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New Correlating Parameter for the Viscosity of Heavy Crude Oils Rashid S. Al-Maamari,† Omar Houache,† and Sabah A. Abdul-Wahab*,‡ Departments of Petroleum & Chemical Engineering and Mechanical & Industrial Engineering, College of Engineering, Sultan Qaboos UniVersity, P.O. Box 33, Al-Khod 123, Oman ReceiVed June 29, 2006. ReVised Manuscript ReceiVed August 4, 2006
A new correlating parameter (CAPI) for heavy crude oils was developed. In this parameter, the API (American Petroleum Institute) oil gravity was corrected using a factor comprising compositional fractions (saturates, aromatics, resins, and asphltenes contents of the heavy crude oil). It was found that relating viscosity to CAPI was more representative than relating the viscosity to the API measurement alone. Using this new correlating parameter for the heavy crude oils considered in this study, two very different tendencies, defining a limiting value of 15, were observed. The viscosities of oils found under the limiting value of 15 were very sensitive to small changes in CAPI. An Omani heavy crude oil that showed higher viscosities compared to other crudes of similar API was found to be normal in terms of viscosity behavior when CAPI was used. In addition, crude oil viscosity could be predicted with more accuracy using this correlating parameter at different temperatures.
Introduction Measurement/estimation of heavy oil viscosity is required in predicting the easiness of fluid flow, selecting a production approach, and predicting oil recovery. Numerous correlations have been proposed in the literature for the estimation of fluid viscosity (e.g., dead-oil viscosity (µod), gas-saturated oil viscosity (µob), and undersaturated oil viscosity (µo)) based on measured fluid properties.1-36 In the majority of cases, these correlations * Corresponding author. E-mail:
[email protected]. † Department of Petroleum & Chemical Engineering. ‡ Department of Mechanical & Industrial Engineering. (1) Braden, W. B. A viscosity-temperature correlation at atmospheric pressure for gas-free oils. J. Pet. Technol. 1966, 1487-1490. (2) Beggs, H. D.; Robinson, J. R. Estimating the viscosity of crude oil systems. J. Pet. Technol. 1975, 27 (Sept), 1140-1141. (3) Viscosity in technical data book-petroleum refining. Data Book; American Petroleum Institute: Washington, D.C., 1978; Chapter 11. (4) Amin, M. B.; Maddox, R. N. Estimating viscosity vs temperature. Hydrocarbon Process. 1980, 59, 31-38. (5) Baltatu, M. E. Prediction of the liquid viscosity of petroleum fractions. Ind. Eng. Chem. Process DeV. 1982, 21, 192-195. (6) Ikram, H. Characterization of Saudi Arabian crude oil fractions with emphasis on viscosity behaviour. MS Thesis, KFUPM University, KSA, 1982. (7) Mehrotra, A. K.; Svrcek, W. Y. Correlations for properties of bitumen saturated with CO2, CH4 and N2, and experiments with combustion gas mixtures. J. Can. Pet. Technol. 1982, 21, 95-104. (8) Khan, M. A. B.; Mehrotra, A. K.; Svrcek, W. Y. Viscosity models of gas-free athabasca bitumen. J. Can. Pet. Technol. 1984, 23, 47-53. (9) Annual Book of ASTM Standards; ASTM (American Society for Testing and Materials): Philadelphia, PA, 1987; Designation D-341, pp 149-153. (10) Johnson, S. E.; Svrcek, W. Y.; Mehrotra, A. K. Viscosity prediction of athabasca bitumen using the extended principle of corresponding states. Ind. Eng. Chem. Res. 1987, 26, 2290-2298. (11) Khan, S. A.; Al-Marhoun, M. A.; Duffuaa, S. O.; Abu-Khamsin, S. A. Viscosity correlations for Saudi Arabian crude oils. Prepared for presentation at the Fifth SPE Middle east Oil show, Manama, Bahrain, Mar 7-10, 1987; SPE 15720. (12) Beg, S. A.; Amin, M. B.; Hussain, I. Generalized kinematic viscosity-temperature correlation for undefined petroleum fractions. Chem. Eng. J. 1988, 38, 123-136. (13) Mehrotra, A. K.; Svrcek, W. Y. Properties of cold lake bitumen saturated with pure gases and gas mixtures. Can. J. Chem. Eng. 1988, 66, 656-665. (14) Al-Blehed, M.; Sayyouh, M. H.; Desouky, S. M. Correlation estimates Saudi crude oil viscosity. Oil Gas J. 1990, 88 (10), 61-62.
