Characteristics of Wax Gel Formation in the Presence of

The x-intercept values obtained from yield stress versus temperature were interpreted as no-flow points, which could be used as alternative measures o...
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Energy & Fuels 2009, 23, 1289–1293

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Characteristics of Wax Gel Formation in the Presence of Asphaltenes† Kyeongseok Oh* and Milind Deo Chemical Engineering Department, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed August 1, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008

When pipelines are shutdown, waxy crude oils tend to form gels, which tend to plug the lines and stop flow. Restart requires sufficient pressure to overcome the yield stress of gelled oils. This study examines the yield strength of well-characterized waxy model oils at temperatures below the pour point. First, the yield stresses of model oils were determined by the vane method at different temperatures. Yield stress values were strongly dependent upon wax amounts and compositions, as expected. The extent of increase in yield stress values with temperature was greater for model oils that had a higher percentage of wax. The x-intercept values obtained from yield stress versus temperature were interpreted as no-flow points, which could be used as alternative measures of pour points. Second, the role of asphaltenes was examined in the evolution of the yield stress as the oil is cooled below the pour point. Asphaltene additions resulted in pour-point reductions, of up to 4 °C for additions of asphaltenes up to 0.1% (w/w). Small amounts of asphaltenes (0.01%, w/w) also played a significant role in yield stress reduction. The concept of steric hindrance and asphaltene aggregation was adapted to explain the yield stress reduction at the different asphaltene concentrations. At lower temperatures, as more wax came out of solution, the slope of the yield stress versus temperature line went back to the slope of the asphaltene-free oil, indicating the dominance of the wax networks at higher wax concentrations.

Introduction High-molecular-weight paraffinic waxes in a crude oil start to precipitate when the surrounding temperature is lower than the wax appearance temperature (WAT). The terms, cloud point and wax precipitation temperature, are often used interchangeably. Dependent upon the method used, WAT measurements can be significantly different. Pedersen et al.1 determined WATs with three different measurement methods: microscopy, viscosity, and differential scanning calorimetry, showing that typically highest WAT values were observed when using the microscopic measurement. The comparison of WAT values and the detection limits with the different methods has also been reported elsewhere.2 The pour point on the other hand is another important characteristic temperature that is usually determined by American Society for Testing and Materials (ASTM) D97.3 Flow discontinuity can occur either with wax deposition or wax gel formation. While the wax deposition can be initiated during flow, wax gel formation occurs under static conditions caused by shutdown. Subsequent to shutdown, if the wax gel develops, a certain level of pressure application upstream is necessary to overcome the yield stress of the gel along the pipeline for † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. Fax: (801) 585-9291. E-mail: [email protected]. (1) Pedersen, K. S.; Skovborg, P.; Rønningsen, H. P. Wax precipitation from North Sea crude oils. 4. Thermodynamic modeling. Energy Fuels 1991, 5, 924–932. (2) Coutinho, J. A. P.; Daridon, J.-L. The limitations of the cloud point measurement techniques and the influence of the oil comparison on its dectection. Pet. Sci. Technol. 2005, 23, 1113–1128. (3) American Society for Testing and Materials (ASTM). Petroleum products, lubrications. Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 1999; section 5.

