melting point, causing wax crystallization and deposition in the production tubing, flow lines, and even in the reservoir.6 On the other hand, asphaltene deposition is a very common problem when light crude oils are produced near the flocculation onset pressure (depleted reservoirs). When pressure declines in the reservoir due to its exploitation, the single phase system may undergo asphaltenes precipitation and deposition, reaching a solubility minimum near bubble point conditions. Severe * Corresponding author. E-mail: [email protected]. (1) Garcı´a, M. C.; Chiaravallo, N. Paper SPE 65009 presented at the SPE International Symposium on Oilfield Chemistry held in Houston, TX, 2001. (2) Misra, S.; Baruah, S.; Singh, K. SPE Prod. Facil. 1995, 50-54. (3) Sutton, G. D.; Roberts, L. D. J. Pet. Technol. 1974, 997-1004. (4) Jorda, R. M. J. Pet. Technol., Trans., Aime 1966, 237, 16051612. (5) Allen, T. Production Operations. Well Completions, Workover and Stimulation 1993, 2, 13-22. (6) Penteado Sanches, C. B. Te´ cn. PETROBRAÄ S 1991, 34, 101-112.

formation damages occur when pressure reduction takes place within the reservoir, and asphaltenes deposit in the porous media.1 Asphaltene plugging problems can also be observed when unstable crude oils are produced, due to the oil intrinsic nature7,8 or incompatible fluid commingled production.1 Also inappropriate stimulations and treatments (cleaning and squeeze procedures with highly paraffinic solvents, as gasoil, kerosene, etc.) are responsible of asphaltene deposition during production.9 Several studies concerning the correlation between crude oil composition and its tendency to wax crystallization, especially the effect of paraffinic hydrocarbon fractions characteristics, have constituted an important research field in oil chemistry.10-15 The fact that flocculated asphaltenes enhance the wax crystallization process in highly paraffinic crude oils was recently demonstrated. This phenomenon was attributed to the (7) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 351-358. (8) Carbognani, L.; Espidel, J.; Izquierdo, A. Chapter 13 in Asphaltenes, Asphalts. 2. Developments in Petroleum Science, 40 B; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science B. V.: Amsterdam, 2000; pp 335-362. (9) Carbognani, L.; Contreras, E.; Guimerans, R.; Leo´n, O.; Flores, E.; Moya, S. Paper SPE 64993 presented at the SPE International Symposium on Oilfield Chemistry held in Houston, TX, 2001. (10) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Pet. Sci., Technol. 1998, 16, 1001-1021. (11) Garcı´a, M. C.; Carbognani, L.; Orea, M.; Urbina, A. Pet. Sci., Eng. 2000, 25, 99-105. (12) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Paper SPE 49200, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, 1998; pp 681-687. (13) Garcı´a, M. C.; Orea, M.; Urbina, A.; Carbognani, L. Paper No. 58 g presented at Spring AIChE Meeting, Second International Symposium on Wax Thermodynamic and Deposition I, at Houston, TX, March 14-18, 1999. (14) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. 3rd International Symposium on Colloids Oil Production. Asphaltenes and Wax Deposition, Huatulco, Oaxaca, Me´xico, 1999. (15) Garcı´a, M. C. Energy Fuels 2000, 14, 1043-1048.