indicated a good prediction of crude oil viscosity for the oils from which they were derived. However, when used with other crude oils from other regions, these correlations are, in most cases, not accurate and certain modifications are needed to obtain acceptable viscosity predictions.36-38 (15) Mehrotra, A. K. Modeling the effects of temperature, pressure, and composition on the viscosity of crude oil mixtures. Ind. Eng. Chem. Res. 1990, 29, 1574. (16) Mehrotra, A. K. A generalized viscosity equation for pure heavy hydrocarbons. Ind. Eng. Chem. Res. 1991, 30, 420-427. (17) Mehrotra, A. K. Modeling temperature and composition dependence for the viscosity of diluted bitumens. J. Pet. Sci. Eng. 1991, 5, 261-272. (18) Miadonye, A.; Singh, B.; Puttagunta, R. V. One-parameter correlation in the estimation of crude oil viscosity. Pap. SPE 1992, 26206. (19) Puttagunta, V. R.; Miadonye, A.; Singh, B. Viscosity temperature correlation for prediction of kinematic viscosity of conventional crude. Trans. Inst. Chem. Eng. 1992, 70, 627-631. (20) Miadonye, A.; Puttagunta, V. R. Prediction of the viscosity of crude oil fractions from a single measurement. Chem. Eng. Commun. 1993, 122, 195-199. (21) Puttagunta, V. R.; Miadonyea, A.; Singh, B. Simple concept predicts viscosity of heavy oil and bitumen. Oil Gas J. 1993, Mar 1, 71-73. (22) Puttagunta, V. R.; Singh, B.; Miadonye, A. Correlation of bitumen viscosity with temperature and pressure. Can. J. Chem. Eng. 1993, 71, 447450. (23) Singh, B.; Miadonye, A.; Puttagunta, V. R. Modeling the viscosity of Middle-East crude oil mixtures. Ind. Eng. Chem. Res. 1993, 32, 21832186. (24) Singh, B.; Miadonye, A.; Puttangunta, V. R. Heavy oil viscosity range from one test. Hydrocarbon Process. 1993, 12 (5), 157-162. (25) Singh, B.; Miadonye, A.; Huang, S. S.; Srivastava, R.; Puttangunta, V. R. Estimating temperature effects on viscosity of Saskatchewan heavy oils. Fuel Sci. Technol. Int. 1994, 12 (5), 693-704. (26) De Ghetto, G.; Paone, F.; Villa, M. Pressure-volume-temperature correlations for heavy and extra heavy oils. Prepared for presentation at the 1995 SPE International Heavy Oil Symposium, Calgary, Alberta, Canada, June 19-21, 1995; SPE 30316. (27) Giambattista, De G.; Villa, M. Pressure-volume-temperature correlations for heavy and extra heavy oils. Prepared for presentation at the International Heavy Oil Symposium, Calgary, Alberta, Canada, June 19-21, 1995; SPE 30316. (28) Petrosky, G. E.; Farshad, F. F. Viscosity correlations for Gulf of Mexico crude oils. Prepared for presentation at the Production Operations Symposium, Oklahoma City, OK, Apr 2-4, 1995; SPE 29468. (29) Wakabayashi, T. Viscosity correlation with specific gravity and molecular weight of crude oil fractions. Fuel 1997, 76 (11), 1049-1056. (30) Bennsison, T. Prediction of heavy oil viscosity. Presented at the IBC Heavy Oil Field Development Conference, London, Dec 2-4, 1998.
10.1021/ef0603030 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006
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Figure 1. Plot of kinematic viscosity versus API at 25 °C.
Figure 2. Plot of kinematic viscosity versus API at 50 °C.
