2504
Energy & Fuels 2006, 20, 2504-2508
A Study of the Effect of Gas Condensate on the Viscosity and Storage Stability of Omani Heavy Crude Oil Naoya Shigemoto,† Rashid S. Al-Maamari,*,‡ Baba Y. Jibril,‡ and Akihiko Hirayama§ Chemical Technology Department, Shikoku Research Institute, Inc., 2109 Yashima-nishimachi, Takamatsu, Kagawa 761-0192, Japan, Petroleum and Chemical Engineering Department, College of Engineering, Sultan Qaboos UniVersity, Post Office Box 33, Al-Khoud, 123 Muscat, Sultanate of Oman, and Department of Energy Solution, Shimizu Corporation, 1-2-3 Shibaura, Minatoku, Tokyo 105-8007, Japan ReceiVed February 18, 2006. ReVised Manuscript ReceiVed July 11, 2006
Heavy crude oil (density of 0.9571 g/cm3 at 288 K and kinematic viscosity of 7160 mm2/s at 303 K) and natural gas condensate (density of 0.7848 g/cm3 at 288 K and kinematic viscosity of 0.764 mm2/s at 303 K) were sampled from Omani oil fields. The oil was mixed with condensate (5-50 vol %), and the mixture viscosities were measured at the temperature range of 293-348 K. A drastic decrease in kinematic viscosity was achieved. An addition of about 12 vol % of condensate gave a viscosity of 265 mm2/s at 303 K. This made it easier to transport the heavy oil mixtures. The experimental data for the kinematic viscosity [ν (mm2/ s)] as a function of the gas-condensate fraction (φ) and temperature [T (K)] could be described by an equation: ln(ln(ν)) ) (k1 + k2‚T) + (k3 + k4‚T)‚φ, with a high accuracy. Blends of the heavy oil and the gas condensate were stored to evaluate their stability. Results showed less than 0.05 wt % sludge formation after 2 months.
1. Introduction There are large heavy crude oil reserves in the world, but the high viscosity of the crude oil restricts its utilization. This has spurred research effort to develop viscosity reducing methods to increase oil mobility in reservoirs and transportability in pipelines. In practice, some of the methods employed to reduce the viscosity are warming,1 emulsifying with an aqueous surfactant solution,1-4 and dilution with a lighter solvent.5 These methods have their merits and demerits for improving oil recovery and transportability.6-8 The dilution method is especially applicable if cheap and large amounts of lighter hydrocarbon are available. Dependent upon the site of an oil field, gas condensate recovered from a petroleum gas could be employed for the dilution of the heavy crude oil.5,9-11 The amount and polarity of asphaltenes and their interactions with * To whom correspondence should be addressed. Fax: +96824413416. E-mail:
[email protected]. † Shikoku Research Institute, Inc. ‡ Sultan Qaboos University. § Shimizu Corporation. (1) Pilehvari, A.; Saadevandi, B.; Halvaci, M.; Clark, P. E. Am. Soc. Mech. Eng. Fluids Eng. DiV. 1988, 75, 161-168. (2) Hernandez-Carstens, E.; Rodriguez, R. Am. Soc. Mech. Eng. Fuels Combust. Technol. DiV. 1991, 11, 1-5. (3) Kennedy, B. A. Am. Soc. Mech. Eng. Fuels Combust. Technol. DiV. 1991, 11, 7-20. (4) Redman, J. Chem. Eng. 1990, 26, 12-13. (5) Adewusi, V. A. Pet. Sci. Technol. 1998, 16, 697-717. (6) Saito, Y. T.; Sato T.; Anazawa, I. J. Am. Oil Chem. Soc. 1990, 67, 145-148. (7) Sun, R.; Shook, C. A. J. Pet. Sci. Eng. 1996, 14, 169-182. (8) Al-Roomi, Y.; George, R.; Elgibaly, A.; Elkamel, A. J. Pet. Sci. Eng. 2004, 42, 235-243. (9) Wyslouzil, B. E.; Kessick, M. A.; Masliyah, J. H. Can. J. Chem. Eng. 1987, 65, 353-360. (10) Del Carmen Garcia, M.; Urbina, A. Pet. Sci. Technol. 2003, 21, 863-878. (11) Schermer, W. E. M.; Melein, P. M. J.; Van Den Berg, F. G. A. Pet. Sci. Technol. 2004, 22, 1045-1054.
