Energy Fuels 2010, 24, 2175–2177 Published on Web 10/13/2009
: DOI:10.1021/ef9009263
Petrophase 2009 Panel Discussion on Standardization of Petroleum Fractions† Received August 26, 2009. Revised Manuscript Received October 5, 2009 fraction of heptane resins are subsets of pentane asphaltenes, the materials constituting the nanoaggregates.3 Analysis definition differences lead to significantly different apparent composition values and perceptions of live or dead oil or resource fraction compositions. For example, one’s perspective on whether asphaltenes and resins associate and the extent to which they associate is dictated by the analysis standard employed! Regrettably, nanofiltrations cannot substitute for solvent standards for routine measurements because of the time as well as the cost involved. The main issues discussed regarding nanoaggregates are summarized in Table 1.
Introduction During the Petroleum Phase Behavior and Fouling Conference held in Rio de Janeiro, Brazil, in June 2009, a panel session was organized on the standardization of petroleum fractions. The session dealt with the need to unify and improve the methods to identify asphaltenes and resins and to assess the representativeness of analysis methods compared to the behavior of asphaltenes in live oils. The motivation for this session was to promote advances because little progress appears to have arisen since the 2004 Petrophase Conference in Banff, Canada, where a discussion webpage was developed to analyze the methodologies applied in asphaltene studies.1 After 5 years, there is no unified view on the asphaltene definition (C5-asphaltenes, C7-asphaltenes, Soxhlet asphaltenes, etc.) or resin definition [C5-C7 fraction, the polar fraction in a saturates, aromatics, resins, and asphaltenes (SARA) separation, etc.]. Without agreement, it is challenging to reach consensus on topics such as asphaltene-resin interaction or the state of asphaltenes in crude oil. Four presenters were invited to address the following topics at Petrophase 2009: (1) On asphaltene-rich nanoaggregate composition and asphaltene þ resin association in hydrocarbon resources (John Shaw, University of Alberta). (2) Toward a better understanding of asphaltenes stability in live oil (Herve Carrier, University of Pau). (3) SARA’s role in oil characterization: Should there be one? (Harvey Yarranton, University of Calgary). (4) Role of resins on asphaltene stability (Lamia Goual, University of Wyoming). The first two speakers discussed the relationship between asphaltenes separated by solvent addition, the asphaltene nanoaggregates that exist in crude oil, and the solids that come out of solution upon pressure drop in the field. The latter two speakers discussed the separation of asphaltenes and resins and the interaction between these two families of compounds. After each talk, a discussion was directed by a series of questions prepared by the authors and the moderator, Daniel Merino-Garcia (Repsol).
Pressure versus Solvent Asphaltenes (H. Carrier and J. L. Daridon, University of Pau) Most asphaltene studies are carried out on dead oils or solvent-separated asphaltenes at ambient conditions. Properties of asphaltenes arising from depressurization of live oils are typically inferred. This presupposes compositional, structural, and behavioral relationships between asphaltenes resulting from solvent extraction and those resulting from pressure depletion, i.e., field deposits. Herve Carrier provided an overview of experimental techniques that can isolate solid asphaltenes by depressurization. To date, the link between the structural properties of asphaltenes separated by solvent addition to dead oils at atmospheric pressure and those obtained by pressure drop has not been established experimentally. Experiments with depressurization equipment4 should help establish this connection. The discussion was centered on how solvent-based separation practices vary from laboratory to laboratory. It is clear that people prefer to maintain their own practices, so that new experimental data can be compared to their own databases. However, differences among laboratories should be minimized, at least in terms of the solvent used (C5 or C7) and washing procedure (Soxhlet extraction is preferred). It was also noted that sampling and sample storage procedures play a critical role in the representativeness of asphaltene samples. Oxidation during handling is one of the greatest concerns. Whether asphaltenes that cause problems during upstream operations are different from those that are relevant in downstream operations was a topic raised during the discussion. Currently, we lump them within the same family of compounds. Table 2 gathers the main issues discussed related to solvent-separated and pressure-drop-separated asphaltenes.
