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Energy & Fuels 2009, 23, 1178–1188
Nanoaggregates of Asphaltenes in a Reservoir Crude Oil and Reservoir Connectivity† Soraya S. Betancourt,‡ G. Todd Ventura,§ Andrew E. Pomerantz,‡ Oswaldo Viloria,| Francois X. Dubost,‡ Julian Zuo,‡ Gene Monson,| Diane Bustamante,| Jeremiah M. Purcell,⊥ Robert K. Nelson,§ Ryan P. Rodgers,⊥ Christopher M. Reddy,§ Alan G. Marshall,⊥ and Oliver C. Mullins*,‡ Schlumberger Doll Research, Cambridge, Massachusetts 02139, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, Pioneer Natural Resources, IrVing, Texas 75039, and National High Magnetic Field Laboratory, Florida State UniVersity, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310-4005 ReceiVed August 9, 2008. ReVised Manuscript ReceiVed NoVember 11, 2008
Recently, asphaltenes have been shown to form nanoaggregates in toluene at very low concentrations (10-4 mass fraction). Subsequently, in situ analysis of a 3000 ft vertical column of crude oil by downhole fluid analysis (DFA) indicated that the asphaltenes in a black crude oil exhibit gravitational sedimentation according to the Boltzmann distribution and that the asphaltene colloidal size is ∼2 nm. Here, we perform a follow-up study of a reservoir black oil from a different field. The black oil in a 658 ft vertical column is analyzed by DFA and advanced laboratory analytical chemical methods. An asphaltene colloidal particle size is found to be ∼2 nm according to the Archimedes buoyancy term in the Boltzmann distribution. In addition, an equation of state (EoS) approach based on literature critical constants and molecular weights for asphaltenes gives an aggregation number of ∼8. Molecular compositional similarities between different oil samples were established with comprehensive two-dimensional gas chromatography (GC × GC). Likewise, results from electrospray ionization Fourier transform ion cyclotron resonance mass spectroscopy (ESI FT-ICR MS) of the samples are consistent with the oils being from the same equilibrium column of oil. The results herein support a growing body of literature indicating that asphaltenes in black oils form relatively tightly bonded nanoaggregates of a single size range. The similarity of results between asphaltenes in crude oil and asphaltenes in toluene points to a very limited role of resins in these nanoaggregates, in contrast to much speculation. The implications of this work on the determination of reservoir connectivity are discussed.
Introduction Asphaltene Molecular Weight. Significant advances have been taking place in asphaltene science in recent years.1 Previously, asphaltene molecular weight had been one of the biggest uncertainties in asphaltene science; at this time, it is now largely resolved. Because the issues of asphaltene molecular weight and asphaltene molecular architecture are critical to understanding asphaltene nanoaggregates, we briefly describe germane developments. The precept for the new field of petroleomics is to understand the properties of crude oils and asphaltenes in terms of their molecular constituents.1 Four molecular diffusion techniques have been used to characterize asphaltene molecular size and weight: time-resolved fluorescence depolarization (TRFD),2-4 Taylor dispersion (TD),5 † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. E-mail: omullins@ houston.oilfield.slb.com. ‡ Schlumberger Doll Research. § Woods Hole Oceanographic Institution. | Pioneer Natural Resources. ⊥ Florida State University. (1) Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007. (2) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and structure. J. Phys. Chem. A 1999, 103, 11237–11245. (3) Goenzin, H.; Mullins, O. C. Molecular sizes of asphaltenes from different origin. Energy Fuels 2000, 14, 677.
nuclear magnetic resonance (NMR),6 and fluorescence correlation spectroscopy (FCS).7-9 The agreement among these four different techniques is excellent. For example, Iino et al. remarked about their TD results on the Tanito Harum coal asphaltene:5 “These results are consistent with the results of Groenzin & Mullins.” The diffusion measurements provide an estimate of petroleum asphaltene molecular weight of ∼750 Da, with a full width half-maximum (fwhm) of 500-1000 Da. (4) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and weight by time-resolved fluorescence depolarization. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 2. (5) Wargadalam, V. J.; Norinaga, K.; Iino, M. Size and shape of a coal asphaltene studied by viscosity and diffusion coefficient measurements. Fuel 2002, 81, 1403. (6) Freed, D. E.; Lisitza, N. V.; Sen, P. N.; Song, Y.-Q. Asphaltene molecular composition and dynamics from NMR diffusion measurements. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 11. (7) Andrews, A. B.; Guerra, R.; Sen, P. N.; Mullins, O. C. Diffusivity of asphaltene molecules by fluorescence correlation spectroscopy. J. Phys. Chem. A 2006, 110, 8095. (8) Guerra, R.; Andrews, A. B.; Mullins, O. C.; Sen, P. N. Comparison of asphaltene molecular diffusivity of various asphaltenes by fluorescence correlation spectroscopy. Fuel 2007, 86, 2016–2020. (9) Schneider, M.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Asphaltene molecular size from translational diffusion constant by fluorescence correlation spectroscopy. Energy Fuels 2007, 21, 2875–2882.