resins in the oil are important factors for understanding the viscosity.12 Therefore, the viscosities of oil-condensate blends depend upon the properties of the original oil and condensates, oil/condensate ratio, and operating temperature. In addition to viscosity reduction, the stability of the blend in storage is important.11 Dependent upon the molecular-weight distribution of the oil components, the wax crystallization tendency changes.10 It was found to be lower for the oil with a wider molecular-weight distribution. Furthermore, although the viscosity of heavy crude oil could be reduced by blending with paraffin-rich light oil, such as the gas condensate, the asphaltenes in the heavy crude oil may be segregated and coagulated, thereby leading to the blend instability. The effects of the condensate volume fraction and operating temperature on the blend flow characteristics have not been fully investigated. In the present study, heavy crude oil and gas condensate were blended at different oil/condensate ratios and different temperatures to reduce the viscosity. To express the simultaneous effects of the dilution by the gas condensate and temperature on the viscosity, an estimation equation for the viscosity as a function of the temperature and the gas-condensate fraction was derived and verified. Blend storage stability was evaluated by observing a sludge formation. 2. Viscosity Correlation Description As demonstrated in different papers,12-14 the viscosity is affected by factors such as temperature, viscosities of the heavy crude oil and the gas condensate, their native components, and mixing fraction. Viscosity of oils increases with increases in (12) Swanson, J. J. Phys. Chem. 1942, 46, 141-150. (13) Hernandez, M. E.; Vives, M. T.; Pasquali, J. Org. Geochem. 1983, 4, 173-178. (14) Priyanto, S.; Mansoori, G. A.; Suwono A. Chem. Eng. Sci. 2001, 56, 6933-6939.
10.1021/ef060074h CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006
Gas Condensate on Omani HeaVy Crude Oil
Energy & Fuels, Vol. 20, No. 6, 2006 2505
aromatics, resins, or asphaltenes or a decrease in saturates.13 The variations in the proportions of these constituents have been shown to have different effects on the oil viscosity. For instance, asphaltenes may increase the viscosity because of their tendency to interact and aggregate.14 An increase in temperature leads to more rapid movements of molecules, reducing their interaction tendency, thereby changing the viscosity. For oil-water emulsions, by modifying Richardson’s equation,15 Broughton and Squires derived the following equation to describe the viscosity as a function of the volume fraction of the dispersed phase (φ) at constant temperature:
ηr ) A‚exp(kφ)
(1)
where ηr is a ratio of the viscosity of the emulsion or blend (ηc) to that of the pure continuous phase (ηo) and A and k are system-dependent constants. This equation was found to give a good description of systems with different types of oils. The logarithmic expression of eq 1 gives the following:
ln(ηr) ) ln(A) + k‚φ
(2)
and coincides with Rønningsen’s equation,16 if ln(A) and k are linear functions of the temperature, T (K):
ln(ηr) ) A + B‚φ ≡ (k1 + k2‚T) + (k3 + k4‚T)‚φ
(3)
where k1-k4 are constants. However, the equation may not be useful for system that differs drastically from oil/water emulsion, such as heavy oils, condensates, and their blends. A modified form of the Walther correlation (eq 4) was proven to be adequate for predicting the variation in kinematic viscosity with temperature for various crude oils and their fractions17
ln(ln(z)) ) A - B‚ln(T)
(4)
where A and B are system-dependent constants, z ) ν + 0.7 + f(ν), and f(ν) ) exp(-1.47 - 1.84ν - 0.51ν2). The exponential term reduces to 0 for kinematic viscosities larger than 2 mm2/ s. This correlation equation was recommended by American Society for Testing and Materials (ASTM)17 and was generally accepted in the industry. We developed alternative correlation equations to estimate the viscosity of the heavy oil and gas-condensate blend by combining eqs 3 and 4 as follows:
ln(ln(ν)) ) A + B‚φ ≡ (k1 + k2‚T) + (k3 + k4‚T)‚φ (5) and
ln(ln(ν)) ) (k1 + k2‚ln(T)) + (k3 + k4‚ln(T))‚φ
(6)
where k1-k4 are constants, φ is the volume fraction of the condensate, and T is temperature (K). The famous Andrade’s equation (eq 7),18 in which ln(ν) is linear to 1/T, was used to describe the temperature effect on the viscosity of many substances. Therefore, in addition to eqs 5 and 6, eq 7 was also tested for our data (15) Richardson, E. G. Colloid J. 1933, 65, 32. (16) Rønningsen, H. P. Proc. SPE Int. Symp. Oil Field Chem. Houston, TX, 1995; p 28968. (17) ASTM. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 2001; Vol. 05, section 05. (18) Society of Chemical Engineers of Japan, Ed. Chemical Engineering Handbook, 5th ed.; Maruzen: Tokyo, Japan, 1988; p 91.