restart.4 The pressure requirement for restart has been predicted on the basis of various rheological studies of gelled waxy oils.5-12 Boger and co-workers11-14 discussed the existence of three definite characteristic responses, which they categorized as elastic, creep, and fracture when the gel was subjected to shear. In the elastic region, the gel strength is fully recovered after a low shear is applied to the gel regardless of the time duration over which the shear is applied. Prior to the static yield stress (the point at which the gel fractures), a creep region is observed, in which the gel strength is partially recovered once the applied stress is released. The gel breakage occurs in the (4) Golczynski, T. S.; Kempton, E. C. Understanding wax problems leads to deepwater flow assurance solutions. World Oil 2006, 227 (3), D7–D10. (5) Davenport, T. C.; Somper, R. S. H. The yield value and breakdown of crude oil gels. J. Inst. Pet. 1971, 57 (554), 86–105. (6) Rønningsen, H. P. Rheological behaviour of gelled, waxy North Sea crude oils. J. Pet. Sci. Eng. 1992, 7, 177–213. (7) Lopes da Silva, J. A.; Coutinho, J. A. P. Dynamic rheological analysis of the gelation behavior of waxy crude oils. Rheol. Acta 2004, 43, 433– 441. (8) Singh, P.; Fogler, H. S.; Nagarajan, N. Prediction of the wax content of the incipient wax-oil gel in a pipeline: An application of the controlledstress rheometer. J. Rheol. 1999, 43, 1437–1459. (9) Venkatesan, R.; Nagarajan, N. R.; Paso, K.; Yi, Y.-B.; Sastry, A. M.; Fogler, H. S. The strength of paraffin gels formed under static and flow conditions. Chem. Eng. Sci. 2005, 60, 3587–3598. (10) Oh, K.; Magda, J.; Deo, M. D. Yield and strength recovery of wax gels. The 8th International Conference on Petroleum Phase Behavior and Fouling, Pau, France, June 10-14, 2007. (11) Chang, C.; Boger, D. V.; Nguyen, Q. D. The yielding of waxy crude oils. Ind. Eng. Chem. Res. 1998, 37, 1551–1559. (12) Wardhaugh, L. T.; Boger, D. V. The measurement and description of the yielding behavior of waxy crude oil. J. Rheol. 1991, 35 (6), 1121– 1156. (13) Nguyen, Q. D.; Boger, D. V. Yield stress measurement for concentrated suspensions. J. Rheol. 1983, 27 (4), 321–349. (14) Nguyen, Q. D.; Boger, D. V. Direct yield stress measurement with the vane method. J. Rheol. 1985, 29 (3), 335–347.

10.1021/ef8006307 CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

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fracture region when the loading stress is high enough to disrupt the gel network. The yield stress varies with temperature, stress duration time, ramping rate of shear, and combinations of all of the above factors. Singh et al.8 pointed out that various factors, such as wax/oil ratio (wax amounts), molecular weight of the wax, cooling rate, and mechanical shear history, affect wax precipitation and deposition characteristics, including gelation temperature. In particular, they reported a depression in the gelation temperature, upon application of shear while cooling, with a greater reduction at higher shear rates. They also compared the effect of cooling rates (ranging from 0.8 to 0.08 °C/min) on the gelation temperatures, showing that slower cooling rates resulted in greater reductions in yield stresses. Venkatesan et al.9 examined the yield stresses with different cooling rates under both quiescent and shear conditions. The highest yield value was observed under quiescent conditions with the slowest cooling rate. Upon application of shear, the yield stress increased, with the highest value observed at the highest cooling rate. Both studies8,9 used the gelation temperature as a flow transition characteristic value instead of the pour-point measurement. The gelation temperature is usually between the WAT and the pour point.10 A lower level of cross-linking (than that required for the detection of the pour point) is often adequate for gelation temperature measurement in rheological studies. It has been shown that wax gel formation is altered in the presence of wax inhibitors15 or asphaltenes.16,17 The influence of the presence of asphaltenes has been examined in determining the WAT, the gelation temperature, and yield stress. Kriz and Andersen16 reported that a dramatic increase in both WAT and yield stress at 0.01% (w/w) asphaltene concentrations followed by a rapid decrease at 0.02% (w/w). They explained this through an asphaltene aggregation concept above a certain critical concentration. Studies of the critical concentration of asphaltenes have been published in various other papers.18-20 The reduction of both gelation temperature and yield stress was also observed at increasing asphaltene concentrations.17 Yield stress is an important property of the oil from restart considerations. It is important to understand the impact of the composition of the oil (wax content, wax type, etc.) on WAT, pour point, and yield stress. Because asphaltenes may coprecipitate with waxes, the consequences of the presence of asphaltenes on properties of importance in managing flow of waxy oils also needs to be established. It is useful to look at yield stresses at temperatures in relation to WAT and pour point because these are two important characteristic mixture temperatures. In this work, we focus on the measurement of yield stresses of carefully formulated model oils with the above considerations. It should be noted that most of the previous rheological data7-9,15-18 were obtained from rheometers with (15) Pedersen, K. S.; Rønningsen, H. P. Influence of wax inhibitors on wax appearance temperature, pour point, and viscosity of waxy crude oils. Energy Fuels 2003, 17, 321–328. (16) Kriz, P.; Andersen, S. I. Effect of asphaltenes on crude oil wax crystallization. Energy Fuels 2005, 19, 948–953. ¨ stlund, J.-A.; Chawla, H.; Wattana, P.; Nyde´n, (17) Venkatesan, R.; O M.; Fogler, H. S. The effect of asphaltenes on the gelation of waxy oils. Energy Fuels 2003, 17, 1630–1640. (18) Groenzin, H.; Mullins, O. C. Molecular size of structure of asphaltenes from various sources. Energy Fuels 2000, 14, 677–684. (19) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Ferna´ndez, A.; Pe´rez, P.; Caetano, M. Thermo-optical studies of asphaltene solutions: Evidence of solvent-solute aggregation formation. Fuel 1999, 78, 997– 1003. (20) Oh, K.; Ring, T. A.; Deo, M. D. Asphaltene aggregation in organic solvents. J. Colloid Interface Sci. 2004, 271, 212–219.