10.1021/ef800598a CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
Nanoaggregates of Asphaltenes
Many different mass spectral (MS) methods have been applied to asphaltenes, including field ionization,10 electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) MS,11,12 atmospheric pressure chemical ionization MS,13 atmospheric pressure photoionization,13-15 field desorption mass spectroscopy,16 and various methods of laser desorption ionization (LDI) MS. All MS methods, except LDI, obtain results roughly in accordance with all diffusion measurements regarding molecular weight. LDI MS has yielded inconsistent results and thus merits a short discussion. Some LDI reports yield very large asphaltene molecular weight;17,18 however, it is quite plausible that these results are artifacts induced by gas-phase aggregation of asphaltenes.19,20 In particular, it has been shown in LDI MS experiments on asphaltenes that gas-phase aggregation of asphaltenes yields very large apparent molecular weights if (1) high laser power is used, (2) high surface concentrations of asphaltenes are used, or (3) if the ions are collected rapidly after the plasma is formed.21-23 If these factors are properly accounted for, then LDI MS results on asphaltenes are consistent with results from all other mass spectral methods and with all diffusion results.21-23 In addition, two-step laser mass spec(10) Boduszynski, M. M. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, D.C., 1981; Chapter 7. (11) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced characterization of petroleum derived materials by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 3. (12) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A.; Asomaning, S. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20, 1965–1972. (13) Merdrignac, I.; Desmazieres, B.; Terrier, P.; Delobel, A.; Laprevote, O. Analysis of raw and hydrotreated asphaltenes using off-line and on-line SEC/MS coupling. Proceedings of the Heavy Organic Deposition, Los Cabos, Baja California, Mexico, 2004. (14) Cunico, R. I.; Sheu, E. Y.; Mullins, O. C. Molecular weight measurement of UG8 asphaltene by APCI mass spectroscopy. Pet. Sci. Technol. 2004, 22, 787. (15) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry for detailed compositional analysis of petroleum In the 9th International Conference on Petroleum Phase Behavior and Fouling, Victoria, British Columbia, Canada, June 15-19, 2008; Abstract 17. (16) Qian, K.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Desorption and ionization of heavy petroleum molecules and measurement of molecular weight distributions. Energy Fuels 2007, 21 (2), 1042–1047. (17) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. On the limitations of UV-fluorescence spectroscopy in the detection of highmass hydrocarbon molecules. Energy Fuels 2005, 19, 164. (18) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Characterization of heavy hydrocarbons by chromatographic and mass spectrometric methods: An overview. Energy Fuels 2007, 21, 2176–2203. (19) Mullins, O. C. Rebuttal to Comment by Professors Herod, Kandiyoti, and Bartle on “Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen” by S. Badre, C. C. Goncalves, K. Norinaga, G. Gustavson and O. C. Mullins. Fuel 2007, 86, 309. (20) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Contrasting perspective on asphaltene molecular weight. This Comment vs the Overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti. Energy Fuels 2008, 22 (3), 1765–1773. (21) Hortal, A. R.; Martı´nez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments. J. Mass Spectrom. 2006, 41, 960. (22) Martı´nez-Haya, B.; Hortal, A. R.; Hurtado, P. M.; Lobato, M. D.; Pedrosa, J. M. Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates. J. Mass Spectrom. 2007, 42, 701–713. (23) Hortal, A. R.; Hurtado, P. M.; Martı´nez-Haya, B.; Mullins, O. C. Molecular weight distributions of coal and petroleum asphaltenes from laser desorption ionization experiments. Energy Fuels 2007, 21, 2863–2868.
