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Energy & Fuels 2008, 22, 1402–1403
New Approach for Characterizing Heavy Oils Abdel M. Kharrat DBR Technology Center, DiVision of Schlumberger Canada Ltd, 9450-17 AVenue, Edmonton Alberta, T6N 1M9 Canada ReceiVed October 25, 2007. ReVised Manuscript ReceiVed December 24, 2007 Introduction The growing global demand for energy has created the need for increased production of heavy oilsthat is, oil defined as having an API gravity of less than 22.3. Production of heavy oil reservoirs, particularly those located in Canada and Venezuela, becomes increasingly important as conventional oil reservoirs continue to be depleted. Many oil companies have begun to shift their activities to include heavy oil and created the need to understand these oils to predict their behavior in reservoirs, pipelines, and refining operations. Heavy oil characterization presents a significant challenge given that heavy oils have a high polar content, polars being resins and asphaltenes, and these polars are not yet wellunderstood.1 The literature shows many studies that have focused on the characterization of asphaltenes using subfractionation approaches based on solubitity,2–4 extrography,5 or chromatography.6,7 One of the most common procedures is to extract asphaltenes from oil samples and fractionate them. The most polar fraction precipitates first, the addition of titrant results in precipitating the second asphaltene subfraction, and so on. However, this approach has drawbacks largely because it does not allow for a correlation between subfractionation and viscosity reduction which is very critical for heavy oil production. The DBR Technology Center chemical laboratory has developed a new approach for heavy oil characterization. The purpose of this communication is to briefly describe this approach. Experimental Section A heavy oil sample was separated, using spinning band distillation, into two fractions: one boiling at a temperature of less than 274 °C and the second boiling at a temperature higher than 274 °C. The second fraction is called residue. A sample of approximately 10 g of the residue was accurately weighed in a 1 L flask. A solvent mixture of 30 volumes of tetrahydrofuran (THF) and 70 volumes of hexane (sample to solvent ratio is 1 g to 40 mL) was added, and the solution was refluxed (68–69 °C) for 2 h and then filtered through a 0.45 µm filter. The asphaltene fraction was washed with the same solvent mixture under reflux using a Soxhlet apparatus. The washed asphaltene fraction was extracted with dichloromethane, dried in a rotary evaporator at 95 °C under a vacuum of 26 in. of (1) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker, Inc.: New York, 1999. (2) Sadeghi, K. M.; Sadeghi, M. A.; Wu, W. H.; Yen., T. F. Fuel 1989, 68, 782–787. (3) Yang, X.; Hamza, H.; Czaenecki, J. Energy Fuels 2004, 18, 770– 777. (4) Wattana, P.; Fogler, H. S. Energy Fuels 2005, 19, 101–110. (5) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 267, 178–193. (6) Boduszynski, M. M. Energy Fuels 1987, 1, 2–11. (7) Dettman, H.; Inman, A. M.; Salmon, S. L.; Scot, K. T.; Fuhr, B. J. 5th International Conference on Petroleum Phase BehaVior and Fouling, Banff, Alberta, Canada, June 13–17, 2004.
Table 1. Results of the Subfractionation of the Canadian Heavy Oil asphaltene % sample
whole oil basis
cumulative
F1 F2 F3 F4 F5 F6 F7 F8
0.04 2.15 2.67 2.25 2.36 3.03 3.33 4.34
0.04 2.19 4.86 7.11 9.47 12.50 15.83 20.17
Table 2. Viscosity of Deasphalted Fractions of the Canadian Heavy Oil sample
viscosity (cP)
overall % reduction
residue residue-F1 residue-F1-F2 residue-F1-F2-F3 residue-F1-F2-F3-F4 residue-F1-F2-F3-F4-F5 residue-F1-F2-F3-F4-F5-F6 residue-F1-F2-F3-F4-F5-F6-F7 residue-F1-F2-F3-F4-F5-F6-F7-F8
7313500 7241220 4234720 2902220 1705590 1075890 641861 383710 175674
1 42 60 76 85 91 95 98
mercury, and weighed in a vial. Viscosity measurements were then performed on the deasphalted fraction after complete removal of solvent. The portion used for viscosity measurements was recovered with dichloromethane, combined with the original one, dried in the rotary evaporator, and weighed. The same procedure was carried out sequentially on the deasphalted fractions with solvent mixtures containing 25/75, 20/80, 15/85, 10/90, 5/95, and 0/100 volumes of THF/hexane and 0/100 volumes of THF/pentane. The precipitated asphaltenes were washed with the corresponding solvent mixtures. Each fraction was separated, dried, and weighed. The viscosity of the residue as well as the deasphalted oil was measured after each fractionation using a Reologica rheometer. Since viscosity measurements had to be performed on a small amount of sample (4-5 g), all fraction measurements were made using a parallel plate system with a 0.3 mm gap. Even though the shear rate has no impact on the viscosity of these oils (Newtonian fluid), a constant shear rate of 0.5 s-1 was chosen for all fractions. All measurements were made at 25 °C. In all cases, 100 data points were collected with a relative standard deviation of not more than 3.0 and the mean value was reported as the fraction viscosity. A known amount of C5 maltenes, deasphalted oil, was loaded into a chromatography column packed with activated alumina. The saturate fraction was then eluted with heptane followed by aromatic fraction with toluene. A mixture of dichloromethane and methanol (50:50) was used to elute the resin-1 fraction. Further elution was continued by dichloromethane to collect the resin-2 fraction
10.1021/ef700633r CCC: $40.75 2008 American Chemical Society Published on Web 01/24/2008
Communications
Energy & Fuels, Vol. 22, No. 2, 2008 1403
Figure 1. Description of the characterization protocol.
followed by methanol for the resin-3 fraction. Each fraction was separated from the solvent, dried, and weighed.
Results and Discussion After distillation of the heavy oil sample using a spinning band distillation apparatus, the oil was separated into two fractions. The volatile fraction was characterized using gas chromatography, and the residue was subfractionated using the approach illustrated in Figure 1. The results presented here were selected from the characterization of a Canadian heavy oil sample. After the spinning band distillation, only 3.9% of the volatile fraction was collected. The residue of the distillation was subjected to subfractionation of asphaltenes and viscosity measurements for each deasphalted fraction. The results are presented in Tables 1 and 2. These fractions were then submitted for elemental, metal, nuclear magnetic resonance, infrared spectroscopy, and mass spectrometry analyses. The column chromatography separation results are presented in Table 3. Conclusions The DBR Technology Center chemical laboratory has developed a protocol to characterize heavy oils where the fractions
Table 3. Column Chromatography Results on C5 Maltenes fraction
yield (%) whole oil basis
saturates aromatics resins 1 resins 2 resins 3
20.3 23.9 29.2 1.3 6.1
were separated while allowing the asphaltenes to remain in their original matrix. This protocol allows for both the assessment of viscosity reduction and the determination of the fraction responsible for lowering viscosity. It is our belief that this protocol could have an important impact on heavy oil production and the flow assurance issues associated with it. Further documents related to these issues will be published in the future. Acknowledgment. The author wishes to thank Dr. Ahmed Hammami for the support and discussions related to the development of this protocol, as well as Jose Zacharia and Amanda Prefontaine for carrying out the lab work with great interest and Schlumberger for the support. EF700633R
