Characteristics of Wax Gel Formation in the Presence ofAsphaltenes†

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 yieldstrength of well-characterized waxy model oils at temperatures below the pour point. First, the yield stressesof model oils were determined by the vane method at different temperatures. Yield stress values were stronglydependent upon wax amounts and compositions, as expected. The extent of increase in yield stress valueswith temperature was greater for model oils that had a higher percentage of wax. The x-intercept values obtainedfrom yield stress versus temperature were interpreted as no-flow points, which could be used as alternativemeasures of pour points. Second, the role of asphaltenes was examined in the evolution of the yield stress asthe oil is cooled below the pour point. Asphaltene additions resulted in pour-point reductions, of up to 4 °Cfor additions of asphaltenes up to 0.1% (w/w). Small amounts of asphaltenes (0.01%, w/w) also played asignificant role in yield stress reduction. The concept of steric hindrance and asphaltene aggregation was adaptedto explain the yield stress reduction at the different asphaltene concentrations. At lower temperatures, as morewax came out of solution, the slope of the yield stress versus temperature line went back to the slope of theasphaltene-free oil, indicating the dominance of the wax networks at higher wax concentrations.

Introduction

High-molecular-weight paraffinic waxes in a crude oil startto precipitate when the surrounding temperature is lower thanthe wax appearance temperature (WAT). The terms, cloud pointand wax precipitation temperature, are often used interchange-ably. Dependent upon the method used, WAT measurementscan be significantly different. Pedersen et al.1 determined WATswith three different measurement methods: microscopy, viscos-ity, and differential scanning calorimetry, showing that typicallyhighest WAT values were observed when using the microscopicmeasurement. The comparison of WAT values and the detectionlimits with the different methods has also been reportedelsewhere.2 The pour point on the other hand is anotherimportant characteristic temperature that is usually determinedby American Society for Testing and Materials (ASTM) D97.3

Flow discontinuity can occur either with wax deposition or waxgel formation. While the wax deposition can be initiated duringflow, wax gel formation occurs under static conditions causedby shutdown. Subsequent to shutdown, if the wax gel develops,a certain level of pressure application upstream is necessary toovercome the yield stress of the gel along the pipeline for

restart.4 The pressure requirement for restart has been predictedon the basis of various rheological studies of gelled waxyoils.5-12 Boger and co-workers11-14 discussed the existence ofthree definite characteristic responses, which they categorizedas elastic, creep, and fracture when the gel was subjected toshear. In the elastic region, the gel strength is fully recoveredafter a low shear is applied to the gel regardless of the timeduration over which the shear is applied. Prior to the static yieldstress (the point at which the gel fractures), a creep region isobserved, in which the gel strength is partially recovered oncethe applied stress is released. The gel breakage occurs in the

† Presented at the 9th International Conference on Petroleum PhaseBehavior and Fouling.

* To whom correspondence should be addressed. Fax: (801) 585-9291.E-mail: asphaltene@yahoo.com.

(1) Pedersen, K. S.; Skovborg, P.; Rønningsen, H. P. Wax precipitationfrom 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 pointmeasurement techniques and the influence of the oil comparison on itsdectection. Pet. Sci. Technol. 2005, 23, 1113–1128.

(3) American Society for Testing and Materials (ASTM). Petroleumproducts, lubrications. Annual Book of ASTM Standards; ASTM: WestConshohocken, PA, 1999; section 5.

(4) Golczynski, T. S.; Kempton, E. C. Understanding wax problems leadsto deepwater flow assurance solutions. World Oil 2006, 227 (3), D7–D10.

(5) Davenport, T. C.; Somper, R. S. H. The yield value and breakdownof crude oil gels. J. Inst. Pet. 1971, 57 (554), 86–105.

(6) Rønningsen, H. P. Rheological behaviour of gelled, waxy North Seacrude oils. J. Pet. Sci. Eng. 1992, 7, 177–213.

(7) Lopes da Silva, J. A.; Coutinho, J. A. P. Dynamic rheological analysisof 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 contentof the incipient wax-oil gel in a pipeline: An application of the controlled-stress 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 flowconditions. Chem. Eng. Sci. 2005, 60, 3587–3598.