A general correlation is nearly always found between oil viscosity and gravity. Therefore, oils are usually characterized by their API (American Petroleum Institute) oil gravity. How-
ever, viscosity-gravity correlations often fail to yield heavy oil viscosity data with sufficient accuracy for engineering applications. They also cannot be used on small samples.39
(31) Carlson, R. M. K.; Pena M. M.; Boduszynski, M. M.; Rechsteiner, C. E.; Shafizadeh, A. S. G.; Henshaw, P. C. Geochemical-Viscosity correlations among heavy crude oils of the San Joaquin Valley, California. In 7th UNITAR heaVy crude and tar sands international conference proceedings [CD-ROM]; Information Center for Heavy Hydrocarbons, United Nations Institute for Training and Research: New York, 1998; Paper 1998.203, p 24.
(32) Miadonye, A.; Puttagunta, V. R. Modeling the viscosity-temperature relationship of Nigerian Niger-Delta crude petroleum. Pet. Sci. Technol. 1998, 16 (5 & 6), 627-638. (33) Dindoruk, B.; Christman, P. G. PVT properties and viscosity correlations for Gulf of Mexico oils. Prepared for presentation at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans, LA, 30 Sept-3 Oct 2001; SPE 71633.
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Figure 3. Oil API gravity vs viscosity under reservoir conditions. Each point represents an oilfield. At any single API gravity value, there maybe variation of 2 orders of magnitude in viscosity.40
Al-Maamari et al.
Available data for an Omani heavy crude oil (OHCO) of 16° API and 1700 cP in situ viscosity showed that the fluid viscosity follows the expected trends with both temperature and pressure. However, based on data available in the open literature, it appears that the OHCO fluid has a higher viscosity compared to other oils of similar or slightly lower APIs and oils with similar or higher asphaltene contents. Although oil viscosity is not always in a direct relationship with gravity, the comparisons are somewhat troubling and it is still not clear why the OHCO viscosity is relatively higher. Therefore, the viscosity-API correlation is the focus of this paper. A new correlating parameter (CAPI) or corrected API that can be used for heavy oil characterization is presented. The reported results are the product of analysis of many heavy oils data collected from the open literature for various heavy oil fields around the world.
Figure 4. Plot of viscosity versus temperature at the same API of 16 (data taken from the USA heavy oil database41).
Figure 5. Plot of log-log kinematic viscosity versus log of the absolute temperature at different API values (data taken from Henshaw et al.42).
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Energy & Fuels, Vol. 20, No. 6, 2006 2589 Table 1. Different Viscosities at the Same API of 16 (Data from the USA Heavy Oil Database41) API
T (°C)
viscosity (cp)
16
32.0
620 1300 339 340 140 296 460 145 420 88 89 99 470 475 12 89 105
35.5 38.0 43.0 49.0 49.5 60.0
% relative change 110 0.3 111 229 190 1 13 1 642 775
Results and Discussion
Figure 6. Kinematic viscosity vs temperature for a group of oils. Their API gravity appears in parentheses.39
Methodology To understand clearly the relationship between the API and viscosity, an extensive literature search on heavy oil viscosity from other regions in the world was carried out. Hence, the results reported in this work are the product of the analysis of many heavy oils data collected for various heavy oil fields from the open literature.29,39-47 The oils used in the present analysis cover an API range of 8.725.83. Distinctive parameters that have been considered are crude oil gravity (API) and compound class distributions (i.e., saturated hydrocarbons (Sa), aromatic hydrocarbons (Ar), resins (Re), and asphaltenes (As)). Experimental data for an Omani heavy crude oil have been used to validate the findings of this work.