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trometry (L2MS), a two color variant of LDI MS, has been performed on asphaltenes.24,25 In this experiment, an IR laser is used to desorb the asphaltenes, while a UV laser is used for ionization. This process decouples desorption from ionization. The lack of dependence of the mass spectral results on the IR laser power, the UV laser power, the surface concentration of asphaltenes, or the laser pulse timing indicates that all asphaltene species are being desorbed and accounted for properly. These experiments yield a most probable asphaltene molecular weight of ∼600 Da.24,25 There persists roughly a 40% discrepancy among all of the different techniques for the most likely molecular weight of asphaltenes. This is reduced from the previous debate over one or more orders of magnitude. Some of these differences might be in the identity of the samples. Asphaltene Molecular Architecture. Asphaltene molecular architecture is of great interest for understanding aggregation. Many previously proposed asphaltene molecular structures are inconsistent with the (now) known asphaltene molecular weights. TRFD results clearly imply that asphaltene molecules have one or perhaps two aromatic fused ring systems because small, bluefluorescing chromophores are found in small molecules while big, red-fluorescing chromophores are found in much bigger molecules.2-4 The most likely molecules correspond to chromophores with seven fused aromatic rings on average.2-4 Direct molecular imaging of asphaltene molecules by scanning tunneling microscopy gave a histogram with 11 Å and ∼6 fused aromatic rings as most likely for asphaltenes.26 The same conclusion is obtained from high-resolution transmission electron microscopy.27 Molecular orbital calculations performed to account for optical absorption and emission spectra of asphaltenes show that seven fused rings is most likely.28,29 Raman spectroscopy has been used to conclude that the range of diameters of aromatic sheets is from 11 to 17 Å, which is slightly larger but still consistent with the other measurements.30 New results from single (asphaltene) molecule decomposition studies are clearly showing the presence of large quantities of asphaltene molecules with single fused ring systems.31 These experiments employ ultrahigh-resolution mass spectroscopy and are uniquely capable of following decomposition at the molecular level without interference from intermoleuclar interactions and other perturbations.31 Coal asphaltenes are known to be of much smaller mass than petroleum asphaltenes.3-5,8,9,22,25 For example, one study found coal asphaltenes to be about half the mass of petroleum (24) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Two-step laser mass spectrometry of asphaltenes. J. Am. Chem. Soc. 2008, 130, 7216–7217. (25) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Asphaltene molecular weight distribution determined by twostep laser mass spectrometry. Energy Fuels, manuscript submitted. (26) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scan. Microsc. 1994, 8, 463. (27) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Probing order in asphaltenes and aromatic ring systems by HRTEM. Energy Fuel 2002, 16, 490. (28) Ruiz-Morales, Y.; Mullins, O. C. Polycyclic aromatic hyodrocarbons of asphaltenes analyzed by molecular orbital calculations with optical spectroscopy. Energy Fuels 2007, 21, 256. (29) Ruiz-Morales, Y.; Wu, X.; Mullins, O. C. Electronic absorption edge of crude oils and asphaltenes analyzed by molecular orbital calculations with optical spectroscopy. Energy Fuels 2007, 21, 944. (30) Bouhadda, Y.; Bormann, D.; Sheu, E. Y.; Bendedouch, D.; Kallafa, A.; Daaou, M. Characterization of Hassi-Messaoud asphaltene structure using Raman spectroscopy and X-ray diffraction. Fuel 2007, 86, 1855– 1864. (31) Rodgers, R. P.; Tan, X.; Ehrmann, B. M.; Juyal, P.; McKenna, A. M.; Purcell, J. M.; Schaub, T. M.; Gray, M. R.; Marshall, A. G. Asphaltene structure determined by mass spectrometry. In the 9th International Conference on Petroleum Phase Behavior and Fouling, Victoria, British Columbia, Canada, June 15-19, 2008; Abstract 102.
