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Energy & Fuels 2003, 17, 1630-1640
The Effect of Asphaltenes on the Gelation of Waxy Oils Ramachandran Venkatesan,† Jenny-Ann O ¨ stlund,‡ Hitesh Chawla,§ † ‡ Piyarat Wattana, Magnus Nyde´n, and H. Scott Fogler*,† Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan, Chalmers University of Technology, Go¨ teborg, Sweden, and Indian Institute of Technology, Delhi, India Received June 2, 2003
The phase stability of crude oil is dependent on a multitude of factors, including temperature, pressure, and component fractions, especially long chain paraffin and polar asphaltene fractions. Paraffins precipitate out of the crude oil during pipeline transportation due to solubility limits, and form paraffin-oil gel deposits on the pipe walls. The presence of asphaltenes in crude oil is postulated to affect the formation of these paraffin gels. To quantify this effect, a controlled stress rheometer was used to study the gelation temperature and the yield stress of a model paraffinoil system. It was observed that the addition of asphaltenes in small proportions (∼0.1 wt %) resulted in a significant decrease both in the gelation temperature and the yield stress of the model system, indicating that the presence of asphaltenes hinders the gelation mechanism. Addition of higher amounts of asphaltenes resulted in macroscopic phase separation: a deposit consisting of asphaltenes and paraffins separated out of the liquid. The effects of operating conditions and the asphaltene polarity on the gelation process were also studied. Polarized light microscopy and nuclear magnetic resonance spectroscopy were used to obtain insights into the rheometric results.
Introduction Crude oil is a complex mixture of hydrocarbons consisting of paraffins, aromatics, naphthenes, asphaltenes, and resins. Among these, high molecular weight n-paraffins and polar asphaltenes are known to cause problems during the production and transportation of crude oil. High molecular weight paraffins (interchangeably referred to as waxes henceforth) cause problems during the transportation of crude oil, especially through cold sub-sea pipelines. At reservoir temperatures (70150 °C) and pressures (50-100 MPa), the solubility of paraffins in crude oil is sufficiently high to keep these paraffin molecules fully dissolved in the mixture. Once the crude oil leaves the reservoir and flows through subsea pipelines, its temperature begins to drop due to the heat loss to the cold sub-sea environment. The solubility of paraffins in crude oil decreases drastically with decreasing temperature, hence paraffins precipitate and deposit on the wall of the pipeline, resulting in decreased flow efficiency. Paraffin deposition is a very serious problem in the oil industry, with the resultant production losses and remediation operations costing millions of dollars.1 Asphaltenes are the most polar fractions of crude oil, consisting of aromatics and heteroatoms. Asphaltenes cause production problems when they precipitate near the well bore due to changes in pressure and temperature. * Corresponding author. Fax: 734 763 0459. E-mail: sfogler@ umich.edu. † University of Michigan. ‡ Chalmers University of Technology. § Indian Institute of Technology. (1) Oil Gas J. 2001, 99, 56.
The paraffin deposit occurs in the form of a paraffinoil gel whose solid wax fraction increases with time in a process termed aging.2 At low temperatures, paraffin molecules precipitate to form stable orthorhombic crystallites.3 The flocculation of crystallized paraffins leads to the formation of paraffin-oil gels with complex morphology. The incipient gel formed on the wall of the pipeline may have as little as 3-4% of solid wax with the remainder being occluded oil. The wax content of the incipient gel then increases with time as more wax molecules diffuse into the gel matrix from the bulk oil flow.2,4 The aging process makes the gel deposit harder, thus mechanical remediation measures such as pigging become more difficult. There have been several attempts at modeling the wax deposition phenomenon based on heat transfer and diffusion mechanisms.5-10 All of these models have been developed under the premise that the wax content of the gel is a constant. More recently, Singh et al.2,4 have developed a comprehensive mathematical model that takes into account the phenomenon (2) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. AIChE J. 2000, 46, 1059-1074. (3) Dirand, M.; Chevallier, V.; Provost, E.; Bouroukba, M.; Petitjean, D. Fuel 1998, 77, 1253-1260. (4) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. AIChE J. 2001, 47, 6-18. (5) Bern, P. A.; Withers, V. R.; Cairns, R. J. R. Proc. Eur. Offshore Pet. Conf. Exhib. 1980, 206, 571-578. (6) Brown, T. S.; Niesen, V. G.; Erickson, D. D. Production Operations and Engineering Proceedings SPE Annual Technical Conference and Exhibition 1993, 353-368. (7) Burger, E. D.; Perkins, T. K.; Striegler, J. H. J. Pet. Technol. 1981, 33, 1075-1086. (8) Majeed, A.; Bringedal, B.; Overa, S. Oil Gas J. 1990, 88, 63-69. (9) Svendsen, J. A. AIChE J. 1993, 39, 1377-1388. (10) Ribeiro, F. S.; Souza Mendes, P. R.; Braga, S. L. Int. J. Heat Mass Transfer 1997, 40, 4319-4328.
