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Different Adsorption/Occlusion Properties of Asphaltenes Associated with Their Secondary Evolution Processes in Oil Reservoirs Zewen Liao,*,†,‡ Ansong Geng,† Alain Graciaa,‡ Patrice Creux,‡ Anna Chrostowska,§ and Yaxue Zhang† State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, People’s Republic of China, Laboratoire des Fluides Complexes, UMR 5150 TOTAL-CNRS-UPPA, BP 1155, 64013 Pau Cedex, France, and Laboratoire de Chimie The´ orique et Physico-Chimie Mole´ culaire, UMR CNRS 5624, UniVersity of Pau, BP 1155, 64013 Pau Cedex, France ReceiVed October 29, 2005. ReVised Manuscript ReceiVed February 12, 2006
In this work, two asphaltenes derived from the same set of source rocks, C1 from a crude oil and C2 from an oil sand, were studied, concerning their adsorption/occlusion properties. From the adsorbed components, saturated and aromatic hydrocarbon distributions are distinctly different for these two asphaltenes. For example, some n-alkanes and alkyl-branched alkanes were found adsorbed in C2 asphaltenes, whereas only some cyclic compounds, particularly terpanes were detected from C1 asphaltenes. This difference is due to their different evolution processes in oil reservoirs, and the exchangeability between asphaltene-sorption compounds and those from outside of the bulk phase is different for these two asphaltenes. However, from the asphalteneocclusion compounds, saturated hydrocarbons show almost the same distribution features. This is consistent with the fact that these two asphaltenes were derived from the same set of source rocks, therefore, occluded almost the same original oils from the source rocks. The experimental results suggest that substantial microporous units exist inside the macromolecular structures of asphaltenes and the asphaltene-derived adsorption/occlusion phenomena extensively occur in oil reservoirs. It seems that asphaltene occlusion should have taken place before asphaltene was detached from kerogen.
Introduction Petroleum asphaltene is a complex admixture, usually presents as a soluble fraction in crude oils. Free-radical ions1 and ζ potential2,3 have been found steadily existing inside the macromolecular structures of asphaltenes. Some saturated hydrocarbons have been detected occluded inside asphaltenes.4,5 All of these results highlighted the report that asphaltenes are highly porous.1,6-9 Owing to these porous structural units inside asphaltenes, adsorption/occlusion for other compounds can steadily take place in oil reservoirs.4,5,10-13 Recently, it has been reported that asphaltene aggregates are * To whom correspondence should be addressed. E-mail: zw_liao@ hotmail.com. Fax: +86-20-85-29-07-06. † Chinese Academy of Sciences. ‡ Laboratoire des Fluides Complexes. § Laboratoire de Chimie The ´ orique et Physico-Chimie Mole´culaire. (1) Acevedo, S.; Escobar, G.; Ranaudo, M. N.; Pinate, J.; Amorin, A. Energy Fuels 1997, 11, 774-778. (2) Leon, O.; Rogel, E.; Torres, G.; Lucas, A. Pet. Sci. Technol. 2000, 18, 913-927. (3) Neves, G.; Sousa, M.; Travalloni-Louvisse, A.; Lucas, E. F.; Gonzalez, G. Pet. Sci. Technol. 2001, 19, 35-43. (4) Liao, Z.; Zhou, H.; Graciaa, A.; Chrostowska, A.; Creux, P.; Geng, A. Energy Fuels 2005, 19, 180-186. (5) Liao, Z.; Geng A.; Graciaa, A.; Creux, P.; Chrostowska, A.; Zhang, Y. Org. Geochem. 2006, 37, 291-303. (6) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278-286. (7) Mujica, V.; Nieto, P.; Puerta, L.; Acevedo, S. Energy Fuels 2000, 14, 632-639. (8) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40-47. (9) Rahmani, N.; Dabros, T.; Masliyah, J. Energy Fuels 2005, 19, 10991108. (10) Murgich, J.; Strausz, O. P. Pet. Sci. Technol. 2001, 19, 231-243.
