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Issues with Comparing SARA Methodologies Abdel M. Kharrat,*,† Jose Zacharia,† V. John Cherian,‡ and Allwell Anyatonwu§ Schlumberger Canada Ltd., 9450-17 AVenue, Edmonton Alberta T6N 1M9, Canada; Dowell Schlumberger Western S.A., P.O. Box 16818, Jebel Ali, Dubai, United Arab Emirates; and 16115 Park Row Suite 150, Houston, Texas 77084 ReceiVed July 10, 2007. ReVised Manuscript ReceiVed August 31, 2007
One of the most common compositional analyses for petroleum samples is known as the SARA (saturates, aromatics, resins, and asphaltenes) fractionation test. SARA fractionation is also used as one of the screening criteria for asphaltene stability of reservoir fluids due to pressure depletion or commingling of different fluids. There are numerous variations of this type of analysis. First, the extraction of asphaltenes is not consistent from method to method. Asphaltenes are extracted using either pentane, hexane, or heptane. There are no specific reasons for selecting one over the other, and usually the users do not associate differences in results with the nature of the solvent. In addition, the extraction temperature could have an impact on the amounts of asphaltenes extracted. The fractionation of maltenes is also a challenge, usually ignored by end-users. Assuring no overlap between fractions and obtaining a very good mass balance are among these challenges. They could be impacted by the type of packing material amount of solvents used for the chromatographic separation. These SARA methods, referred to as standard methods, usually generate different results leading to confusion if the users are not that familiar with analytical details of each method. This paper discusses the role of the major parameters involved in generating the four fractions and how these parameters affect results, thus impacting decision for the end-users. It also shows that it is impossible to perform any prediction of results when changing from one method to another.
Introduction Characterization of fluids is required for many reasons as given in the literature. The fluid behavior inside the reservoir depends on composition. Modeling this behavior with time requires compositional analysis along with other physical parameters.1 Proper reservoir management necessitates the knowledge of reservoir conditions, such as pressure and temperature in addition to composition.2 During transportation and storage, the mixing of different fluids could cause perturbation of the fluids system. The presence of noncompatible fluids leads to precipitation and deposition. The refining process is also dependent on the nature of the fluids. One of the most common methods is the separation of oil into four fractions: saturates, aromatics, resins, and asphaltenes (known as SARA). Jewel et al.3 were the first researchers who worked out this type of fractionation method. Development of other approaches * To whom correspondence should be addressed. E-mail:
[email protected]. † Schlumberger Canada Ltd. ‡ Dowell Schlumberger Western S.A. § 16115 Park Row Suite 150. (1) Pina, A.; Mougin, P.; Behar, E. Oil Gas Sci. Technol. 2006, 61, 319–343. (2) Karan, K.; Hammami, A.; Flannery, M.; Stankiewicz, A. Systematic Evaluation of Asphaltenes Instability and Control During Production of Live Oils: A flow Assurance Study. Pet. Sci. Technol. 2003, 21, 629–645. (3) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972, 44, 1391–1395. (4) Suatoni, J. C.; Swab, R. E. J. Chromatogr. Sci. 1975, 13, 361–366. (5) Miller, R. Anal. Chem. 1982, 54, 1742–1746. (6) Radke, M.; Willish, H.; Welte, D. H. Anal. Chem. 1984, 56, 2538– 2546. (7) Grizzle, P. L.; Sablotny, D. M. Anal. Chem. 1986, 58, 2389–2396. (8) Chaffin, J. M.; Lin, M. S.; Liu, M.; Davison, R. R.; Glover, C. J.; Bullin, J. A. J. Liq. Chromatogr. Relat. Technol. 1996, 19, 1669–1682.
was the focus of later studies.4–9 This effort led to the development of the ASTM D 2007 method,10 a rather inconvenient method because it consumes large quantity of solvents, adsorbent, and oil. Consequently, other methods were developed.8,11–13 During the Fifth International Conference on Petroleum Phase Behavior and Fouling at Banff in 2004, a discussion was held on standardizing a method for asphaltene determination. The discussion led to no agreement because of the complexity of the available methodologies and lack of understanding of correlation among them. As a consequence, we undertook this study to investigate the possibility of establishing a correlation between methods and studying the impact of the major method parameters on the results. Three laboratories were involved in this work to study the effect of most of the parameters. Experimental Section Data from three laboratories (referred to as Lab 1, Lab 2, and Lab 3) using three different methods for SARA fractionation are presented here. Sample Size. Lab 1 uses 2 g of oil for asphaltene precipitation and 300 mg of maltenes for SAR separation. Lab 2 uses 2 g of oil for asphaltene precipitation and 350 mg of maltenes for SAR (9) Felix, G.; Thoumazeau, E.; Colin, J. M.; Vion, G. J. Liq. Chromatogr. 1987, 10, 2115–2132. (10) ASTM D 2007. Standard Test Method for Characteristic Groups in Rubber Extender and Processing Oils by Clay-Gel Adsorption Chromatography Method, 1993. (11) Fan, T.; Buckley, J. S. Energy Fuels 2002, 16, 1571–1575. (12) Fuhr, B. J.; Hawrelechko, C.; Holloway, L. R.; Huang, H. Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling, Banff, Canada, June 13–17, 2004. (13) Carbognani, L.; Buenrostro-Gonzalez, E. Energy Fuels 2006, 20, 1137–1144.