10.1021/ef034013k CCC: $25.00 © 2003 American Chemical Society Published on Web 10/24/2003
Effect of Asphaltenes on the Gelation of Waxy Oils
of aging as well as the evolution of the deposit morphology due to aging. Two of the fundamental properties of paraffinic oils that determine the precipitation, gelation, and deposition processes are (i) the cloud point temperature (TCP), and (ii) the gelation temperature (Tgel). The cloud point temperature is defined as the temperature at which the first paraffin molecules precipitate out of the oil when it is subjected to cooling; thus, it is also termed as the “wax appearance temperature”. Theoretically, the cloud point is a purely thermodynamic quantity that does not depend on the operating conditions. Operational definitions of the cloud point could, however, lead to small variations with operating conditions. The gelation temperature is the temperature at which the paraffinic oil transforms into a “gel”, and is best determined with the help of rheometric studies.11 Unlike the cloud point temperature, the gelation temperature does depend on the operating conditions, and is a strong function of the shear and thermal histories to which the paraffinic oil is subjected.12 When a temperature lower than the gelation temperature is imposed on paraffinic oil, it forms a gel consisting of a solid paraffin crystal network and entrapped liquid oil. The yield stress (τy) is the rheological property that quantifies the strength of such paraffin-oil gels. The knowledge of the yield stress is critical in determining either the force required for pigging the wax-oil gels or the pressure required for the restart of a completely plugged oil pipeline. Studies with model paraffin-oil systems have shown that the gelation process is a complex function of the shear and thermal histories imposed on the system.11,12 When crude oil flows through sub-sea pipelines, the flow rate, the oil temperature, the ambient temperature, the thermal conductivities of the oil, the pipe material, etc., are the factors that determine the shear and cooling histories of the gel deposit. In addition to these operating conditions, the composition of the crude oil is the most important factor that affects the gelation process. The cloud point, the gelation temperature, the yield stress, the gelation kinetics and the extent of gel deposition all depend on the oil composition. As mentioned before, crude oil consists of paraffins, aromatics, resins, and asphaltenes. The paraffin gelation process would depend on the relative amounts and characteristics of these components of the crude oil. For example, we have determined previously13 that the yield stress of a wax-oil system increases exponentially with the wax content. The present article describes our investigation on the effect of the presence of asphaltenes on the gelation temperature and the yield stress of paraffinic oils. It has been reported in the literature that the presence of flocculated asphaltenes affects the wax crystallization process. Garcia14 has observed that the flocculated asphaltenes provide nucleation sites for the crystallization of waxes. Thus, the cloud point temperature of a “reconstructed” crude oil was observed to increase with (11) Venkatesan, R.; Singh, P.; Fogler, H. S. Soc. Pet. Eng. J. 2002, 7, 349-352. (12) Singh, P.; Fogler, H. S.; Nagarajan, N. R. J. Rheol. 1999, 43, 1437-1459. (13) Venkatesan, R.; Fogler, H. S. Proceedingss2001 Annual AIChE meeting, Reno, NV, 2001. (14) Garcia, M. d. C. Energy Fuels 2000, 14, 1043-1048.
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the addition of asphaltenes. Further, the flocculation of asphaltenes was found to reduce the effectiveness of wax inhibitors, due to the formation of complex asphalteneparaffin solid aggregates.15 Chanda et al.16 have observed that the effect of the molecular weight of pour point depressants on the extent of pour point depression was dependent on whether asphaltenes were present in the crude oil. Further, some oil field observations have shown that crude oils that contain higher amounts of asphaltenes typically form paraffin deposits that are easier to remove.17 Thus, the presence of asphaltenes is known to affect the formation and properties of paraffin-oil gels. It is, therefore, instructive to scientifically quantify the effect of asphaltenes on the gelation of paraffinic oils. As mentioned earlier, the gelation and gel properties can be best described by two rheological quantities: namely the gelation temperature (Tgel) and the yield stress (τy). In the research described in this article, a model paraffin-oil system was doped with minute quantities (∼ 0.1 wt %) of asphaltenes and the effects of this addition on these rheological properties were studied. Rheometric experiments, using a controlled stress rheometer, were performed to identify Tgel and τy of the system. Optical microscopy and nuclear magnetic resonance spectroscopy were used to study the structure of the gel system. Experimental Section The Model Asphaltene-Paraffin-Oil System. A mixture of paraffins and asphaltenes in an oil solvent was used as the model system for gelation temperature and yield stress studies. The chemical description of these components