10.1021/ef0100303 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001

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presence of nucleation sites in the crude oil matrix, corresponding to the asphaltenic particles which accelerate the wax crystal growing and increase the cloud point.15 Recently, the elution of paraffin concentrates through solid asphaltenes packed inside HPLC columns was not possible to achieve due to the formation of composite materials that prevented solvent flow.16 This finding suggested the formation of molecular complexes among these hydrocarbon types, resembling those isolated from bottom sediments found in storage vessels. One of such sediments was assessed to be composed of very long n-alkanes and highly aromatic asphaltenes.17 To the best of our knowledge, evidences of interactions between paraffins and asphaltenes were first reported by Graves and Tuggle18 and by Schuster and Irani.19 The last study showed important implications for the pour-point depression and effective wax crystallization inhibition of paraffinic oils. Further studies carried out along this line showed that differences in behavior are to be found depending on the physical state of asphaltenes, namely in solid precipitate or in colloidal form.15 Some paraffin-polar hydrocarbon molecular complexes have been formerly studied in bottom layers separated from large storage facilities.20-22 Large alkane hydrocarbons, long-chain alkyl-naphthenes, and alkylbenzenes have been identified as important components in these mixtures.21,23 High proportions of large nparaffins were commonly assessed to be present in bottom sediments isolated from crude oil storage tanks, in which inorganic and polar hydrocarbon species also participate.24-27 Special protocols for isolation of waxes associated with asphaltenes have been described. Hot extraction with xylene was described for the isolation of presumed ozokerites associated with a tar sand bitumen.28 The xylene extraction was adopted in another study that, in addition, described a separation of precipitated waxes in terms of macro and microcrystalline types, obtained by fractional solubilization in cold pentane.29 Hot filtration with methyl-isobutyl ketone was recently reported for the isolation of the waxes that coprecipitate with asphaltenes, separated during the (16) Carbognani, L.; Orea, M. Pet. Sci., Technol. 1999, 17, 165-187. (17) Carbognani, L.; De Lima, L.; Orea, M.; Ehrmann, U. Pet. Sci., Technol. 2000, 18, 607-634. (18) Graves, R. H.; Tuggle, R. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1968, 13, 46-52. (19) Schuster, D. S.; Irani, C. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30, 169-177. (20) Green, J. B.; Woodward, P. W.; Thomson, J. S.; Shay, J.; Giles, H. N. Proc. 4th Int. Conf. Stability Handling Liq. Fuels 1992, 1, 5064. (21) Thomson, J. S.; Grigsby, R. D.; Doughty, D. A.; Woodward, P. W.; Giles, H. N. Proc. 4th Int. Conf. Stability Handling Liq. Fuels 1992, 1, 65-78. (22) Green, J. B.; Yu, S. K. T.; Woodward, P. W.; Giles, H. N. Proc. 4th Int. Conf. Stability Handling f Liq. Fuels 1992, 1, 79-93. (23) Thomson, J. S.; Grigsby, R. D.; Doughty, D. A.; Woodward, P. W.; Giles, H. N. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1989, 34, 275-281. (24) Fazal, S. A.; Zarapkar, S. S.; Joshi, G. C. Fuel Sci., Technol. Int. 1995, 13, 881-893. (25) Fazal, S. A.; Zarapkar, S. S.; Joshi, G. C. Fuel Sci., Technol. Int. 1995, 13, 1239-1249. (26) Agrawal, K. M.; Joshi, G. C. Fuel Sci., Technol. Int. 1994, 12, 1105-1113. (27) Fazal, S. A.; Rai, R.; Joshi, G. C. Pet. Sci., Technol. 1997, 15, 755-764. (28) Branthaver, J. F.; Thomas, K. P.; Dorrence, S. M.; Heppner, R. A.; Ryan, M. J. Liquid Fuels Technol. 1983, 1, 127-146. (29) Thanh, N. X.; Hsieh, M.; Philp, R. P. Org. Geochem. 1999, 30, 119-132.

Garcı´a and Carbognani Table 1. Asphaltene and Wax Properties asphaltene

origin

% alkyl chains a

H/C

fab

M29 FPO550 DTJ9 C9535

stable oil stable oil deposit unstable oil

26 28 24 21

1.22 1.13 1.00 1.05

0.46 0.51 0.62 0.62

wax

spanned carbon rangec

wt % n-paraffinsc

melting range (°C)

macrocrystalline microcrystalline

C20-C49 C20-C138

80 150

8 7 13 13

13 7 9 20

12 9 15 28

14 28 10 75

8 12

a Values interpolated from the curves shown in Figures 3 and 4. b Asphaltene/macrocrystalline composite. c Asphaltene/microcrystalline composite. d Predissolved composite. e Pre-structured composite.