Oh and Deo Table 1. Elemental Analysis and Molecular Weight of Asphaltene Used in This Study analyses

C (%)

H (%)

N (%)

S (%)

H/C

Mna

ASP (field asphaltenes)

87.13

7.33

0.79

2.81

1.01

930

a

Mn ) number average molecular weight.

Figure 1. Carbon number distributions of W1 and W2 determined by SIMDIS. Table 2. List of the Model Oils Used in This Studya wax ASP TOL wax (wt %) K (v/v) (wt %) (wt %)

model oil 10% W1-K-MO1 20% W1-K-MO1 3% W2-K-MO2 5% W2-K-MO2 5% W2-TOL-MO1 5% W2-ASP0.01-TOL-MO1 5% W2-ASP0.1-TOL-MO1

W1 W1 W2 W2 W2 W2 W2

10 20 3 5 5 5 5

3:1 3:1 3:1 3:1 0.01 0.1

10 9.99 9.9

a K, ASP, TOL, and MO represent kerosene, field asphaltene, toluene, and mineral oil, respectively.

the cone-and-plate geometry. The measurements in this paper were performed using a rheometer with a vane fixture. Experimental Section Model Oil. Various model oils were prepared by using different combination of waxes (W1 and W2), different white mineral oils (MO1 and MO2), deordorized kerosene (K), toluene (TOL), and field asphaltenes (ASP). The asphaltenes were from the Rangely field,20 in northwestern Colorado. These had been deposited in production tubing. Elemental analysis and molecular weight of these asphaltenes are presented in Table 1. The compositions of waxes (W1 and W2) were measured using hightemperature gas chromatography. The carbon number distributions of W1 and W2 are shown in Figure 1. The model oils were prepared by using the procedure introduced in prior papers.8-10 In the case of asphaltene addition, toluene was used as a solvent before mixing with white mineral oil. Detailed compositions are presented in Table 2. Measurement of WAT and Pour Point. Modified ASTM methods were used to determine WAT and pour point using a temperature-controlled bath and a cell-type jacket for coolant circulation. The test cell used had the same size range (33.2-34.8 mm outer diameter and 115-125 mm height) as described in the ASTM methods (D2500 and D97). The bath temperature was set to 0 °C for all model oils, except for the 10% W1-K-MO1, to determine both the WAT and the pour point. In the case of 10% W1-K-MO1, the bath temperature was set to 0 °C initially to