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asphaltenes.25 13C NMR shows that, in contrast to petroleum asphaltenes, coal asphaltenes have almost no alkyl carbons, similar to their source material.32 The smaller intermolecular repulsion of the small alkane content of coal asphaltenes mandates a smaller aromatic fused ring system in the molecule. With less alkane and a smaller single polycyclic aromatic hydrocarbon (PAH) per molecule, the coal asphaltene molecules are much smaller, as observed by many techniques.3-5,8,9,22,25 That is, the argument is advanced that all asphaltenes, a solubility fraction (toluene-soluble-n-heptane-insoluble), are governed by a balance of attractive and repulsive forces. Larger repulsive forces would cause too much solubility, while large attraction would result in insufficient solubility to be asphaltene. The two primary forces are van der Waals attraction of the (predominantly) single heteroatom-substituted PAH per molecule versus steric repulsion of alkane chains.32 The prediction then is that increased mild cracking of petroleum source material for asphaltenes, thus increasing the removal of alkane substituents, should result in smaller, blue-shifted PAHs and smaller asphaltene molecules. The bigger asphaltenes with the loss of alkane end up as coke, totally insoluble. All of these effects have been shown.33 The tight correlation of PAH ring size with asphaltene molecular attraction follows only if the primary asphaltene molecular architecture consists of one or perhaps two fused ring systems per molecule. If there were many fused ring systems per molecule, then the number of different fused ring systems would strongly determine attractive forces, thereby obscuring the observed tight correlation. If one constructs a molecule consisting of seven fused rings, 60% of the molecular carbon representing alkyl substituents with one or two heteroatoms, the most likely resulting asphaltene molecular weight will be 750 Da. Single molecule decomposition studies yield results consistent with this picture.31 Nevertheless, decomposition studies carried out in condensed phase indicate the presence of some asphaltene molecular architecture with more than one fused ring system per molecule.34 Perhaps intermolecular interactions are playing a role in these results or perhaps these multi-PAH species are present in small mass fractions in asphaltenes. Asphaltene Nanoaggregation. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) of asphaltenes consistently indicate that asphaltic materials contain a small particle of several nanometer dimensions.35-39 This particle size is missing from SANS on maltenes35 (maltenes are deasphaltened crude oils). In addition, high-Q ultrasonics was recently (32) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. The overriding chemical principles that define asphaltenes. Energy Fuel 2001, 15, 972. (33) Buch, L.; Groenzin, H.; Buenrostro-Gonzalez, E.; Andersen, S. I.; Lira-Galeana, C.; Mullins, O. C. Effect of hydrotreatment on asphaltene fractions. Fuel 2003, 82, 1075. (34) Gray, M. R. Consistency of asphaltene chemical structures with pyrolysis and coking behavior. Energy Fuels 2006, 17, 1566–1569. (35) Sheu, E. Y. Phys. ReV. A: At., Mol., Opt. Phys. 1992, 45, 2428– 2438. (36) Sheu, E. Y. Petroleomics and characterization of asphaltene aggregates using small angle scattering. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 14. (37) Barre, L.; Simon, S.; Palermo, T. Solution properties of asphaltenes. Langmuir 2008, 24 (8), 3709–3717. (38) Fenistein, D.; Barre, L.; Espinat, D.; Livet, A.; Roux, J.-N.; Scarcella, M. Langmuir 1998, 14, 1013–1020. (39) Gawrys, K.; Kilpatrick, P. J. Colloid Interface Sci. 2005, 288, 325– 334.
Betancourt et al.