Asphaltene Nano-Aggregates (B. Zhao, M. Becerra, and J. Shaw, University of Alberta) John Shaw focused on techniques that permit the separation of asphaltene nanoaggregates (aggregates of a few molecules with a mean diameter of around 2-3 nm) without the addition of solvent, namely, ultracentrifugation and nanofiltration. Nanofiltration experiments with 5-200 nm mesh show that, for Maya crude oil and Athabasca bitumen, the composition of nanoaggregated asphaltene-rich species closely approximate asphaltenes separated by pentane addition over a broad temperature range.2 Heptane asphaltene-resin association is expected because heptane asphaltenes and a
SARA Fractionation (H. Yarranton, University of Calgary) Harvey Yarranton addressed how the very definition of SARA has added confusion to the role of heavy aromatic species in phase behavior and emulsion stability. SARA fractionation is a convenient tool to characterize heavy oils because other characterization techniques, such as distillation,
*To whom correspondence should be addressed: Centro de Tecnologı´ a Repsol, Carretera N-V Km 18, M ostoles, Madrid 28931, Spain. Telephone: þ34-91348-1368. Fax: þ34-91348-8613. E-mail: merinog@ repsol.com. (1) www.ualberta.ca/dept/chemeng/asphaltenes/. (2) Zhao, B.; Shaw, J. M. Energy Fuels 2007, 21 (5), 2795–2804. r 2009 American Chemical Society
(3) Zhao, B.; Becerra, M.; Shaw, J. M. Energy Fuels 2009, 23 (9), 4431–4437. (4) Pina, A.; Mougin, P.; Behar, E. Oil Gas Sci. Technol. 2006, 61 (3), 319–343.
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: DOI:10.1021/ef9009263
Table 1. Discussion Summary on Asphaltene Nanoaggregates
Table 3. Discussion Summary on SARA Fractionation
CONSENSUS ON
CONSENSUS ON
• the C5 standard seems to match the composition of nanoaggregates better than the C7 standard • nanoaggregates cannot be described as micelles • it is not sensible to substitute IP-143 with nanofiltration for routine measurements
• SARA fractionation has repeatability problems even within the same laboratory • a comparison of SARA values among different laboratories has to be performed with caution • SARA fractionation is of little use in water-oil emulsion stability analysis
DEBATE ON
DEBATE ON
• what is the relationship between nanoaggregates and the solids that come out of solution upon pressure drop? • an agreement on a simple method to separate associating material is needed • are all asphaltenes (C5 or C7 standard) nanoaggregated? • how dynamic are asphaltene-rich nanoaggregates in terms of aggregate lifetime?
• can SARA fractionation data provide reliable information for asphaltene stability analysis? • does SARA fractionation permit a good separation of saturates from the rest of the oil? where do the long-chain alkyl-benzenes go? • would it be more useful to define asphaltenes and resins as one single fraction based on association capacity rather than solubility?
Table 2. Discussion Summary on Solvent versus Pressure Asphaltenes
Table 4. Discussion Summary on the Resin-Asphaltene Interaction
CONSENSUS ON
CONSENSUS ON
• minimize differences among solvent standards (specially n-alkane used and washing procedure) • solvent destabilization is equivalent to pressure drop at least in the onset region • the importance of sampling and sample preservation cannot be underestimated
• a fraction of resins participates in the nanoaggregates together with asphaltenes • proof exists on the resin-asphaltene interaction in model solvents • caution is needed when extrapolating the behavior from model solvents to crude oil
DEBATE ON
• how critical is the presence of resins in the nanoaggregates? • what is the role of heteroatoms (N, O, S, and metals) on the stability of nanoaggregates? • what is the role of aromatic rings on the stability of nanoaggregates?
DEBATE ON
• how similar are dead oil asphaltenes to the material that precipitate from live oils upon pressure drop? is it just a question of quantity that precipitates, or are deposits enriched in a certain asphaltene family of molecules? • how relevant are the average asphaltene properties measured in asphaltenes precipitated with excess heptane to the analysis of asphaltene deposition problems upon pressure drop? • are up- and downstream asphaltene problems caused by the same familiy of molecules inside the asphaltene fraction?