(10) Oh, K.; Magda, J.; Deo, M. D. Yield and strength recovery of waxgels. The 8th International Conference on Petroleum Phase Behavior andFouling, Pau, France, June 10-14, 2007.

(11) Chang, C.; Boger, D. V.; Nguyen, Q. D. The yielding of waxycrude oils. Ind. Eng. Chem. Res. 1998, 37, 1551–1559.

(12) Wardhaugh, L. T.; Boger, D. V. The measurement and descriptionof the yielding behavior of waxy crude oil. J. Rheol. 1991, 35 (6), 1121–1156.

(13) Nguyen, Q. D.; Boger, D. V. Yield stress measurement forconcentrated suspensions. J. Rheol. 1983, 27 (4), 321–349.

(14) Nguyen, Q. D.; Boger, D. V. Direct yield stress measurement withthe vane method. J. Rheol. 1985, 29 (3), 335–347.

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10.1021/ef8006307 CCC: $40.75 2009 American Chemical SocietyPublished on Web 12/29/2008

fracture region when the loading stress is high enough to disruptthe gel network.

The yield stress varies with temperature, stress duration time,ramping rate of shear, and combinations of all of the abovefactors. Singh et al.8 pointed out that various factors, such aswax/oil ratio (wax amounts), molecular weight of the wax,cooling rate, and mechanical shear history, affect wax precipita-tion and deposition characteristics, including gelation temper-ature. In particular, they reported a depression in the gelationtemperature, upon application of shear while cooling, with agreater reduction at higher shear rates. They also compared theeffect of cooling rates (ranging from 0.8 to 0.08 °C/min) onthe gelation temperatures, showing that slower cooling ratesresulted in greater reductions in yield stresses. Venkatesan etal.9 examined the yield stresses with different cooling rates underboth quiescent and shear conditions. The highest yield valuewas observed under quiescent conditions with the slowestcooling rate. Upon application of shear, the yield stressincreased, with the highest value observed at the highest coolingrate. Both studies8,9 used the gelation temperature as a flowtransition characteristic value instead of the pour-point measure-ment. The gelation temperature is usually between the WATand the pour point.10 A lower level of cross-linking (than thatrequired for the detection of the pour point) is often adequatefor gelation temperature measurement in rheological studies.

It has been shown that wax gel formation is altered in thepresence of wax inhibitors15 or asphaltenes.16,17 The influenceof the presence of asphaltenes has been examined in determiningthe WAT, the gelation temperature, and yield stress. Kriz andAndersen16 reported that a dramatic increase in both WAT andyield stress at 0.01% (w/w) asphaltene concentrations followedby a rapid decrease at 0.02% (w/w). They explained this throughan asphaltene aggregation concept above a certain criticalconcentration. Studies of the critical concentration of asphalteneshave been published in various other papers.18-20 The reductionof both gelation temperature and yield stress was also observedat increasing asphaltene concentrations.17

Yield stress is an important property of the oil from restartconsiderations. It is important to understand the impact of thecomposition of the oil (wax content, wax type, etc.) on WAT,pour point, and yield stress. Because asphaltenes may co-precipitate with waxes, the consequences of the presence ofasphaltenes on properties of importance in managing flow ofwaxy oils also needs to be established. It is useful to look atyield stresses at temperatures in relation to WAT and pour pointbecause these are two important characteristic mixture temper-atures. In this work, we focus on the measurement of yieldstresses of carefully formulated model oils with the aboveconsiderations. It should be noted that most of the previousrheological data7-9,15-18 were obtained from rheometers with

the cone-and-plate geometry. The measurements in this paperwere performed using a rheometer with a vane fixture.

Experimental Section

Model Oil. Various model oils were prepared by usingdifferent combination of waxes (W1 and W2), different whitemineral oils (MO1 and MO2), deordorized kerosene (K), toluene(TOL), and field asphaltenes (ASP). The asphaltenes were fromthe Rangely field,20 in northwestern Colorado. These had beendeposited in production tubing. Elemental analysis and molecularweight of these asphaltenes are presented in Table 1. Thecompositions of waxes (W1 and W2) were measured using high-temperature gas chromatography. The carbon number distribu-tions of W1 and W2 are shown in Figure 1. The model oilswere prepared by using the procedure introduced in priorpapers.8-10 In the case of asphaltene addition, toluene was used asa solvent before mixing with white mineral oil. Detailed composi-tions are presented in Table 2.