still stable even under a high temperature, up to 300 °C or more.14-17 For example, in the asphaltene solutions (5 wt % asphaltenes in decalin, 1-methylnaphthalene, or quinoline), asphaltene aggregates are found in the form of a prolate ellipsoid with a high aspect ratio at 25 °C and become a compact sphere with a size of around 25 Å in radius at 350 °C.14 Molecular dynamics simulation has showed that the hydrogen bond between asphaltene molecules dissociated at 523 K, while aromaticaromatic stacking interactions appeared to be still stable.15 These results demonstrated that the core aggregates of asphaltenes are stable and can be sustained up to 300 °C.17 Therefore, the occluded compounds inside the core of asphaltene aggregates should be stable even up to 300 °C, too. These results suggested that asphaltene occlusion could stably exist even under a hightemperature/high-pressure environment of reservoired oils. Our previous work has reported some adsorption/occlusion characteristics of asphaltenes;4,5 the preliminary results indicated that the adsorbed components can be exchanged with the outside (11) Leon, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5106-5112. (12) Pan, C.; Geng, A.; Liao, Z.; Xiong, Y.; Fu, J.; Sheng, G. Mar. Pet. Geol. 2002, 19, 619-632. (13) Russell, C. A.; Snape, C. E.; Meredith, W.; Love, G. D.; Clarke, E.; Moffatt, B. Org. Geochem. 2004, 35, 1441-1459. (14) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127-134. (15) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135-139. (16) Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Energy Fuels 2004, 18, 1243-1249. (17) Tanaka, R.; Sato, E.; Hunt, J.; Winans, R.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118-1125.
10.1021/ef050355+ CCC: $33.50 © 2006 American Chemical Society Published on Web 03/14/2006
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Table 1. Group Composition (wt %) of the Crude Oil (C1) and Chloroform Extracts of Oil Sand (C2) samples
saturated hydrocarbons
aromatic hydrocarbons
resins
asphaltenes
C1 C2
39.6 7.15
30.7 9.86
26.4 38.4
3.3 44.6
bulk phase in oil reservoirs, whereas the occluded compounds cannot. Here, this work is aimed to study the adsorption/ occlusion behaviors of asphaltenes associated with their different secondary evolution processes in oil reservoirs. Two asphaltenes derived from the same set of source rocks, one from a crude oil and the other from an oil sand, were studied in this work. Acetone extraction was used to obtain the adsorbed components from n-heptane asphaltenes, and the occluded compounds were released by H2O2 oxidation. Experimental Section Samples. A crude oil and an oil sand used in this work were supplied by the Total Company (Total, CSTJF, Avenue Larribau, 64018 Pau Cedex, France). These two samples were collected from Congo; the previous work from Total Company indicated that they were derived from the same set of source rocks bearing type-I kerogens, and the oil has been subjected to moderate biodegradation after being expelled from the source rocks. Table 1 shows the group composition of the crude oil and chloroform extracts of the oil sand. Resins and especially asphaltenes represent the dominant compounds in the oil sand extracts, whereas the other fractions have been heavily depleted during the evolution processes of oil sand. In comparison to the oil sample, the high asphaltene content in the oil sand extracts should be ascribed to the oil sand that has experienced strong alteration processes such as biodegradation, water washing/oxidation, evaporation, and so on. Asphaltene Preparation. The preparation method of asphaltenes has been described in details in our previous paper.18 In short, 1:1 (v/v) of crude oil or oil sand extracts/toluene were precipitated by n-heptane (all solvents used in this work were redistilled prior to use). The obtained n-heptane asphaltene solids, C1 from the crude oil and C2 from the oil sand extracts, appear brittle with a metallic luster. Acetone Extraction of n-Heptane Asphaltene. Acetone extraction of asphaltenes has been reported to separate asphaltenes into LMA (low molar mass asphaltene) and HMA (high molar mass asphaltene) subfractions,19 and the adsorbed hydrocarbons were believed to be removed from asphaltenes during this extraction process. Thus, in this work, from the acetone extracts, the hydrocarbons were interpreted as the adsorbed compounds inside asphaltene deposits. Then, the isolated n-heptane asphaltenes were subjected to Soxhlet extraction with acetone continuously for 10 days, and the extracts and residues were further fractionated into the other subfractions according to the flowchart depicted in Figure 1. The acetone extracts were separated by n-heptane into residue and maltene fractions. The maltenes were further separated into saturates, aromatics, and resins using SiO2/Al2O3 column chromatography. The acetone-extracted asphaltenes (asphaltene residue in Figure 1) were subsequently subjected to the oxidation process as described below. Oxidation of Aspaltene Residues. Fatty acids in high yields and high selectivity were obtained by H2O2 oxidation of low-rank coals.20,21 These reactions can be performed under mild conditions at room temperature and ambient pressure. The oxidation mechanism has been proposed as follows:22 (1) The oxidation depoly(18) Liao, Z.; Geng, A. Org. Geochem. 2002, 33, 1477-1486. (19) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171-1187. (20) Miura, K.; Mae, K.; Okutsu, H.; Mizutani, N. Energy Fuels 1996, 10, 1196-1201. (21) Mae, K.; Maki, T.; Araki, J.; Miura, K. Energy Fuels 1997, 11, 825-831.