Figure 4. Dissolution kinetics for asphaltenes, pre-structured composites and waxes of FPO550, C9535, DTJ9, and M29 crude oils. Asphaltene (b), micro-composite (9), macrocomposite (0), microcrystalline wax (×), macrocrystalline wax (+).

hydrogen deficiency, and aromaticity, Table 1). However, it can be observed that one of the asphaltenes isolated from a stable crude (M29), did not show major changes after being mixed with waxes. On the other hand, asphaltenes separated from the rest of the samples (FPO550, DTJ9, and C9535), formed composites whose solubility differs in comparison with the original asphaltene. A numerical treatment of data from Figures 3 and 4 was conducted for better understanding of the previous findings (Table 2). M29 asphaltenes and derived composites showed similar solubility. Dissolution of com-

posites generated from the FPO550 asphaltenes required less than half toluene in comparison with the asphaltenes, with one exception (pre-structured asphaltene/microcrystalline composite). Asphaltenes from crude C9535 and deposit DTJ9, noticeably increased their solubility when mixed with both types of waxes under any conditions (predissolved or pre-structured). From the last two cases (C9535 and DTJ9), it is speculated that formed paraffin-asphaltene complexes bear alkyl appendages pointing outside the aggregate structure. Composite solubilization can be viewed as the elution of aggregates whose effective surface is supposed to be paraffinic in nature, facilitating in this way their incorporation into the toluene carrier. These aggregates must be smaller than 2 µm, since they passed through the pores of the retaining screen. Trying to shed some light on possible synergistic effects governing composite dissolution, a comparison was made between measured composite solubility and the calculated value based on the solubility of asphaltenes and paraffins and their relative amount in the original mixture. Dissolution values were selected for 15 mL of eluted toluene, attending to the fact that greater differences are detected in this region (see Figures 3 and 4). The following example illustrates the way this calculation was performed. Measured solubility values for DTJ9 asphaltene, macrocrystalline wax, and the 75:25 (wt) composite were 4.3, 87.8, and 44.2 wt %. The calculated value for the composite is 25.2 wt % (4.3 × 0.75 + 87.7 × 0.25). The difference between the measured and calculated value is 19.0 wt % (44.2-25.2) Results from all the comparisons were plotted on Figure 5. From the results of Figure 5 it can be observed that composites from stable asphaltenes either showed no variation within the error range (M29), or became harder to dissolve (FPO550). On the other hand, composites from unstable materials (DTJ9 and C9535) showed synergistic effects (in general became easier to dissolve). From the results it appears that the wax nature (micro- and macrocrystalline) and the mixing procedure (predissolved and pre-structured) did not significantly affect the solubility of the composites. These results suggest, in a similar way as discussed in the preceding paragraph, that the most important effect is brought by asphaltene properties (aromaticity, alkyl chains, hydrogen content). However, more asphaltenes selected among available types (stable/unstable crudes and deposits), and composites prepared with varying

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Figure 5. Difference between the measured composite solubility and the calculated value based on the relative amount of asphaltenes and paraffins in the original mixture. Predissolved micro-composite (]), pre-structured micro-composite (0), predissolved macro-composite ([), pre-structured macrocomposite (9). Dissolution values for 15 mL of toluene. Replicate analysis of some samples suggests that results within (9 wt % are not significant.