Measurement of WAT and Pour Point. Modified ASTMmethods were used to determine WAT and pour point using atemperature-controlled bath and a cell-type jacket for coolantcirculation. The test cell used had the same size range (33.2-34.8mm outer diameter and 115-125 mm height) as described in theASTM methods (D2500 and D97). The bath temperature was setto 0 °C for all model oils, except for the 10% W1-K-MO1, todetermine 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

(15) Pedersen, K. S.; Rønningsen, H. P. Influence of wax inhibitors onwax 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 waxcrystallization. Energy Fuels 2005, 19, 948–953.

(17) Venkatesan, R.; Ostlund, J.-A.; Chawla, H.; Wattana, P.; Nyden,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 ofasphaltenes from various sources. Energy Fuels 2000, 14, 677–684.

(19) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernandez,A.; Perez, 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 organicsolvents. J. Colloid Interface Sci. 2004, 271, 212–219.

Table 1. Elemental Analysis and Molecular Weight ofAsphaltene 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 bySIMDIS.

Table 2. List of the Model Oils Used in This Studya

model oil waxwax

(wt %) K (v/v)ASP

(wt %)TOL

(wt %)

10% W1-K-MO1 W1 10 3:120% W1-K-MO1 W1 20 3:13% W2-K-MO2 W2 3 3:15% W2-K-MO2 W2 5 3:15% W2-TOL-MO1 W2 5 105% W2-ASP0.01-TOL-MO1 W2 5 0.01 9.995% W2-ASP0.1-TOL-MO1 W2 5 0.1 9.9

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

1290 Energy & Fuels, Vol. 23, 2009 Oh and Deo

measure WAT and then reset to -5 °C for the pour point. Themeasurement was carried out at 3 °C intervals at first and then innarrower temperature intervals as the measurements became closerto the target. WAT was determined by the appearance of cloudinessvisually at the bottom of the jar that held the oil sample. It is mucheasier to determine WAT in model oils than in dark crude oils.The pour point was determined by observing no flow when the jarwas tilted for 3 s and held horizontal for 5 s. The pour point wasdefined as the temperature at 1 °C higher than the temperature ofno-flow observation. WAT and pour-point measurements wererepeated 3 times to ensure reproducibility.

Yield Stress Measurements. Brookfield RVDV-II+ was usedto measure the yield stress using the vane method at differenttemperatures. Built-in maximum torque of RVDV-II+ is 0.7187mN m. The bath temperature was set at 45 °C before cooling themodel oil to the designated temperature. The cooling rate was fixedat 0.8 °C/min to the designated temperature, and the torque readingwas recorded after aging the gelled oil for 1 h duration at eachtemperature interval. Dimensions of the four-bladed vane spindlewere 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 diameterof the jacketed cylinder was 33 mm, for preventing the slip of gelledoil during vane rotation. The oil level placed in the vane cell wasabout 60 mm. The actual temperature of the vane trajectory in thegel was measured by an external thermocouple. The temperaturevariation between the core of the gel and the outside was less than0.3 °C.

Results and Discussion

The yield stresses at different temperatures below the pourpoint were measured using the vane rheometer. The results areorganized 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 vanemethod between the maximum torque reading obtained fromthe rheometer and the yield stress, which is shown in eq 1.

τy ) Tmax[π2

d3(Hd+ 1

3)]-1(1)

Here, H and d represent the length (or height) of the vane bladeand diameter of the vane rotation, respectively. Tmax representsthe maximum torque reading. On the basis of the maximumtorque rating of the viscometer, the maximum yield stress thatcould be measured with this arrangement was 380 Pa.