Wax Gel Formation in Asphaltenes

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Figure 2. Yield stresses of model oil (20% W1-K-MO1) at two different aging times as measured by the vane method.

measure WAT and then reset to -5 °C for the pour point. The measurement was carried out at 3 °C intervals at first and then in narrower temperature intervals as the measurements became closer to the target. WAT was determined by the appearance of cloudiness visually at the bottom of the jar that held the oil sample. It is much easier to determine WAT in model oils than in dark crude oils. The pour point was determined by observing no flow when the jar was tilted for 3 s and held horizontal for 5 s. The pour point was defined as the temperature at 1 °C higher than the temperature of no-flow observation. WAT and pour-point measurements were repeated 3 times to ensure reproducibility. Yield Stress Measurements. Brookfield RVDV-II+ was used to measure the yield stress using the vane method at different temperatures. Built-in maximum torque of RVDV-II+ is 0.7187 mN m. The bath temperature was set at 45 °C before cooling the model oil to the designated temperature. The cooling rate was fixed at 0.8 °C/min to the designated temperature, and the torque reading was recorded after aging the gelled oil for 1 h duration at each temperature interval. Dimensions of the four-bladed vane spindle were 8.026 mm (0.312 in.) in diameter and 16.053 mm (0.632 in.) in length, with the ratio of length/diameter of 2. The inner diameter of the jacketed cylinder was 33 mm, for preventing the slip of gelled oil during vane rotation. The oil level placed in the vane cell was about 60 mm. The actual temperature of the vane trajectory in the gel was measured by an external thermocouple. The temperature variation between the core of the gel and the outside was less than 0.3 °C.

Results and Discussion The yield stresses at different temperatures below the pour point were measured using the vane rheometer. The results are organized as follows: (1) effect of aging on yield stresses, (2) dependence of yield stresses on wax amounts and compositions, and (3) yield behavior in the presence of asphaltenes. Nguyen and Boger13,14 derived a correlation using the vane method between the maximum torque reading obtained from the rheometer and the yield stress, which is shown in eq 1. -1

[ π2 d ( Hd + 31 )]

τy ) Tmax

3

(1)

Here, H and d represent the length (or height) of the vane blade and diameter of the vane rotation, respectively. Tmax represents the maximum torque reading. On the basis of the maximum torque rating of the viscometer, the maximum yield stress that could be measured with this arrangement was 380 Pa. Yield Behavior of Gelled Waxy Oils. Figure 2 shows the torque readings obtained at the vane rotation speed of 0.3 rpm

Figure 3. Yield stresses of model oils. A linear increase in yield stress was observed for all four model oils, with the slope dependent upon the amount and type of wax in the oil.

(the minimum). The measured torque readings are presented as a function of the vane rotation angle. The torque values were determined after 2, 12, and 16 h aging of the 20% W1-K-MO1 gel at 6.7 ( 0.3 °C. The aging time clock began after the water bath temperature reached the set point. The aging effect on gelation has been reported previously by Lopez da Silva and Coutinho.7 Their study focused on the relationship between the gelation time and gelation temperature. Elastic and viscous moduli were measured in the temperature range of 40-52 °C with two different oils. It was observed that the gelation time was becoming shorter at colder temperatures. However, the aging effect has not been fully examined with gels developed much below the pour point. In this work, the yield stress values increased about 7% for 2 h of aging (253 Pa) to 16 h of aging (271 Pa). The effect of aging from 2 to 16 h was not significant as shown in Figure 2. Only an increase of 7% (18 Pa) was observed for the 20% W1-K-MO1 sample. Yield stress values for the four different model oils were determined as functions of temperature and are plotted in Figure 3. A linear increase in yield stress was observed in all model oils as the temperature decreased in the temperature range studied in this paper. The yield stress of the gel is a meaningful property only at or below the pour point. It is observed from Figure 3 that the slope is characteristic of the wax amount and wax composition. For model oils with W1, the yield stress of 20% W1-K-MO1 is much higher than that of 10% W1-K-MO1 at the same temperature (For instance, 564 Pa for 20% W1-K-MO1 and 42 Pa for 10% W1-K-MO1 calculated from the regression at 2 °C shown in Figure 3). The slope of the temperature versus yield stress line is steeper for 20% W1-K-MO1 than for 10% W1-K-MO1. Similarly, for model oils with W2, 5% W2-K-MO2 has higher yield values and steeper yield stress relationships at the temperature ranges measured than those of 3% W2-K-MO2. The slopes in Figure 3 are 61.7, 30.3, 41.6, and 18.7 Pa/°C for 20% W1-K-MO1, 10% W1-K-MO1, 5% W2-K-MO2, and 3% W2-K-MO2, respectively. Wax W2 is comprised of higher molecular-weight compounds, in general, than W1. The yield stress values of 5% W2-K-MO2 were higher than the yield values of 20% W1-K-MO1 in this temperature range. However, the gel strength increased more rapidly in 20% W1-K-MO1 as the temperature decreased. The model oil with a higher content of higher carbon number wax shows higher yield stress values and rapid gel strength buildup at temperatures below the pour point. Because the temperature effect on yield stress is pronounced,