10.1021/ef700393a CCC: $37.00 2007 American Chemical Society Published on Web 10/23/2007
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Energy & Fuels, Vol. 21, No. 6, 2007 3619 Table 1. Summary of the Analytical Parameters Performed by the Three Laboratories
parameter
Lab 1
sample size for asphaltenes for SAR topping aspahltene precipitation asphaltene filtration asphaltene washing asphaltene extraction column chromatography chromatography saturates aromatics resins solvent removal a
Lab 2
Lab 3
2 g of oil 300 mg of maltenes concentrator 1:40 heptane 98 °C 0.45 µm filter hot heptane dichloromethane alumina activated at 430 °C
2 g of oil 350 mg of maltenes spinning band 1:30 heptane room temperature 0.45 µm filter heptane at RTa chloroform alumina and silica activated at 250 °C
1 g of oil 200 mg of maltenes no topping 1:30 pentane 39 °C 2.5 µm filter pentane at RTa chloroform alumina and silica activated at 180 °C
250 mL of heptane 250 mL of toluene 250 mL of 1:1 dichloromethane– methanol rotary evaporator
140 mL of heptane 180 mL of 2:1 heptane–toluene 60 mL of 1:1:1 toluene–chloroform– methanol hot plate
70 mL of pentane 70 mL of 1:1 pentane–dichloromethane 70 mL of methanol hot plate
RT: room temperature.
separation. Lab 3 uses 1 g of oil for asphaltene precipitation and 200 mg of maltenes for SAR separation. Topping. Lab 1 uses solvent evaporator (nonrotating) at 80° and at a vacuum of 26.5 in. of mercury while Lab 2 uses spinning band distillation and Lab 3 does not perform any sort of topping. Asphaltene Precipitation and Filtration. Lab 1 precipitates asphaltene using heptane (1:40 ratio) by refluxing at the boiling point of heptane for 2 h. Lab 2 also precipitates with heptane (1:30 ratio) but at room temperature, whereas Lab 3 precipitates asphaltenes with pentane (1:30 ratio) by refluxing at the boiling point of pentane followed by an incubation time of 24 h. For filtration, Labs 1 and 2 use 0.45 µm pore size filters whereas Lab 3 uses a 2.5 µm filter. Asphaltene Washing. Lab 1 uses hot heptane washing using a Soxhlet extractor. Lab 2 uses heptane at room temperature until solvent is clear. Lab 3 uses normal pentane at room temperature. Asphaltene Extraction. Lab 1 uses dichloromethane to extract the asphaltene from the filter paper using a Soxhlet extractor. Labs 2 and 3 use chloroform to extract asphaltene deposited on the filter. Chromatography Column. Lab 1 uses a stainless steel (SS) column (1 cm × 60 cm) packed with alumina activated at 430 °C for 24 h. Lab 2 uses an SS column (1 cm × 110 cm) packed with alumina and silica gel activated at 250 °C for 24 h. Lab 3 uses a glass column (1 cm × 50 cm) packed with alumina and silica gel activated at 180 °C for 24 h. Chromatographic Elution. Labs 1 and 2 use a pump to deliver solvent at the rate of 2 mL/min. Lab 3 uses manual gravity drain method. For eluting saturates Lab 1 uses 250 mL of heptane whereas Lab 2 uses 140 mL of heptane. Lab 3 uses 70 mL of pentane to elute saturates. Lab 1 uses 250 mL of toluene to elute the aromatic fraction, while laboratory 2 uses 180 mL of a mixture of heptane and toluene in the ratio 2:1. Lab 3 uses 70 mL of a mixture of pentane and dichloromethane in the ratio 1:1. For resins, Lab 1 uses 250 mL of a mixture of dichloromethane and methanol in the ratio 1:1. Lab 2 uses 60 mL of a mixture of toluene, chloroform, and methanol in the ratio 1:1:1, and Lab 3 performs resin elution with 70 mL of methanol. Lab 1 uses a final column flush with 100 mL of dichloromethane followed by 100 mL of methanol. This fraction is added up to the resin fraction. This step is not followed by Labs 2 and 3. All these parameters are tabulated in Table 1. Recovery of Separated Fractions. After chromatography separation, each fraction needs to be dried to remove the solvent and weighed to calculate the final percentage composition. Lab 1 used rotary evaporator under vacuum followed by hot plate with nitrogen purging to dry the fractions. Labs 2 and 3 used a hot plate for the drying.