performed on asphaltene solutions.40-42 Ultrasonic velocity u is given by u ) (1/βF)1/2, where β is the compressibility and F is the density. With a high-Q system, small changes in the velocity can be measured; aggregation causes the differential quantity, compressibility, to change. The high-Q ultrasonics experiments established that (1) nanoaggregates form at 10-4 mass fraction and (2) once formed, the nanoaggregates do not change in size with increasing concentration.40-42 These measurements did not detect any aggregation in maltenes alone.41,42 NMR measurements have confirmed the ultrasonics results.6 The hydrogen index shows a robust and unmistakable break at the critical nanoaggregate concentration (CNAC).6 In addition, the NMR diffusion measurements showed a factor of 2 reduction in the diffusion constant at the CNAC.6 The translational diffusion constant is linear in radius, indicating a volume change for nanoaggregation of a factor of 8, thus roughly 8 molecules per nanoaggregate. This volume is consistent with the above predictions based on molecular architecture. Most importantly, both the hydrogen index measurements and the diffusion measurements show no change in the nanoaggregate size with asphaltene concentration, confirming the ultrasonics measurements. AC conductivity measurements also show the same CNAC.43 In addition, centrifugation studies of asphaltene in toluene have now confirmed that the nanoaggregates can be collected at the bottom of a centrifuge tube, while the molecules remain suspended in a molecular solution.44 The corresponding CNAC is ∼100 mg/L, which is close to the value from other measurements.44 We note that the SANS and SAXS results give a dimension (radius of gyration), which is roughly a factor of 2 larger than the hydrodynamic radius. It may be that the SANS and SAXS determined sizes of the nanoaggregates and aggregation numbers are overestimated. Previous surface tension data and microcalorimetry data had been interpreted to give much larger CNACs of asphaltenes, several grams per liter. It is plausible that these results were misinterpreted. Specifically, for the surface tension, a highenergy molecule on the surface of toluene, a low-surface-tension liquid, would cause an increase and not a decrease in surface tension.45 At several grams per liter, asphaltene nanoaggregates in toluene exhibit clustering.46,47 Indeed, recent work on a model system shows that inverse micelles loaded onto the surface of (40) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants. Langmuir 2005, 21 (7), 2728–2736. (41) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga-Larios, F.; Perez, E.; Mullins, O. C. Nanoaggregates and structure-function relations in asphaltenes. Energy Fuels 2005, 19, 1282– 1289. (42) Andreatta, G.; Bostrom, N.; Mullins, O. C. Ultrasonic study of asphaltene aggregation. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 9. (43) Sheu, E. Y.; Long, Y.; Hamza, H. Asphaltene self-association and precipitation in solventssAC conductivity measurements. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 10. (44) Mostowfi, F.; Indo, K.; Mullins, O. C.; McFarlane, R. Critical nanoaggregate concentration of asphaltenes by centrifugation. Energy Fuels, in press. (45) Friberg, S. E.; Mullins, O. C.; Sheu, E. Y. Surface activity of an amphiphilic association structure. J. Dispersion Sci. Technol. 2005, 26, 513. (46) Yudin, I. K.; Anisimov, M. A. Dynamic light scattering monitoring of asphaltene aggregation in crude oils and hydrocarbon solutions. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 18. (47) Oh, K.; Deo, M. D. Near-infrared spectroscopy to study asphaltene aggregation solvents. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 19.
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Energy & Fuels, Vol. 23, 2009 1181
organic solvents result in a decrease of surface tension.48 Inverse micelles have primarily alkane groups projecting outward; these alkane groups become exposed to the liquid surface, thereby lowering surface tension. It is likely that the reported grams per liter “CMCs” were actually obtaining the concentration of nanoaggregate clustering. In heavy oils, there is evidence for larger asphaltene structures.49 These are plausibly clusters of asphaltene nanoaggregates, which would be expected to form at the high asphaltene concentrations in heavy oil. Asphaltene Nanoaggregates in Crude Oil. X-ray scattering measurements on crude oil have established the presence of the 002 scattering peak, the graphitic layering peak, strongly indicating at least some aggregation of asphaltenes in crude oil.50 Recently, a 3000 ft vertical column of oil in the Tahiti Reservoir, deepwater Gulf of Mexico, was examined for possible asphaltene sedimentation effects.51 The oil column was examined by downhole fluid analysis (DFA)52 and by various laboratory methods. The oil color that one visually sees was shown to relate proportionally to the asphaltene content. Coloration is a standard measurement of DFA.53 The concentration of asphaltenes was shown to vary by a factor of 2.5 over the 3000 ft vertical column in a tilted sheet reservoir. For each of the three distinct reservoir sand bodies,54 the asphaltenes were shown to obey eq 1, the Boltzmann distribution with Archimedes buoyancy giving the energy of excitation
{
ODh V∆Fgh ) exp OD0 kT
}
(1)