is given below. For all the experiments, a mixture of 5% wax in oil was used and different amounts of asphaltenes were added. The oil solvent used was a mixture of toluene and a mineral oil (Coray-15, an Exxon product). This mineral oil consists of paraffinic, aromatic, and naphthenic components. Figure 1 shows the analysis of Coray-15 using a high-temperature gas chromatograph (HTGC). Two different paraffins were used in this study: (i) a food-grade paraffin wax obtained locally (referred to as Wax1 henceforth), and (ii) a laboratory-grade paraffin wax from Aldrich Chemicals (referred to as Wax2 henceforth). The melting point of Wax1 was about 55 °C and that of Wax2 was 65 °C. The carbon number distribution of these two waxes, as determined by HTGC, is compared in Figure 2. Wax2 had longer paraffins, with the maximum carbon number detected to be C49, whereas Wax1 had a greater abundance of shorter paraffins. Hence the higher melting point of Wax2. The cloud point temperature of the mixture consisting of 5% Wax1, 85% Coray-15, and 10% toluene (all percentages by weight) was TCP ) 29 °C. In this article, the mixture of Coray-15 and toluene is referred to as “oil” henceforth. TCP of the 5% Wax2-oil mixture was 39 °C. The higher TCP of the Wax2 mixture is again attributed to the presence of longer n-paraffins which precipitate at higher temperatures. The asphaltenes were extracted from crude oil (Zuata crude oil, from Venezuela) by the addition of heptane. Further, the asphaltenes were fractionated into various polarity-based fractions using a technique developed previously in our laboratory.18 Three fractions were isolated: (i) F30/70, the most polar (15) Garcia, M. d. C.; Carbognani, L. Energy Fuels 2001, 15, 10211027. (16) Chanda, D.; Sarmah, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam, B.; Das, H. C. Fuel 1998, 77, 1163-1167. (17) Thomason, W. Personal communication.
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Venkatesan et al.
Figure 1. GC signal of Coray-15.
Figure 2. Carbon number distribution of Wax1 and Wax2. fraction, (ii) F20/80, the mid-fraction, and (iii) F10/90, the least polar fraction. The terminology “Fx/y” etc are explained by Kaminski et al.18 The fractionation was carried out in order to study the effect of asphaltene polarity on the gelation temperature and yield stress of the paraffin-oil mixture. The asphaltenes were first dissolved in toluene and then mixed with Coray-15 in appropriate amounts. When Coray-15 was added, there was a certain extent of asphaltene flocculation and precipitation, which could be observed by microscopy as explained in a later section. Finally, the wax was added and the entire system was heated and stirred so that a homogeneous solution was obtained. Rheometric Studies. Rheological studies were carried out using a controlled-stress rheometersAR1000 from TA Instrumentssto determine Tgel andτy. The rheometer had a cone and plate geometry and was mounted with a Peltier plate cooling system for accurate temperature control. A sample of asphaltene-paraffin-oil mixture was heated well above the cloud point temperature and loaded on the rheometer. Then the (18) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14, 25-30.
sample was cooled at a specified rate (1 °C/min except where mentioned otherwise). A low-amplitude (0.15 Pa) oscillatory shear stress of frequency 0.1 Hz was applied and the resultant motion was studied as a function of the temperature. The rheological definition of the “gel point” or the “gelation temperature” (Tgel) is the point at which the solidlike behavior of a complex fluid takes predominance over its liquidlike behavior. When a complex fluid, such as waxy crude oil, is subjected to an oscillatory shear stress, there is an in-phase response and an out-of-phase response in the resultant strain rate of the fluid. The liquidlike behavior of the complex fluid is characterized by the loss modulus, which is determined by the in-phase response (similar to a Newtonian liquid). The solidlike behavior is characterized by the storage modulus, which is determined by the out-of-phase response (similar to a Hookean solid). The ratio of the loss modulus (identified as G′′) to the storage modulus (identified as G′) is the loss tangent “tan (δ)”. When a wax-oil mixture is at a temperature well above the cloud point temperature, it behaves as a Newtonian liquid. At this temperature, the loss modulus is much higher than the storage modulus, hence tan (δ) > 1. As the temperature of the waxy oil decreases, both G′ and G′′ increase in value. At temperatures below TCP, as wax molecules precipitate out, the solidlike behavior of the mixture increases and is manifested in the form of a sharp increase in the magnitude of G′. The temperature at which G′ becomes equal to G′′, i.e., the point at which tan (δ) ) 1, is defined as the Tgel of the waxoil mixture. G′ becomes higher than G′′ if the mixture is subjected to further cooling. The yield stress (τy) of the wax-oil system was studied by a two-step process in the rheometer: (i) After initial heating to a temperature well above the cloud point, the sample was cooled under quiescent conditions at a cooling rate of 1 °C/min until a desired final temperature (Tys) was reached. Tys was chosen to be less thanTgel. Hence, at this final temperature, the sample was a gel that exhibited a yield stress. (ii) The sample was maintained isothermally at Tys, and a shear stress ramp was applied at a rate of 5 Pa/min. τy of the