Figures 1 and 2 show the kinematic viscosity versus API gravity for different crude oils at 25 and 50 °C, respectively. It can be seen that the viscosity remains fairly low across the 2520° API gravity range but increases below 18° API. It has been noticed that some heavy oils show anomalous behavior with regard to their API. A wide range of viscosity values for the same API were observed.40,41 At any single API gravity value, there may be variation of 2 orders of magnitude in viscosity (Figure 3). This behavior can be clearly seen in Table 1 and Figure 4. For the considered 17 crude oils with the same API of 16, a wide range of viscosities can be encountered at the same temperature. The relative variation in viscosity values could be as high as 775%. Figures 5 and 6 show that viscosity versus temperature has different slopes which means that there could be cases where two different oils (say, oil1 and oil2) with API1 less than API2 will have µ1 greater than µ2 at one temperature, µ1 equal to µ2 at another temperature, and even µ1 less than µ2 at other tem-
Figure 7. Mass fraction of Sa, Ar, Re, and As versus API (data taken from the Enviromental Technology Centre database43).
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Figure 8. Plot of viscosity versus CAPI.
Figure 9. Plot of log(log(viscosity)) versus API.
peratures. This indicates that serious errors are introduced by the assumption that the viscosity of a heavy oil is only a function of API.
Due to their complexity, the composition of crudes is not expressed in terms of pure components. Instead, their composition is commonly described as percentages of saturated hydro-
(34) Shanshool, J.; Hashim, E. T. Kinematic Viscosity-temperature correlation for undefined petroleum fraction of a wide boiling range. J. Pet. Technol. 2001, 19 (13 & 14), 257-268. (35) Elsharkawy, A. M.; Hassan, S. A.; Hashim, Y. S. Kh.; Fahim, M. A. New compositional models for calculating the viscosity of crude oils. Ind. Eng. Chem. Res. 2003, 42, 4132-4142. (36) Al-Marhoun, M. A. Evaluation of empirically derived pvt properties for Middle East crude oils. J. Pet. Sci. Eng. 2004, 42, 209-221.
(37) Barrufet, M. A.; Dexheimer, D. Use of an automatic data quality control algorithm for crude oil viscosity. Fluid Phase Equilib. 2004, 219, 113-121. (38) Naseri, A.; Nikazar, M.; Dehghani, S. A. M. A correlation approach for prediction of crude oil viscosities. J. Pet. Sci. Eng. 2005, 47, 163-174. (39) Hernandez, M. E.; Vives, M. T.; Pasquali, J. Relationships among viscosity, composition, and temperature for two groups of heavy crudes from Eastern Venezuelan basin. Org. Geochem. 1983, 4 (3/4), 173-178.
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Figure 10. Plot of log(log(viscosity)) versus CAPI.
Table 2. Ranking of Heavy Oils Taken from Henshaw et al.42 by the New Correlating Parameter (CAPI) kinematic viscosity, mm2/s field
API
CAPI
40 °C
50 °C
60 °C
70 °C
100 °C
135 °C
177 °C
Cymric Cymric Midway Sunset Midway Sunset Cymric Cymric Cymric Cymric Cymric Coalinga Coalinga Cymric Kern River Kern River Kern River Kern River Kern River Kern River Kern River Coalinga
8.7 9.4 10.4 10.5 9.2 10 9.7 10.2 10.9 10.3 9.7 11.7 11.9 13.1 12.5 12.3 14.5 14.1 13.4 14.4
1.69 1.95 2.01 2.03 2.20 2.22 2.31 2.44 2.45 2.79 2.80 2.80 3.41 3.58 3.60 3.75 4.03 4.17 4.84 5.34
23280 22380 12190 9530 9170 13020 9340 9420 4400 9270 9890 2070 3360 1840 3050 2170 710 600 1150 380
6420 5990 3670 3200 2950 3720 3100 2920 1520 2830 2950 800 1250 740 1140 890 330 280 490 190
2260 2050 1380 1220 1170 1350 1150 1120 640 1080 1100 370 550 340 510 420 170 150 240 100
1000 890 610 540 540 580 510 510 310 480 490 190 280 180 260 220 100 90 130 60
145.5 126.9 103.9 91.5 97.7 94.2 84.8 86 61.6 79.7 78.3 42 58 41.7 54.3 52.1 26.6 24.9 33.6 19.1
32.6 31.2 26.3 24.2 24.4 23.3 21.1 23.4 17.4 22.2 22 13.6 17.1 13.7 16.3 15.8 10 7.4 12.9 7.3
10.37 10.18 9.43 8.72 8.71 8.01 7.59 8.62 6.62 7.99 7.83 5.48 6.86 5.52 6.87 6.49 4.4 4.15 4.92 3.4