where V is the asphaltene colloidal particle volume, ∆F is the density contrast between the liquid oil phase and the asphaltene nanoaggregate, g is earth’s gravitational acceleration, h is the height above the oil column base at height 0, k is Boltzmann’s constant, and T is the reservoir temperature. ODi is the optical density (optical absorption) of the oil color at height i, measured by DFA and confirmed in the laboratory. From a fit to the DFA data, the only adjustable parameter, the asphaltene particle size, is determined. The Tahiti field results for each of the three separate reservoir sands are that the asphaltene colloidal particle diameter is ∼1.6 nm.51,54 One of the biggest concerns with the analysis is whether or not the sands are in fact continuous across the reservoir, a prerequisite to using the equilibrium model in eq 1. Indeed, compartmentalization, the lack of reservoir continuity, is one of the biggest problems in the oil industry (48) Friberg, S. E.; Al Bawab, A.; Abdoh, A. A. Surface active inverse micelles. Colloid Polym. Sci. 2007, 285, 1625–1630. (49) Zhao, B.; Shaw, J. M. Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007, 21, 2795. (50) Chianelli, R. R.; Siadati, M.; Mehta, A.; Pople, J.; Ortega, L. C.; Chiang, L. Y. Self-assembly of asphaltene micelles: Synchrotron, simulation and chemical modeling techniques applied to problems in the structure and reactivity of asphaltenes. Near-infrared spectroscopy to study asphaltene aggregation solvents. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 15. (51) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Creek, J. L.; Andrews, B. A.; Dubost, F.; Venkataramanan, L. The colloidal structure of crude oil and the structure of reservoirs. Energy Fuels 2007, 21, 2785– 2794. (52) Fujisawa, G.; Mullins, O. C. Live oil sample acquisition and downhole fluid analysis. Asphaltenes, HeaVy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 22. (53) Mullins, O. C.; Mitra-Kirtley, S.; Zhu, Y. Electronic absorption edge of petroleum. Appl. Spectrosc. 1992, 46, 1405. (54) Betancourt, S. S.; Dubost, F. X.; Mullins, O. C.; Cribbs, M. E.; Creek, J. L.; Mathews, S. G. Predicting downhole fluid analysis logs to investigate reservoir connectivity. SPE IPTC 11488, Dubai, United Arab Emirates, 2007.
today.55,56 In particular, compartmentalization is plausibly indicated if there is a discontinuous change of a fluid property in the reservoir (except expected phase boundaries) or if there is a density inversion in the oil column with higher density fluids higher in the column.55,56 In this paper, we analyze a tilted sheet reservoir containing a 658 ft vertical column from a different deepwater field. We use DFA and standard laboratory methods to analyze the asphaltene content as a function of height. The reservoir parameters are known to be conducive to an equilibrium distribution of fluids. The results of this analysis show that asphaltene colloidal size in crude oils is ∼1.6 nm. Equation of state (EoS) methods are employed to obtain an aggregation number of ∼8 for the asphaltene nanoaggregate. We analyze the reservoir connectivity issue by linking DFA with comprehensive two-dimensional gas chromatography (GC × GC)57 and ESI FT-ICR MS.58 These advanced laboratory analytic methods support the assertion of fluid connectivity within the reservoir. Implications of the nanocolloidal size are discussed. Experimental Section MDT Sample Acquisition and DFA. The DFA measurements are described elsewhere.52,56 Downhole samples are acquired with the MDT tool that is in a section of an open borehole that has not yet been cased. The MDT has a stout tube that presses against the borehole wall with great force, establishing hydraulic communication with the permeable zone of interest. A rubber packer around the probe and mud cake formed by the drilling mud create hydraulic seals that prevent borehole fluids from getting into the MDT. An onboard pump drops the pressure in the MDT sampling tube or probe, thereby causing reservoir fluids to enter the MDT. A 10channel filter spectrometer in the LFA tool performs the spectral measurement at 3 Hz frequency on the reservoir fluids as they are being pumped through the MDT.52 Measurement errors in OD for the DFA are typically less than 0.02. The oil samples were from two wells; the shallower sample PER-1 is from a depth of xx335 feet, while the deeper sample PER-2 is from xx993 feet. xx is used to conceal the actual depth of the samples; the relative depth difference is what matters here, and that difference is 658 feet. DFA results from a third well are also discussed herein. The asphaltene content of the dead reservoir crude oils was determined by standard n-heptane precipitation and from a 40:1 n-heptane solution. After 24 h, the solution was filtered and washed with n-heptane. After redissolving/suspending the filtered asphaltene in a minimum of toluene, this precipitation process was repeated. A dead crude oil has been exposed to atmospheric pressure; thus, dissolved gases under pressure are lost in contrast to a live crude oil, which possesses all its dissolved gases (at pressure), as under reservoir conditions. GC × GC Chemical Preparation. The asphaltenes of each oil sample were removed prior to gas chromatographic analysis. The (55) Mullins, O. C.; Fujisawa, G.; Hashem, M. N.; Elshahawi, H. Determination of coarse and ultra-fine scale compartmentalization by downhole fluid analysis coupled with other logs. Society of Petroleum Engineers (SPE) International Petroleum Technology Conference Paper 10036, 2005. (56) Mullins, O.C. The Physics of ReserVoir Fluids, DiscoVery through Downhole Fluid Analysis; Schlumberger Press: Cambridge, MA, 2008. (57) Mullins, O. C.; Nelson, R. K.; Ventura, G. T.; Raghuraman, B.; Reddy, C. M. Oil reservoir characterization by coupling downhole fluid analysis with comprehensive laboratory 2D-GC analysis of crude oils. Energy Fuels 2008, 22, 496–503. (58) Mullins, O. C.; Rodgers, R. P.; Weinheber, P.; Klein, G. C.; Venkatramanan, L.; Andrews, A. B.; Marshall, A. G. Oil reservoir characterization via crude oil analysis by downhole fluid analysis in oil wells with visible-near-infrared spectroscopy and by laboratory analysis with electrospray ionization-Fourier transform ion cyclotron resonance mass spectroscopy. Energy Fuels 2007, 21, 256.