Yield Behavior of Gelled Waxy Oils. Figure 2 shows thetorque readings obtained at the vane rotation speed of 0.3 rpm

(the minimum). The measured torque readings are presented asa function of the vane rotation angle. The torque values weredetermined after 2, 12, and 16 h aging of the 20% W1-K-MO1gel at 6.7 ( 0.3 °C. The aging time clock began after the waterbath temperature reached the set point. The aging effect ongelation has been reported previously by Lopez da Silva andCoutinho.7 Their study focused on the relationship between thegelation time and gelation temperature. Elastic and viscousmoduli were measured in the temperature range of 40-52 °Cwith two different oils. It was observed that the gelation timewas becoming shorter at colder temperatures. However, theaging effect has not been fully examined with gels developedmuch below the pour point. In this work, the yield stress valuesincreased 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 significantas shown in Figure 2. Only an increase of 7% (18 Pa) wasobserved for the 20% W1-K-MO1 sample.

Yield stress values for the four different model oils weredetermined as functions of temperature and are plotted in Figure3. A linear increase in yield stress was observed in all modeloils as the temperature decreased in the temperature rangestudied in this paper. The yield stress of the gel is a meaningfulproperty only at or below the pour point. It is observed fromFigure 3 that the slope is characteristic of the wax amount andwax composition. For model oils with W1, the yield stress of20%W1-K-MO1ismuchhigher thanthatof10%W1-K-MO1at the same temperature (For instance, 564 Pa for 20%W1-K-MO1 and 42 Pa for 10% W1-K-MO1 calculatedfrom the regression at 2 °C shown in Figure 3). The slope ofthe temperature versus yield stress line is steeper for 20%W1-K-MO1 than for 10% W1-K-MO1. Similarly, formodel oils with W2, 5% W2-K-MO2 has higher yield valuesand steeper yield stress relationships at the temperature rangesmeasured than those of 3% W2-K-MO2. The slopes in Figure3 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-weightcompounds, 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 gelstrength increased more rapidly in 20% W1-K-MO1 as thetemperature decreased. The model oil with a higher content ofhigher carbon number wax shows higher yield stress values andrapid gel strength buildup at temperatures below the pour point.Because the temperature effect on yield stress is pronounced,

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

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

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it can be concluded that aging time has relatively little impacton gel yield strength below the pour point.

The x-intercept value from the yield stress versus thetemperature plot provides an alternative method of determiningthe pour point. WAT, observation of no flow, pour point, andthe x-intercept values obtained by extrapolation of the yieldstress versus the temperature line from Figure 3 are summarizedin Table 3. The x-intercept values are close to the temperatureof no-flow observation during pour-point measurements butalways below the temperature measured by the ASTM method.In the ASTM method, a series of bath temperatures rangingfrom 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 isreached. This may result in a slightly higher value of the pourpoint. The alternative method described in the paper is one moreway of ascertaining the no-flow condition and may be particu-larly suitable for oils with higher wax content.

Yield Behavior in the Presence of Asphaltenes. Venkatesanet al.17 examined the effect of the presence of asphaltenes onthe yield stress at different temperatures. They presented theyield stress values with respect to the wax percent at differenttemperatures. Significant reductions in yield stress values werereported in the waxy oil containing asphaltenes. They observeda linear increase in yield stress in both asphaltene-free andasphaltene-containing samples (0.05%, w/w and 0.1%, w/w) astemperatures decreased.

In this study, we examined the yield stress values below thepour point in the presence of asphaltenes at low concentrations(0.01%, w/w and 0.1%, w/w). The model oils with asphaltenesstudied in this paper are presented in Table 2. Toluene was usedas a solvent for asphaltenes. The yield stresses (as functions ofthe temperature) of 5% W2-TOL-MO1 (asphaltene-free), 5%W2-ASP0.01-TOL-MO1, and 5% W2-ASP0.1-TOL-MO1are shown in Figure 4. When asphaltenes were added, yieldstress values were lower and the rate of increase of yield stresswith temperature decrease was lower. However, as the temper-