carbons, aromatic hydrocarbons, resins, and asphaltenes. Since these groups of compounds change in composition from one crude to another, they are usually not used to predict the viscosity of crudes. Figure 7 relates the API values of the studied (40) Smalley, C. Heavy oil and viscous oil. In Modern Petroleum Technology: Volume 1: Upstream; Dawe, R. Ed.; John Wiley & Sons Ltd.: New York, 2000; Chapter 11. (41) USA Heavy Oil Database, International Centre for Heavy Hydrocarbons http://databases.oildrop.org/default.htm. (42) Henshaw, P. C.; Carlson, R. M. K.; Pena, M. M. Evaluation of geochemical approaches to heavy oil viscosity mapping in San Joaquin Valley, California. Prepared for presentation at the 1998 SPE Western Regional Meeting, Bakersfield, CA, May 10-13, 1998; SPE 46205. (43) Environmental Technology Centre (ETC). Oil properties database. http://www.etc.ec.gc.ca or http://www.etcentre.org/databases/spills_e.html. (44) Wu, W.; Chen, J. Characteristics of Chinese Heavy Crudes. J. Pet. Sci. Eng. 1999, 22, 25-30.
oils to their Sa, Ar, Re, and As content. In general, these plots indicate a decrease in API with an increase in Sa and decreases in Ar, Re, and As. It is interesting to note the relationship between viscosity and the (Sa/(Ar + Re + As)) ratio multiplied by the API, the new correlating parameter (CAPI) (Figure 8). (45) Pirela, L. Influence of Time in the Viscosity Determination of Heavy Crudes and Their Mixtures. Presented at the Third International Conference on Heavy Crudes and Tar sands, UNITAR/UNDP Information Centre for Heavy Crude & Tar Sands, New York, USA, 1985; HCTS/CF.3/8.7, 559A559T. (46) Srinivasan, M.; Watkinson, A. P. Fouling of Some Canadian Crude Oils. Presented at the ECI Conference on Heat Exchanger Fouling and Cleaning: Fundamentals and Applications, Watkinson, P., Mu¨llerSteinhagen, H., Malayeri, M. R., Eds.; Santa Fe, NM, USA, 2003; Paper 26. (47) Pereira, P. How to Extend Existing Heavy Oil Resources through Aquaconversion Technology. Presented at the World Energy Council, 2004.
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Figure 11. Plot of log(log(viscosity)) versus log(temperature) for data taken from Henshaw et al.42
It can be seen that the viscosities of oils under the limiting value of ∼15 are very sensitive to small changes in CAPI. Furthermore, it is also interesting to note that the Omani heavy crude oil viscosity can be predicted from this graph at any temperature. Unfortunately, no complete consistent data is available to explain the existence of this limiting value. Figure 9 shows a plot of viscosity versus API. Looking at this figure, it can be seen that Omani heavy crude oil viscosity does not follow the general trend of other crude oils. However, when the viscosity is plotted versus CAPI, (Sa/(Ar + Re + As)) × API, the Omani heavy crude oil seems to behave like other heavy oils (Figure 10). This new method of ranking heavy oils was applied to data taken from Henshaw et al.,42 and the results are shown in Table 2 and Figure 11. It seems that heavy oils are better characterized by this correlating parameter than by the API gravity alone, as is usually done.
Conclusion Considerable errors may be introduced when the viscosity of heavy oils is estimated from general viscosity trends and the API gravity. The method of ranking heavy oil using API gravity was not always valid. Compositional terms such as the percent saturates, aromatics, resins, and asphaltenes can reasonably correct these apparently anomalous behaviors. In a plot of viscosity versus the new correlating parameter (CAPI), two very different tendencies, defining a limiting value of 15, were observed. The viscosities of oils found under the “limiting value” of 15 were very sensitive to small changes in CAPI. On the basis of the above points, heavy oil that showed higher viscosity compared to oils of similar API was found to be normal when CAPI was used. EF0603030