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Table 3. WAT, Observation Temperature of No Flow, and Pour Point Data of Model Oils model oil

WAT (°C)

no flow (°C)

pour point (°C)

x intercept from Figure 3 (°C)

10% W1-K-MO1 20% W1-K-MO1 3% W2-K-MO2 5% W2-K-MO2

17 25 25 30

4 13 10 18

5 14 11 19

3.4 11.1 10.7 16.8

it can be concluded that aging time has relatively little impact on gel yield strength below the pour point. The x-intercept value from the yield stress versus the temperature plot provides an alternative method of determining the pour point. WAT, observation of no flow, pour point, and the x-intercept values obtained by extrapolation of the yield stress versus the temperature line from Figure 3 are summarized in Table 3. The x-intercept values are close to the temperature of no-flow observation during pour-point measurements but always below the temperature measured by the ASTM method. In the ASTM method, a series of bath temperatures ranging from 0 °C are prescribed as bath temperatures. As a result, samples with higher pour points in general (of the order of 20 °C) may undergo supercooling before the no-flow point is reached. This may result in a slightly higher value of the pour point. The alternative method described in the paper is one more way of ascertaining the no-flow condition and may be particularly suitable for oils with higher wax content. Yield Behavior in the Presence of Asphaltenes. Venkatesan et al.17 examined the effect of the presence of asphaltenes on the yield stress at different temperatures. They presented the yield stress values with respect to the wax percent at different temperatures. Significant reductions in yield stress values were reported in the waxy oil containing asphaltenes. They observed a linear increase in yield stress in both asphaltene-free and asphaltene-containing samples (0.05%, w/w and 0.1%, w/w) as temperatures decreased. In this study, we examined the yield stress values below the pour point in the presence of asphaltenes at low concentrations (0.01%, w/w and 0.1%, w/w). The model oils with asphaltenes studied in this paper are presented in Table 2. Toluene was used as a solvent for asphaltenes. The yield stresses (as functions of the temperature) of 5% W2-TOL-MO1 (asphaltene-free), 5% W2-ASP0.01-TOL-MO1, and 5% W2-ASP0.1-TOL-MO1 are shown in Figure 4. When asphaltenes were added, yield stress values were lower and the rate of increase of yield stress with temperature decrease was lower. However, as the temper-

Figure 4. Yield behavior of model oils in the presence of asphaltenes compared to the model oils without asphaltenes.