Table 2. Solvent Effect on Asphaltene Precipitation crude oil oil oil oil oil oil
% asphaltene precipitated with pentane
% asphaltene precipitated with heptane
asphaltene ratio C5/C7
20.2 8.8 25.0 10.5 3.7
10 4.1 17.1 0.04 0.04
2.0 2.2 1.5 263 94
1 2 3 4 5
Table 3. Effect of Washing on Percentage Asphaltene Calculated % asphaltene calculated with crude oil
room temperature washing
heptane Soxhlet washing
ratio
oil 6 oil 7 oil 8 oil 9 oil 10 oil 11 oil 12 oil 13
2.76 8.80 0.44 0.64 1.96 2.05 18.2 1.60
2.43 8.67 0.40 0.38 0.37 1.5 12.1 0.30
1.1 1.0 1.1 1.7 5.3 1.4 1.5 5.3
Table 4. SARA Fractions for Oils 14 and 15 crude oil oil 14 oil 15
SARA method Lab 1 Lab 2 Lab 3 Lab 1 Lab 2 Lab 3 normalized Lab 3 C
% % % % % saturates aromatics resins asphaltenes recovery 25.6 23.7 25.7 61.2 60.2 49.7 60.1 61.1
23.6 33.9 39.1 18.8 25.3 23.5 28.4 26.3
38.9 25.5 11.6 17.6 13.5 9.2 11.1 9.2
10.8 16.6 12.1 0.10 0.8 0.3 0.4 0.3
98.9 99.6 88.5 97.7 99.9 82.7 100 96.9
Results and Discussion
and extract asphaltenes from oils and bitumen are pentane and heptane. It is well established in the literature14 that the higher the carbon number of the solvent, the less asphaltenes are precipitating. Because both solvents are being used by laboratories, users of the data are always interested in how to interpret the data from two different laboratories, one using pentane and the second using heptane. We determined asphaltenes using heptane and pentane for different oils having different asphaltene contents. We computed the ratios of amount of asphaltenes generated by pentane over the amount of asphaltenes generated by heptane. The results are presented in Table 2.
Asphaltene Precipitation and Extraction. Correlation between SolVents. The most common solvents used to precipitate
(14) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999.
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Table 5. Effect of Solvent Amount on Separation and Recovery Using the Lab 1 Method % saturates with crude oil oil oil oil oil
16 17 18 19
% aromatics with
% resins
% recovery
250 mL (method 1)
100 mL (method 2)
250 mL (method 1)
100 mL (method 2)
250 mL (method 1)
70 mL (method 2)
method 1
method 2
72.7 85.1 49.7 59.7
71.8 84.4 48.4 45.7
15.8 8.2 18.0 26.5
15.4 8.2 17.0 27.7
10.6 5.8 27.3 14.0
4.7 0.2 14.1 12.2
99.1 99.0 97.4 98.8
92.0 92.8 81.9 85.6
It is expected to have asphaltenes content higher using pentane than heptane,14 but the ratios shown here are not constant. It was reported that these ratios are in the range of 1.2–1.2515 and 1.38,16 much lower in some cases than ours. These ratios are oil dependent, and it is not possible to extrapolate them from the literature. The lack of correlation is the first proof that standardization based on analytical methodology will be very hard to accomplish. When comparing asphaltene content data generated from different methodologies, the users should pay attention to this parameter and not use the ratio published in the literature. Effect of Washing. Washing is rinsing the asphaltene fraction with the precipitating solvent to eliminate the oil that could be trapped inside the asphaltene cake during filtration. Since there are numerous precipitation methods, it is expected that washing will have different effects. Many methods describe this step as rinsing the asphaltenes filtered until the solvent going through the filter becomes clear. Calculating the asphaltene content from the weight of unwashed filter cake is a common practice in the industry. Even though this saves a lot of time, the accuracy of the method is jeopardized. An independent study was done on 10 crude oils with varying asphaltene content to understand the criticality of the washing. The C7 asphaltene content measured with Soxhlet washing with hot heptane as outlined in the Experimental Section (Lab 1 method) is compared with the one without washing in Table 3. Anderson and Stenby17 when studying the temperature effect on precipitation of asphaltenes found a ratio of 1.33 between 24 and 80 °C, after performing washing. In our case, ratios were between 1 and 5.3, presented in Table 3, leading to believe that extraction of trapped maltenes in the asphaltene cake is more efficient at the reflux temperature of heptane than at room temperature. Saturates Aromatics and Resins Extraction. Saturates, aromatics, and resins (SAR) results for two crude oil samples performed with three different approaches as outlined in the Experimental Section are presented in Table 4 for comparison. Oil 14 has low volatiles (0.6% topping) compared with oil 15 (14.2% topping). Lab 3 normalized results are calculated