to de-asphalt.7 Table 3 gathers the main issues discussed regarding SARA fractionation. Resin-Asphaltene Interaction (L. Goual, University of Wyoming)
do not apply to much of the fluid. However, the SARA method fractionates the fluid partly by solubility (asphaltenes) and partly by adsorption (saturates, aromatics, and resins). The method creates an artificial distinction between resins and asphaltenes yet provides no information on more meaningful distinctions, such as between associating and non-associating species or surface-active versus non-surface-active species. The discussion was centered on the two main issues faced with SARA fractionation: (1) the repeatability of the adsorption measurements and (2) the usefulness of these classes. A single operator can achieve consistent results, but significant differences are observed between operators and also among different laboratories with different variations of the method (e.g., ASTM 2007-025 and ASTM D4124-976). Yarranton highlighted up to 10% relative error, particularly in the aromatic and resin fractions, based on unpublished data. Limitations in SARA-based characterization include (a) no separation between paraffins and naphthenes in saturates, (b) little useful distinction between aromatics and resins, (c) a misleading distinction between resins and asphaltenes, and (d) asphaltene fraction properties, which are strongly dependent upon the protocol used
Because of the divergent views among research groups on the importance of resins in asphaltene stability, there is a pressing need to clarify the properties of resins and their role in asphaltene aggregation and precipitation. Lamia Goual presented previous experimental work in this area.2,8-12 Novel experimental data were also provided from impedance analysis, highlighting the aggregation of asphaltenes in toluene and the possible presence of resins in the aggregates.13,14 One of the conclusions was that the amount of C7 resins in C7 asphaltene nanoaggregates is variable and depends upon several factors, such as the solubility of resins in the medium. As described in the first presentation, resins may be present in crude oil nanoaggregates according to the definition of C7 asphaltenes. The ensuing discussion centered on whether there is enough experimental evidence to prove that resins can coat asphaltene nanoaggregates as stipulated by the Nellensteyn hypothetical model.15 The discussion was raised on the importance of the presence of resins and the mechanism of association. Highlights of the discussion are summarized in Table 4. (8) Gonzalez, G.; Neves, G. B. M.; Saraiva, S.; Lucas, E. F.; dos Anjos de Sousa, M. Energy Fuels 2003, 17, 879–886. (9) Merino-Garcia, D.; Andersen, S. I. Langmuir 2004, 20 (11), 4559– 4565. (10) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy Fuels 2007, 21 (5), 2785–2794. (11) Rogel, E. Energy Fuels 2008, 22 (6), 3922–3929. (12) Goual, L.; Firrozabadi, A. AIChE J. 2004, 50 (2), 470–479. (13) Goual, L. Energy Fuels 2009, 23 (4), 2090–2094. (14) Sedghi, M.; Goual, L. Energy Fuels 2009, manuscript submitted. (15) Nellensteyn, F. I. In The Science of Petroleum; Dunstan, A. E., Ed.; Oxford University Press: London, U.K., 1938; Vol. 4.
(5) American Society for Testing and Materials (ASTM). ASTM D2007-02. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2009. (6) American Society for Testing and Materials (ASTM) ASTM D4124-97. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2009. (7) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzedeh, K. Energy Fuels 2002, 16, 462–469.
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: DOI:10.1021/ef9009263 impact future research, directing it toward clarification of the issues still open for debate.
Conclusions Tables 1-4 summarize the main issues discussed during the session. The panelists and conference participants were able to identify issues where there was consensus, areas where the field would benefit from additional experiments, and conceptual insights. Key shortcomings of the SARA analysis approach were highlighted. The diversity of operating definitions employed and measurement variability affect the ability of researchers to determine whether compound classes are present and to draw cross-comparisons among measurements from different laboratories. The diversity of the definitions also color researchers’ perspectives on whether and to what extent asphaltenes and resins associate. An experimental link between the properties of chemically separated asphaltenes (dead oil measurements) and those obtained by pressure drop (live oil measurements) remains to be established. Although not addressed specifically in the session, these uncertainties in the databases underlying predictive models underscore the risk in basing property prediction models on SARA analysis and the need for an improved basis for defining oil fractions. We hope that the session benefited new and current researchers and will
Acknowledgment. The corresponding author thanks John Shaw (University of Alberta), Herve Carrier (University of Pau), Harvey Yarranton (University of Calgary), and Lamia Goual (University of Wyoming) for their contributions as well as for fruitful discussion before and during the panel session.† Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling.
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Daniel Merino-Garcia,*,‡ John Shaw,§ Herv e Carrier, Harvey Yarranton,^ and Lamia Goual# ‡
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Centro de Tecnologı´a Repsol, N-V Km 18, M ostoles, Madrid 28931, Spain, §Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2R3, Canada, Laboratoire des Fluides Complexes, Universit e Pau et des Pays de l’Adour, 64012 Pau, France, ^ Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada, and #Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071
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