merizes the coal by eliminating aromatic clusters. (2) When a cluster is eliminated, the intercluster bridges are converted into peripheral chains with carboxyl groups at the ends. According to this oxidation mechanism, H2O2/CH3COOH has been reported4,5 to release the occluded hydrocarbons from asphaltene structures without interference from the cleaved aliphatic chains covalent bonded to asphaltene molecules based on the interpretation that those covalent-bonded chains should be cleaved as polar compounds such as carboxyl acids under the oxidation conditions. Consequently, from the oxidation products of asphaltenes, the saturated hydrocarbons, if there are any, are not generated by covalent-bond cracking but just from those components occluded inside asphaltene structures. However, because the functionalized compounds (e.g., compounds bearing oxygen derived from the oxidation procedure) could be introduced into the other fractions (for example, aromatics) of the oxidation products, this work is then focused on the saturated hydrocarbons obtained from the asphaltene oxidation products. To release the occluded components from the asphaltenes, about 200 mg of asphaltene residues (Figure 1) were first transferred into a 250 mL flask using 20 mL of toluene, and then H2O2 (4 mL) and CH3COOH (15 mL) were mixed in a 50 mL beaker and slowly transferred into the flask containing the asphaltene residues, with stirring throughout. The reaction was carried out at ambient temperature (25 °C) with vigorous stirring for 48 h. The reaction products were then transferred into a 250 mL separating funnel using 50 mL of CHCl3, and 40 mL of saturated aqueous NaCl was added. The organic layer was collected and dried using Na2SO4. The organic phase was reduced to a constant weight using a rotary evaporator. This organic phase can be further separated into CH2Cl2-soluble and CH2Cl2-insoluble fractions. The CH2Cl2-soluble fraction was precipitated by n-heptane into residues and maltenes, and the latter subfraction was then further separated into saturates, aromatics, and resins using SiO2/Al2O3 column chromatography, eluted by n-heptane, toluene, and ethanol, respectively. Elemental Analysis. Carbon, hydrogen, and nitrogen contents of asphaltenes were obtained by combustion at 1050 °C on a Nitromatic 500 analyzer. The sulfur content was measured by combustion at 1320 °C on a sulfur coulometric analyzer. The oxygen content was determined by a coulometric analyzer. At 1120 °C, through the reaction with amorphous carbon, the oxygen was initially transformed to CO and then, with CuO, transformed to CO2, the content of which was determined by a coulometric analysis method. Gas Chromatography-Mass Spectrometry (GC-MS) Analysis. Full-scan GC-MS was performed using a Platform II MS detector combined with an HP6890 GC analyzer. The GC was fitted with a split/splitless injector, and a DB-1 MS column (60 m × 0.32 mm × 0.25 µm) was used. Helium was used as the carrier gas (1.2 mL/min). For saturated hydrocarbons, the oven temperature was initially set at 80 °C for 4 min, programmed to 290 °C at 4 °C/min, and then held isothermally for 45 min. To analyze aromatic hydrocarbons, the oven temperature was initially set at 80 °C for 4 min, programmed to 290 °C at 3 °C/min, and then held isothermally for 20 min. The MS was operated with an ionization energy of 70 eV, a source temperature of 155 °C, an electron multiplier voltage of 1765 V, and a mass range from 19 to 500 amu.
Results and Discussion Composition of the Acetone Extracts from n-Heptane Asphaltenes. All of the numbers in Figure 1 represent percentage yield values (wt %) based on the initial n-heptane asphaltene. There is a big difference concerning the amounts of acetone extracts from C1 (17.09%) versus C2 (6.22%) asphaltenes. This is owing to their different evolution processes; in the crude oil, the adsorbed compounds in C1 asphaltene can exchange with outside crude oils and then can maintain an approximate mass (22) Hayashi, J.; Chiba, T. Energy Fuels 1999, 13, 1230-1238.