ln(ln(ν)) ) A + B‚φ ≡ (k1 + k2/T) + (k3 + k4/T)‚φ (7) 3. Experimental Section 3.1. Materials and Characterization. Samples of Omani heavy crude oil and gas condensate were collected from Mukhaizna and Qarn Alam oil fields in Oman, respectively. The basic characteristics of the samples were obtained on the basis of Japan Industrial Standards (JIS). The density was measured using the vibrating densitometer method (JIS K 224919). The elemental composition, C, H, N, and S, was measured by employing combustion gas analysis. By combustion of a small amount of the samples in an oxygen atmosphere, carbon and hydrogen in the samples were converted to carbon dioxide and water, respectively. The differences of the thermal conductivities of the formed gases before and after carbon dioxide adsorption or dehumidifying were detected to determine the concentrations of the element. The concentration of nitrogen in the samples was measured by oxidative combustion and the chemiluminescence method (JIS K 260920). The sulfur content in the samples was measured by the combustion-absorption method (JIS K 254121), where sulfur combusted in oxygen flow, to form SO2. The SO2 was then absorbed in a H2O2 solution to afford H2SO4. Titration with a NaOH solution was used to determine the acid concentration. Water content was measured by the conventional method of coulometric Karl Fisher titration (JIS K 227522). The asphaltene content was measured gravimetrically after the separation of the asphaltene from the crude oil (IP 14723). The crude oil was mixed with heptane, and the mixture was heated. Then, the precipitated material was collected on a filter paper and washed with hot heptane to remove waxy substances. The asphaltene material in the residual was dissolved with hot toluene; the solvent was evaporated; and then the asphaltenes were weighted. The hydrocarbon type for both the heavy oil and the condensate was analyzed by thin-layer chromatography with flame-ionization detection. Ash in the samples was obtained by weighting the combustion (1048 K) residual (JIS K 227224). The kinematic viscosity was measured by means of a capillary tube viscometer (JIS K 228325). 3.2. Mixed Oil Storage Stability Testing. The condensate was mixed with the crude oil at different condensate volume percent of 0-50 vol % in a step of 5%. The viscosities of the mixtures were measured after keeping their temperature at 293, 303, 323, or 348 K. A blend with more than 50 vol % gas condensate was not considered in this study. To evaluate storage stability by means of sludge formation, the mixture was stored in a metal container (500 mL) at room temperature (303 K) for 2 months. The resultant dry sludge measurement was started immediately after storage and then after a period of 1 week, 1 month, and 2 months. The storage stability of the samples was evaluated by measuring the increases in the amounts of dry sludge. A warmed sample was poured into a filter vessel with a filter at the bottom. It was then pressurized to separate the sludge. To remove any oil that might have remained, the filter (Wattman No. 5 filter, pore diameter of 1 µm) was washed with warm heptane, dried, and weighed.
4. Results and Discussion 4.1. Properties of Heavy Crude Oil and Gas Condensate. Table 1 shows basic properties of the heavy crude oil and gas condensate, which were used in the experiments. The kinematic viscosities of the crude oil at 303, 323, 348, and 373 K were 7160, 1360, 303, and 101 mm2/s, respectively, showing a decrease of the viscosity with increasing temperature. The (19) JIS. JIS K 2249 (Japanese). JIS: Tokyo, Japan, 1995. (20) JIS. JIS K 2609 (Japanese). JIS: Tokyo, Japan, 1998. (21) JIS. JIS K 2541 (Japanese). JIS: Tokyo, Japan, 1996. (22) JIS. JIS K 2275 (Japanese). JIS: Tokyo, Japan, 1996. (23) The Institute of Petroleum (U.K.) Standard Methods IP 147. The Energy Institute: London, U.K., 2006. (24) JIS. JIS K 2272 (Japanese). JIS: Tokyo, Japan, 1998. (25) JIS. JIS K 2283 (Japanese). JIS: Tokyo, Japan, 2000.
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Shigemoto et al.
Table 1. Properties of Crude Oil and Gas Condensate item density element analysis
unit 288 K C H N S
C/H ratio asphaltene content water content ash content kinematic viscosity pour point residual carbon hydrocarbon composition analysis
253 K 273 K 303 K 323 K 348 K 373 K saturated olefin aromatic
g/cm3 wt % wt % wt % wt % wt % vol % wt % mm2/s mm2/s mm2/s mm2/s mm2/s mm2/s K wt % wt % wt % wt %
Mukhaizna crude oil 0.9571 84.4 11.5 0.27 3.46 7.34 4.70 0.16 0.033 7160 1360 303 101 253.0 12.3
gas condensate 0.7848 87.4 12.6