1182 Energy & Fuels, Vol. 23, 2009 maltene fractions were collected by vacuum filtration by passing 40 mL of heptane/g of oil through a 0.5 µm Millipore GF/F fiberglass filter. The asphaltene fractions were then collected by a second solvent rinse using 40 mL of DCM/MeOH (1:1, v/v). Each fraction was rotovapped and dried under a continuous stream of compressed nitrogen. GC × GC-Flame Ionization Detector (FID). Each extract was analyzed with a GC × GC-FID that employed a loop-jet modulator, purchased from the Zoex Corporation, Lincoln, NE. The complete system included an Agilent 6890 gas chromatograph configured with a 7683 series split/splitless autoinjector, two capillary gas chromatography columns, and an FID. Extracts were injected in splitless mode. The purge vent was opened at 0.5 min. The front inlet temperature was set to 300 °C. The FID signal was sampled at 100 Hz. The carrier gas was H2, at a constant flow rate of 1.0 mL/min. The first-dimension column was a nonpolar 100% dimethylpolysiloxane phase (Restek Rtx-1 Crossbond, 20 m length, 0.25 mm inner diameter, 0.25 µm film thickness) that was programmed to remain isothermal at 45 °C for 3.5 min and then ramped from 45 to 320 °C at 1.60 °C/min. The modulation loop was deactivated fused silica (1.0 m length, 0.20 mm inner diameter). The thermal modulator cold jet gas was dry N2, chilled with liquid Ar, set to a constant flow rate of 2.0 L/min. The thermal modulator hot jet air was heated to 100 °C above the temperature of the first oven. The hot jet pulse width was 350 ms every 12.5 s (0.08 Hz), producing a 12.5 s modulation period. Second-dimension separations were performed with a 50% phenyl polysilphenylene-siloxane column (SGE BPX-50, 0.9 m length, 0.10 mm inner diameter, 0.1 µm film thickness) that was programmed to remain isothermal at 85 °C for 3.5 min and then ramped from 85 to 340 °C at 1.53 °C/min. GC × GC-Time of Flight Mass Spectrometer (ToF MS) Analysis. Each sample was injected into a Leco Pegasus IVD GC × GC-ToF MS that is partially housed in a Hewlett-Packard 6890 gas chromatograph configured with a split/splitless injector, two chromatography columns, and a two-stage liquid-nitrogen-cooled modulator coupled to a ToF MS. A total of 1 mg of sample was dissolved in 1 mL of hexane, of which 1.0 µL was injected into a 300 °C splitless injector (2 min purge time). The first-dimension separation was performed with a 10 m, 5% phenyl-substituted polydimethylsiloxane phase capillary column (Restek, Rtx-5, 180 µm inner diameter, 0.2 µm film thickness) and temperatureprogrammed from 50 °C (5 min) to 300 °C at 3 °C/min, followed by a 10 °C/min temperature increase to 335 °C. Second-dimension separation was performed with a 0.7 m capillary column by use of a SGE BPX-50, 100 µm inner diameter, 0.1 µm film thickness, housed in a secondary oven programmed to offset the firstdimension oven temperature program by an additional 20 °C. Analytes were modulated on a deactivated fused silica column (0.5 m × 0.22 mm inner diameter), which was programmed to offset the second-dimension oven by 60 °C. The GC × GC modulation period was 8 s. Helium was used as the carrier gas in constant flow mode (1.0 mL/min). A 1.0 m × 0.10 mm inner diameter deactivated transfer line column connecting the second-dimension column to the ToF MS was heated to 270 °C. After a 5.83 min solvent delay, the ToF MS collected 50 spectra/s from m/z 50 to 776 units, with a detector voltage of 1575 V and a 230 °C ion source temperature. The mass defect was set to 110 milliunits/100 units. Sample data acquisition and data processing were performed with ChromaToF software. GC × GC Data Processing. GC × GC-ToF MS is used to identify the chromatographic space occupied by each chemical family with the software ChromaToF. Alkenes added from the chemical components of OBM filtrate were isolated from the GC × GC chromatogram, and the effect of influence of their signal intensity was quantitated prior to the comparative data analysis.59 (59) Reddy, C. M.; Nelson, R. K.; Sylva, S. P.; Xu, L.; Peacock, E. A.; Raghuraman, B.; Mullins, O. C. Identification and quantification of alkenebased drilling fluids in crude oils by comprehensive two-dimensional gas chromatography with flame ionization detection. J. Chromatogr., A 2007, 1148 (1), 100–107.