Adsorption/Occlusion Properties of Asphaltenes
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Figure 1. Flowchart for the acetone extraction of n-heptane asphaltenes and the subsequent oxidation processes. All of the numbers represent percentage yield values (wt %) based on the initial n-heptane asphaltene. Table 2. Results (wt %) of the Elemental Analyses of the n-Heptane Asphaltenes samples
carbon C
hydrogen H
nitrogen N
oxygen O
sulfur S
H/C
N/C
O/C
S/C
C1 C2
84.48 79.50
8.45 7.94
1.56 1.10
2.45 1.99
2.86 7.21
1.200 1.198
0.0158 0.0119
0.0218 0.0188
0.0127 0.0340
balance for the adsorbed fraction. In crude oils, exchanges between the asphaltene-adsorbed components and crude oil maltenes have been proven by some other work.4,5 However, for C2 asphaltene from the oil sand, the adsorbed components can only be unilaterally released into the outside bulk phase and cannot obtain replenishment during the evolution processes of oil sand. Thus, much less has been found adsorbed in C2 asphaltene. From the acetone extracts, residues precipitated by n-heptane have been referred to as the HSA (higher soluble asphaltene) fraction.4 From the adsorbed maltenes within C1 and C2 asphaltenes, the big difference of their amounts (13.42 versus 4.11%) is mainly caused from their hydrocarbon compositions (Figure 1). This indicated that most of the hydrocarbons (both saturates and aromatics) adsorbed in C2 asphaltene have been lost during the oil sand evolution processes, while in C1 asphaltene, they can be recruited from the crude oils. Table 2 shows the elemental results of C1 and C2 n-heptane asphaltenes. For C2 asphaltene, C and H contents are lower than C1, whereas the S% content (7.21 versus 2.86%) is much higher than C1 asphaltene. The big difference of the S% content between these two asphaltenes should be ascribed to their different postalteration processes occurring in oil reservoirs. The oil sand may have experienced strong alterations such as biodegradation, water washing/oxidation, evaporation, mineralization, and so on. During these oil sand evolution processes, in C2 asphaltene,
the adsorbed compounds, especially hydrocarbons, have then been released into the outside bulk sand. Distribution of Hydrocarbons from the Acetone Extracts. For all of the following parts, in the GC-MS chromatograms, the peak identifications were made by comparing their mass spectra with the available standard compounds from the MassLynx Library (NIST Libraries and Structures, version 1.0) appended to the MS analyzer or by comparing their mass spectra with those reported in the listed references. From the saturated hydrocarbons of acetone extracts from C1 asphaltene, only cyclic hydrocarbons, especially pentacyclic terpane compounds, have been detected (Figure 2). This is because the crude oil has experienced biodegradation in the oil reservoir. During the biodegradation process, the n-alkanes and alkyl-branched alkanes were heavily depleted and nearly only the cyclic hydrocarbon remnants have been exchanged into C1 asphaltene structures as the adsorbed fraction. This result also suggested that exchange between the adsorbed components and the crude oil maltenes can effectively take place upon geological time. However, from the C2 acetone extracts, although the amount of saturated hydrocarbons is much less than the C1 sample (0.65 versus 5.27%, Figure 1), some n-alkanes and alkyl-branched alkanes have been detected as the dominant compounds in the saturates (Figure 2). This is consistent with the interpretation that the adsorbed components in C2 asphaltene can only be unilaterally released into the outside bulk sand. Without
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Figure 2. Saturated hydrocarbons from acetone extracts of C1 and C2 asphaltenes.
Figure 3. Aromatic hydrocarbons from acetone extracts of C1 and C2 asphaltenes. Peak identifications are given in Table 3.
replenishment from the outside oil sand phase, only a small amount of hydrocarbon remnants as the earlier adsorbed components have survived the oil sand evolution processes. Similarly, for the aromatic hydrocarbons from acetone extracts of C1 and C2 asphaltenes, a big difference was found from each other concerning either their amount (3.01 versus 0.43%, Figure 1) or the distributions (Figure 3, with peak identifications given in Table 3). After the different secondary evolution processes between C1 and C2 asphaltenes, Figure 3a represented the aromatic information derived from the crude oil maltenes, showing a relatively complete distribution for a series of naphthalene, phenanthrene, and triphenylene compounds (Table 3), which can be steadily detected from the crude oil maltenes. However, only some aromatic remnants have been found
Table 3. Identification of Peaks in Figure 3 peak
identification
peak
identification
1 2 3 4 5 6 7 8
Figure 3a dimethyl-naphthalene trimethyl-naphthalene phenanthrene methyl-phenanthrene dimethyl-phenanthrene trimethyl-phenanthrene tetramethyl-phenanthrene triphenylene
9 10 11
methyl-triphenylene dimethyl-triphenylene trimethyl-triphenylene Figure 3b butylated hydroxytoluene tetramethyl-biphenyl phenanthrene pyrene
1 2 3 4
adsorbed in C2 asphaltene (Figure 3b), most of which have been lost during the oil sand evolution processes. Oxidation Products from the Asphaltene Residues. From the asphaltene oxidation products, most of which are CH2Cl2-
Adsorption/Occlusion Properties of Asphaltenes
Energy & Fuels, Vol. 20, No. 3, 2006 1135
Figure 4. Saturated hydrocarbons from oxidation products of C1 and C2 asphaltenes.