asphaltene/paraffin ratios should be analyzed aiming to confirm the tendencies already observed. Some of the collected fractions during the kinetic dissolution studies were analyzed in order to gain insights into relative distributions of paraffins and asphaltenes. High-temperature TLC/FID proved suitable for this purpose, achieving good resolution between these two hydrocarbon types. High temperature overcomes low solubility of microcrystalline paraffins, giving a single signal for this hydrocarbon group as can be observed in Figure 6a. When room temperature was initially adopted, large alkanes from microcrystalline waxes were found to elute within time windows corresponding to aromatics and polars as well, displaying a multi-signal chromatogram. On the other hand, hightemperature TLC/FID allows detecting the presence of saturates within the precipitated asphaltene (Figure 6b), in a similar way as observed by Gomes and Aleixo with a TG approach.31,32 Determined amounts of saturates found by TLC/FID, were used to improve the accuracy of group-type distributions calculated for the prepared composites. Figure 6c illustrates TLC/FID separation of one paraffin-asphaltene composite material. Calibration carried out with wax and asphaltene pure fractions, indicated that the FID detector response for macro and microcrystalline waxes was similar. However, asphaltenes were observed to display noticeable lower signals. The quantitative determination of the relative distribution of paraffins and asphaltenes in collected fractions was possible to achieve correcting asphaltene areas with a response factor of 1.41. Figure 7 shows one example of paraffin/asphaltene distributions for fractions collected along the dissolution process of a composite. The pre-structured material generated by C9535 asphaltenes + macrocrystalline paraffins was selected since it showed the greatest observed kinetic differences compared with the original asphaltene (Figure 4). Considering that this asphaltene sample already contained 4 wt % saturates, as deter-

Figure 6. High-temperature TLC/FID chromatograms of (a) microcrystalline wax, (b) C9535 asphaltene fraction, and (c) C9535 asphaltene/macrocrystalline wax pre-structured composite. Bands shown correspond to paraffins (0.26 min) and asphaltenes (0.52 min).

mined by TLC/FID (previous paragraph), the aimed 75/ 25 wt % mixture actually corresponds to 72/28 wt %. One fact that suggests that diverse asphaltene and paraffin components were solubilized in a selective way, is the variable relative distribution of hydrocarbon types observed in the collected fractions (Figure 7). It appears that highly soluble paraffins are eluted in the first fraction (F1), dragging more asphaltenes than expected, based on their intrinsic solubility. From quantitative

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plex, after 60 mL of toluene were collected. Pure wax is observed to elute in 45 mL (Figures 3 and 4). These findings suggest that different molecular mixtures were eluted along the dissolution process. Studies are currently under development in order to ascertain which types of asphaltenes were selectively isolated in each fraction and also which paraffin classes and carbon range distributions are contributing to each of these fractions. Conclusions

Figure 7. Relative distribution of paraffins and asphaltenes by high-temperature TLC/FID. Fractions were isolated from asphaltene C9535/macrocrystalline wax pre-structured composite. F ) eluted fractions (F1 ) with 15 mL, F2 ) with 30 mL, F3 ) with 45 mL, and F4 ) with 60 mL). Residue shown pertains to the nondissolved component remaining after collecting 60 mL of toluene.

results derived from Figures 4 and 7, it can be calculated that F1 should represent 40 wt % of the collected sample, based only on the solubility of the pure components. However, it is observed that 70 wt % was eluted in this fraction (Figure 4), clearly showing the cited effect of asphaltene dragging exerted by paraffins. It should be pointed out that this behavior was more pronounced for studied complexes derived from asphaltenes of unstable materials. Another fact that points toward a selective elution of components from the composite is deduced from the results shown in Figure 7. It was found that paraffins were present in the nondissolved residue for the C9535 asphaltene/macrocrystalline pre-structured studied com-

Flocculated asphaltenes in bulk crude oil behave like wax crystallization sites interfering with the inhibition mechanism of a maleic anhydride crystal modifier. Asphaltenes and paraffins generate complex solid aggregates when mixed together. The formation mechanisms remain unknown, but complex generation appears to strongly depend on asphaltene properties and less on wax composition and the complex formation pathway. Dissolution of complexes formed by highly aromatic and hydrogen-deficient asphaltenes was remarkably easier to achieve, in comparison with pure asphaltenes contributing for their generation. Selective dissolution of asphaltenes and paraffins from solid composites, was evidenced by combined kinetic dissolution profiles and high temperature TLC/ FID group-type hydrocarbon analysis. Room temperature TLC/FID analysis proved to be useless for the highly insoluble samples studied. Acknowledgment. The authors thank Petro´leos de Venezuela, S.A. for funding and permission to publish this work. Microphotographs provided by Argelia Urbina and TLC/FID performed by Mario Lattanzio are also acknowledged. EF0100303