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gelled sample was the value of the shear stress at which there was a rapid decrease in the viscosity of the sample. NMR Self-Diffusion Studies. NMR self-diffusion experiments were performed to probe the structural aspects of paraffin gels. The 1H NMR self-diffusion technique is a component resolved technique that provides an accurate determination of the diffusion coefficients of the components in a sample.19-21 Hence, it is possible to study the diffusion of solvent and/or probe molecules in, as in this work, a gel system, and the results may give valuable information about the microstructure of the investigated system. Wax-oil samples with and without asphaltenes were studied using this technique. The wax was initially dissolved in Coray-15 and then toluene-d8 solutions containing the asphaltenes were added (pure toluene-d8 was added in the case of no asphaltenes). The mixtures were then heated and transferred to 5 mm NMR tubes, and probe molecules of tetramethylsiloxane (TMS) were added. The tubes were thereafter flame sealed. After sealing, the samples were once again heated and shaken to ensure homogeneity. The final samples contained 4.6% wax, 83.5% Coray-15 oil, 2.7% TMS, 9.2% pure toluene-d8, or a mixture of toluene-d8 and asphaltenes (all by weight). For the samples with asphaltenes, the asphaltene content was 0.05 wt %. For the NMR self-diffusion experiments, a standard Hahn echo,22 consisting of two radio frequency pulses, was used. The first pulse in the Hahn echo is a 90° pulse and, after t ) τ, a 180° pulse is applied. The latter pulse gives rise to an echo signal (at t ) 2τ) due to refocusing of the transverse magnetization. Furthermore, the pulse sequence consists of two pulsed field gradients that are applied after each of the pulses. The time separation between the pulsed field gradients is called the effective diffusion time, ∆. In all experiments the gradient pulse duration (δ) was 4 ms and the pulses were sineshaped. The echo attenuation due to diffusion of the molecules studied is for a Hahn-echo with sine-shaped pulses described by -kD
I ) I0′e
Figure 3. Determination of gelation temperature of wax-oil system by rheometry.
Figure 4. Typical yielding curves showing a breakdown in viscosity.
(1)
In this equation I is the measured intensity of the echo, k ) γ2g2δ2(4∆ - δ)/π2, where g is the gradient strength and γ the gyromagnetic ratio of the 1H nucleus, I0′ is the intensity at g)0, and D is the diffusion coefficient. By increasing g, the echo intensity decreases and, by applying a nonlinear leastsquares fit of eq 1 to the experimental data, the diffusion coefficient can be determined. If the echo-decay is nonlinear, a more complex function containing more than one exponential may be used to describe the experimental data. The diffusion experiments were performed on a Unity Inova 500 MHz spectrometer and an Oxford magnet equipped with a diffusion probe from DOTY Sci. Inc. Microscope Studies. Cross-polarized light microscopy was used to study the wax crystal structure formed in the presence and absence of asphaltenes. A Nikon Eclipse microscope (TE 2000-S) was used for these studies. When using polarized light, the anisotropic domains such as crystals are seen as bright domains inasmuch as they scatter light. A Bionomic cooling system (from 20-20 Technology) was mounted onto the microscope for temperature control, and an aluminum/sapphire slide was used for good thermal conductivity. Images were captured using a video camera attached to the microscope.
Results and Discussion Determination of Gelation Temperature and Yield Stress. The gelation temperature of 5% wax in (19) Callaghan, P. T. Aust. J. Phys. 1984, 37, 359-397. (20) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1-45. (21) Price, W. S. Concepts of Magnetic Resonance; John Wiley and Sons: New York, 1997. (22) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288-292.
oil mixture was first studied by oscillatory stress tests in the rheometer. The 5% Wax1 mixture sample was initially heated to 40 °C, whereas the 5% Wax2 sample was initially heated to 47 °C, the temperatures being well above their respective cloud points. A typical rheometric result is shown in Figure 3. As observed from this figure, for the Wax2 system, Tgel measured under static conditions at a cooling rate of 1 °C/min was 35.8 °C. Tgel of the Wax1 system was measured to be 27.8 °C. We have shown previously that the gelation temperature of a given waxy oil is a function of the shear and thermal histories.11,12 To measure τy of the wax-oil mixture, it was initially heated to a temperature well above TCP (40 °C for the Wax1 mixture and 47 °C for the Wax2 mixture) and was cooled to Tys ) 5, 10, or 15 °C. A stress ramp was applied while maintaining the mixture isothermally at Tys. Figure 4 shows the plot of viscosity as a function of the applied shear stress for two different systems. At low values of the shear stress (