insoluble (Figure 1). Out of the CH2Cl2-soluble compounds, some maltenes were obtained, from which a small amount of hydrocarbons were obtained. Because the experimental results indicated that the oxidation procedure can produce some functionalized compounds, e.g., heteroatom compounds, into the aromatics and resins, this work is then focused on the saturates obtained from the oxidation products. These saturated hydrocarbons represented the original oils occluded inside asphaltenes, thus bearing some important earlier stage information of the oil reservoirs.5,18 Although there is a distinct difference between the hydrocarbons adsorbed in C1 and C2 asphaltenes (Figures 2 and 3), the occluded saturated hydrocarbons show almost the same distribution features for these two samples (Figure 4). This is because these two asphaltenes (C1 from crude oil and C2 from oil sand) were derived from the same set of source rocks and have occluded almost the same original oils inside the macromolecular structures of asphaltenes. These results also suggested that these saturated hydrocarbons should have been occluded inside asphaltenes before asphaltenes were detached from kerogen, because these hydrocarbons have been minimally affected by the postalteration processes occurring in oil reservoirs, as a result of the effective protection from asphaltene macromolecular structures. In Figure 4, a series of n-alkanes and some alkyl-branched alkanes were detected, occluded inside C1 and C2 asphaltenes, among which exceptional abundant C21 and C22 highly branched isoprenoid alkanes have been detected for both samples. When their mass spectra were compared to those reported by Sinninghe Damste et al.,23 these two branched alkanes should be 2,6,10trimethyl-7-(3-methylpentyl)dodecane and 3,7,11-trimethyl-6(3-methylpentyl)tridecane, respectively. These two compounds that have been reported may have derived from the methylation of the parent C20 highly branched alkane by stereospecific (23) Sinninghe Damste, J.; Baas, M.; Geenevasen, J.; Kenig, F. Org. Geochem. 2005, 36, 511-517.
enzyme-controlled biosynthesis from an as yet unidentified organism.23-25 The presence of these two highly branched alkanes occluded inside asphaltenes will be further addressed in our future work. Conclusions Two asphaltenes derived from the same set of source rocks, one from a crude oil and the other from an oil sand, have been studied in this work. The experimental results indicated that there is a big difference between the asphaltene-adsorption compounds concerning either their amounts or distributions for these two asphaltenes. This is ascribed to their different secondary evolution processes in oil reservoirs; from the crudeoil-derived asphaltene, the adsorbed compounds can exchange with the outside oil maltenes and then maintain an approximate mass balance from the bulk crude oil phase. However, much less adsorbed components, especially for hydrocarbons, were obtained from the oil-sand-derived asphaltene, because most of the adsorbed compounds have been unilaterally released into the outside bulk sand during the oil sand evolution processes. Nevertheless, the occluded saturated hydrocarbons show almost the same distribution features for these two asphaltenes. This is consistent with the fact that they were derived from the same set of source rocks and have occluded almost the same original oils inside the macromolecular structures of asphaltenes. This result also suggested that these saturated hydrocarbons should have been occluded inside asphaltenes before asphaltenes were detached from kerogen, because these hydrocarbons have been minimally affected by the postalteration processes occurring in oil reservoirs, as a result of the effective protection from asphaltene macromolecular structures. Acknowledgment. We are very grateful to Mr. Honggang Zhou from the Total Company for supplying the samples and some (24) Kenig, F.; Huc, A.; Purser, B.; Oudin, J. Org. Geochem. 1990, 16, 735-747. (25) Kenig, F.; Sinninghe Damste, J.; van Dalen, A.; Rijpstra, W.; Huc, A.; de Leeuw, J. Geochim. Cosmochim. Acta 1995, 59, 2999-3015.
1136 Energy & Fuels, Vol. 20, No. 3, 2006 helpful discussion. Z. L. gratefully acknowledges the receipt of a postdoctorate research scholarship from the FFCSA project (Fondation Franco-Chinoise pour la Science et ses Applications). This work has been financed by grants from the National Science Foundation of China (NSFC 40302035), from the Project of
Liao et al. President Foundation of Chinese Academy of Sciences, CAS JiJi 904, and from Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCX-04-08). EF050355+