to force recovery to 100%. This is common practice, and it is easy to detect because the recovery would be always 100.00%, impossible to achieve in all the cases. Lab 3 C results were corrected on the basis of the fact that they lost 14.2% volatiles, and those volatiles were assumed to be 80% saturates and 20% aromatics. Effect of Topping. Topping is the removal of volatiles by means of distillation, rotary evaporation, or heating under atmospheric or reduced pressure. Topping should be performed at a temperature and pressure that do not allow further (15) Buenrostro-Gonzalez, E.; Lira-Galeana, C.; Gil-Villegas, A.; Wu, J. AIChE J. 2004, 50, 2552–2570. (16) Alboudwarej, H.; Akbarzadeh, K.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W. AIChE J. 2003, 49, 2948–2956. (17) Anderson, S. I.; Stenby, E. H. Fuel Sci. Technol. Int. 1996, 14, 261–287.
Table 6. Normalization of Results for Oils 20 and 21 crude oil oil oil oil oil
18 18 normalized 19 19 normalized
% saturates
% aromatics
% resins
% recovery
48.4 59.1 45.7 53.4
17.0 20.7 27.7 32.4
14.1 17.2 12.2 14.2
81.9 100 85.6 100
hydrocarbons to be lost in the saturates, aromatics, and resins analysis when the solvent is removed. The method used in Lab 3 produced low recovery for both oils. This is because under the Lab 3 method topping is not performed, and hence some hydrocarbons are lost during the evaporation of the solvents after fractionation. This is evident from the low saturate value for oil 15 using the Lab 3 method. Since Labs 1 and 2 performed topping, both laboratories produced comparable results for saturates and obtained a higher recovery. These results show clearly that the separation of volatiles is a critical step to perform during SARA analysis. It is important though to highlight that spinning band distillation would be preferred to the rotary evaporator or concentrator as these techniques do not allow collection of the light fraction, and consequently it would be impossible to characterize it. Effect of SolVent and Packing Material. The nature and amount of solvent and the packing material have an impact on the column separation. Since saturates are nonpolar, their interaction with the adsorbent is minimal. Differences in saturates recovery is attributed to the difference in solvent (type and quantity). Labs 1 and 2 used heptane in 250 and 140 mL quantities while Lab 3 used only 70 mL of pentane. For these two oils, it seems that 140 mL of heptane was sufficient enough to extract all saturates from both oils; when 70 mL of pentane is used, all saturates seemed to be recovered after correction due to topping. The recoveries for the aromatic fractions were not consistent. Labs 2 and 3 had higher recovery than Lab 1 probably due to the lack of separation between the aromatics and the resins. Lab 3 obtained low recovery of resins. Lab 3 used 70 mL of methanol to elute this fraction. This amount is most probably not enough to elute the entire fraction of resins. Solvent amount and packing material both could be the origin of these differences. We decided to study the impact of the amount of eluting solvent on four oils using the same packing material. The results are presented in Table 5. For oil 19, 100 mL of heptane was not enough to extract the entire saturate fraction. For oil 18, 70 mL was not enough to extract the entire resin fraction. These results impacted the recovery. Only 81.9% and 85.6% recoveries were obtained for these two oils. The practice of normalization, a common practice, generates erroneous results.18 Method 2 in Table 5 has 81.9% and 85.6% recovery for oils 18 and 19, respectively. The normalized results are presented in Table 6. Forcing the recovery to 100% (18) Kharrat, A. M. Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling, Banff, Canada, June 13–17, 2004.
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Energy & Fuels, Vol. 21, No. 6, 2007 3621
drastically changes the results. The change is also oil- and recovery-dependent. Conclusions SARA methodologies do not generate similar results. Comparing results from different methodologies is very risky and could lead to erroneous conclusions. There are a few points to watch for when reading SARA results:
1. The recovery should be close to 100%. 2. Sample topping should be performed initially to avoid loss of volatiles during solvent removal. This helps to achieve a mass balance close to 100%. It is recommended to use a spinning band distillation to perform this step. 3. Solvents should be used in quantities that do not allow low recoveries. 4. Correlation between methods is shown to be very difficult. EF700393A