Table 4. WAT, Observation Temperature of No Flow, and Pour Point Data of Asphaltene-Added Model Oils

model oil

WAT (°C)

no flow (°C)

pour point (°C)

x intercept from Figure 4 (°C)

5% W2-MO1 5% W2-ASP0.01-TOL-MO1 5% W2-ASP0.1-TOL-MO1

33 34 35

20 19 16

21 20 17

18.8 16.9 12.6

atures decreased further, the slope of the yield stress versus temperature line resorted to the original slope (with no asphaltene), indicating the dominance of wax structures as more wax came out of solution. Pedersen and Rønningsen15 reported that the wax inhibitor acts as a steric hindrance agent in the wax network structure. Venkatesan et al.17 also interpreted the reduction in yield stress because of asphaltene addition to a possible hindrance on wax gel network. Observations that are not entirely consistent with those in Figure 4 have also been reported previously. Kriz and Andersen16 observed the highest WAT and yield stress at the lowest concentration of asphaltenes in their experiments (0.01%, w/w). They showed a clear demarcation between yield stress values in oils with asphaltene concentrations below and above the “critical” concentrations. A significant decrease in oil yield stress was observed above a certain asphaltene concentration. They interpreted this to be the concentration necessary for flocculation of asphaltenes, at which point asphaltenes start hindering the formation of gel networks rather than providing sites to generate wax crystallites, resulting in decreases of WAT and yield stress. Below the “critical” concentration, they theorized that asphaltenes may even act as nucleation sites for wax crystallization, hastening precipitation and providing higher yield stresses. Because shear was applied during cooling in all of their experiments, their yield stress values were far lower than those seen in this study (of the order of 10 Pa). In this study, we found that the yield stresses decreased by the addition of asphaltenes, even at very low concentrations of 0.01 wt %. Table 4 shows WAT, no-flow point, and pour point data of 5% W2-TOL-MO1, 5% W2-ASP0.01-TOL-MO1, and 5% W2-ASP0.1-TOL-MO1. Asphaltene additions resulted in pour-point reductions, of up to 4 °C for additions of asphaltenes up to 0.1 wt %. Small amounts of asphaltenes (0.01 wt %) also resulted in significant reductions of the yield stress. Asphaltenes hinder the formation of continuous, consistent gel networks, causing reduction in yield stress. The magnitude of yield stress reduction at 0.01 wt % compared to 0.1 wt % indicates that asphaltene aggregation introduces the hindrance in gel formation by reducing the wax generation sites, which is consistent with the observations of Kriz and Andersen.16 The slopes of the yield stress versus temperature lines went back to the original slopes of the asphaltene-free oils at lower temperatures. In Table 4, the x-intercept values in model oils with asphaltenes are obtained by the extrapolation of three data points near zero yields. The difference between the no-flow temperatures and x-intercept values are larger in both samples with asphaltenes. This may be caused by the fact that the oil at temperatures between the pour point and x intercept has negligible yield stress values but enough gel strength to prevent the flow. This is implicit evidence that asphaltenes intervene in the wax network during the initial network formation, which results in gel weakness near the pour point. This study also provides insight concerning the yield behavior of complex mixtures. The yield stress of an oil may be higher than predicted in the presence of asphaltene at temperatures much below its pour point, even though asphalt-

Wax Gel Formation in Asphaltenes

enes may depress the pour point and yield stress in the vicinity of the pour point. Conclusions Gel strength development at temperatures below the pour point was examined by measuring the yield stresses of a variety of model oils using a vane rheometer. For the model oils without asphaltenes, the yield stress increased linearly as the temperature decreased. The extent of increase in yield stress values was greater for model oils that had a higher percentage of wax. A steeper increase in yield stress values with a decrease in temperature was observed in model oils with a higher wax content and with the wax containing a higher carbon number distribution. Second, the role of asphaltenes in the evolution of

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the yield stress was explored. With asphaltene addition, yield stress values were reduced and the rate of increase of the yield stress with temperature decrease was lower. However, as the temperatures decreased further, the slope of the yield stress versus temperature resorted to the original slope (asphaltenefree model oil), indicating the dominance of wax structures as more wax came out of solution. The x-intercept values obtained from yield stress versus temperature were interpreted as noflow points, which could be used as alternative measures of pour points. However, the deviation between the no-flow point and x-intercept values was larger in asphaltene-added model oils. EF8006307