Betancourt et al. GC × GC-FID data is used to calculate integrated peak volumes with GC-Image software. Biomarkers were identified by mass spectral analysis and the comparison of their relative elution order with published data. Peaks of chemical compound classes were identified with commercially available standards from Aldrich, U.S. National Institute of Standards and Technology (NIST), and Chiron (Trondheim, Norway). FT-ICR MS. High-resolution FT-ICR MS is a powerful analytic technique capable of resolving >10 000 distinct components of crude oil without the need for prior chromatographic separation. The large number of species that can be resolved and identified in FT-ICR MS provides a detailed fingerprint of the composition of an oil, making this technique especially appropriate for identifying potential compartmentalization. In this experiment, the oils are ionized with electrospray ionization (ESI), which has been shown to be sensitive to a substantial fraction of the organic composition of similar samples.60 FT-ICR MS complements the GC work presented here because FT-ICR MS with ESI is sensitive to the heavier, more polar, and infusible fractions of crude oil that are not efficiently detected with GC. FT-ICR MS Data Acquisition. Whole, dead crude oils are analyzed by FT-ICR MS. Samples are diluted to a final concentration of 1 mg/mL in a 2:1 mixture of toluene and methanol, and NH4OH base is added at 1% by volume. Negative-ion ESI FTICR MS measurements are performed with a custom-built 9.4 T mass spectrometer located at the National High Magnetic Field Laboratory at Florida State University, Tallahassee, FL. Data acquisition is controlled by a modular ICR data system that was developed in-house. The sample is sprayed into the inlet of the mass spectrometer at a flow rate of 500 nL/min across a potential of -3.5 kV. Ions are drawn into the vacuum chamber via a heated metal capillary, leading to a skimmer region that allows for differential pumping. Ions are collected in a pair of RF-only octapole traps, where they are collisionally cooled and accumulated to enhance the experimental signal-to-noise ratio. After sufficient accumulation, the ions are injected via a RF-only octapole into an open cylindrical Penning ion trap in the magnetic field. To excite the ions to a detectable cyclotron radius, a broadband sweptfrequency excitation is applied. ICR signals from the orbiting ions are induced on two opposed detection electrodes of the ICR trap. Typically, 100 time domain transients are co-added to improve the experimental signal-to-noise ratio. FT-ICR MS Data Analysis. The time-domain acquisitions are summed, Hanning-apodized, zero-filled, and then subjected to a fast Fourier transform. The magnitude spectrum is then taken from the Fourier transform. Peaks in the range of 272-775 Da and above 6σ of baseline noise are analyzed by converting the mass values to Kendrick mass and sorting by Kendrick mass defect. In this format, homologous alkylation series can be readily identified and assigned. Homologous series identified in this manner serve as internal standards for cailbration of the instrument. After calibration, mass resolving power >300 000 and mass accuracy