atures decreased further, the slope of the yield stress versustemperature line resorted to the original slope (with no asphalt-ene), indicating the dominance of wax structures as more waxcame out of solution. Pedersen and Rønningsen15 reported thatthe wax inhibitor acts as a steric hindrance agent in the waxnetwork structure. Venkatesan et al.17 also interpreted thereduction in yield stress because of asphaltene addition to apossible hindrance on wax gel network. Observations that arenot entirely consistent with those in Figure 4 have also beenreported previously. Kriz and Andersen16 observed the highestWAT and yield stress at the lowest concentration of asphaltenesin their experiments (0.01%, w/w). They showed a cleardemarcation between yield stress values in oils with asphalteneconcentrations below and above the “critical” concentrations.A significant decrease in oil yield stress was observed above acertain asphaltene concentration. They interpreted this to be theconcentration necessary for flocculation of asphaltenes, at whichpoint asphaltenes start hindering the formation of gel networksrather than providing sites to generate wax crystallites, resultingin decreases of WAT and yield stress. Below the “critical”concentration, they theorized that asphaltenes may even act asnucleation sites for wax crystallization, hastening precipitationand providing higher yield stresses. Because shear was appliedduring cooling in all of their experiments, their yield stressvalues were far lower than those seen in this study (of the orderof 10 Pa). In this study, we found that the yield stressesdecreased by the addition of asphaltenes, even at very lowconcentrations of 0.01 wt %. Table 4 shows WAT, no-flowpoint, 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 %. Smallamounts of asphaltenes (0.01 wt %) also resulted in significantreductions of the yield stress. Asphaltenes hinder the formationof continuous, consistent gel networks, causing reduction in yieldstress. The magnitude of yield stress reduction at 0.01 wt %compared to 0.1 wt % indicates that asphaltene aggregationintroduces the hindrance in gel formation by reducing the waxgeneration sites, which is consistent with the observations ofKriz and Andersen.16 The slopes of the yield stress versustemperature lines went back to the original slopes of theasphaltene-free oils at lower temperatures. In Table 4, thex-intercept values in model oils with asphaltenes are obtainedby the extrapolation of three data points near zero yields. Thedifference between the no-flow temperatures and x-interceptvalues are larger in both samples with asphaltenes. This maybe caused by the fact that the oil at temperatures between thepour point and x intercept has negligible yield stress values butenough gel strength to prevent the flow. This is implicit evidencethat asphaltenes intervene in the wax network during the initialnetwork formation, which results in gel weakness near the pourpoint. This study also provides insight concerning the yieldbehavior of complex mixtures. The yield stress of an oil maybe higher than predicted in the presence of asphaltene attemperatures much below its pour point, even though asphalt-

Table 3. WAT, Observation Temperature of No Flow, and PourPoint Data of Model Oils

modeloil

WAT(°C)

no flow(°C)

pour point(°C)

x intercept fromFigure 3 (°C)

10% W1-K-MO1 17 4 5 3.420% W1-K-MO1 25 13 14 11.13% W2-K-MO2 25 10 11 10.75% W2-K-MO2 30 18 19 16.8

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

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

model oilWAT(°C)

no flow(°C)

pourpoint(°C)

x interceptfrom Figure

4 (°C)

5% W2-MO1 33 20 21 18.85% W2-ASP0.01-TOL-MO1 34 19 20 16.95% W2-ASP0.1-TOL-MO1 35 16 17 12.6

1292 Energy & Fuels, Vol. 23, 2009 Oh and Deo

enes may depress the pour point and yield stress in the vicinityof the pour point.

Conclusions

Gel strength development at temperatures below the pourpoint was examined by measuring the yield stresses of a varietyof model oils using a vane rheometer. For the model oils withoutasphaltenes, the yield stress increased linearly as the temperaturedecreased. The extent of increase in yield stress values wasgreater for model oils that had a higher percentage of wax. Asteeper increase in yield stress values with a decrease intemperature was observed in model oils with a higher waxcontent and with the wax containing a higher carbon numberdistribution. Second, the role of asphaltenes in the evolution of

the yield stress was explored. With asphaltene addition, yieldstress values were reduced and the rate of increase of the yieldstress with temperature decrease was lower. However, as thetemperatures decreased further, the slope of the yield stressversus temperature resorted to the original slope (asphaltene-free model oil), indicating the dominance of wax structures asmore wax came out of solution. The x-intercept values obtainedfrom yield stress versus temperature were interpreted as no-flow points, which could be used as alternative measures ofpour points. However, the deviation between the no-flow pointand x-intercept values was larger in asphaltene-added modeloils.

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