Effects of Shear and Temperature on Wax Deposition - American

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Energy & Fuels 2005, 19, 1376-1386

Effects of Shear and Temperature on Wax Deposition: Coldfinger Investigation with a Gulf of Mexico Crude Oil† David W. Jennings* and Klaus Weispfennig Baker Petrolite, Oilfield Technology, 12645 W. Airport Blvd., Sugar Land, Texas 77478 Received August 30, 2004. Revised Manuscript Received April 12, 2005

The ability to determine the severity of wax deposition is an extremely important issue for the petroleum industry, particularly in the design and development of deepwater fields. Unfortunately, wax deposition is a complex process for which the mechanism is not fully understood. Furthermore, although much progress has been made in the last few decades in better understanding this complex process, the ability to accurately account for all the factors that affect deposition does not currently exist in the wax deposition simulators used in industry today. This paper examines the effects of two factors on the deposition process: shear and temperature. The effects are illustrated through results from coldfinger experiments, which are often used as a simple means to approximate the deposition process in flow lines. Shown are the effects in influencing both the amount of deposition occurring and nature of deposits formed from a medium-gravity Gulf of Mexico crude oil. Although the results do not directly represent flow-line deposition data, the data provide a relatively comprehensive set of illustrative examples on how deposition can vary with changing conditions. In the study, increases in shear have been observed to result in decreases in the amount of total deposition, primarily through a reduction in the amount of entrained crude oil contained in the deposits. The amount of wax in the deposits was determined to be relatively constant, within the range of variation in shear examined. However, the concentration of wax increased as the shear increased, because of the reduction in entrained crude oil. The effect of increasing the temperature differential (between bulk oil and cold surface) led to an expected increase in total deposition. Both the amount of wax and entrained crude oil increased. The concentration of entrained crude oil in the deposits was observed to increase at a greater rate. Hence, higher entrained oil concentrations occurred with larger temperature differentials.

Study Objectives The objectives of this study were 2-fold. The first objective was to provide more information toward improving the understanding of wax deposition. Although great strides have been made in the past few decades toward understanding and assessing wax deposition, wax deposition is a complex process for which the mechanism is still not fully understood, and the existing deposition simulators used in field design and development are viewed as not being accurate.1 In particular, the effects of various factors (such as shear and temperature differentials) that affect the characteristics of the wax deposits are not well-captured. The results from this study, although not flow-line deposition data, provide insight into the effects of shear and temperature on wax deposition. As such, the results may help in guiding development of more-appropriate † Presented at the 5th Internationlal Conference on Petroleum Phase Behavior and Fouling. * Author to whom correspondence should be addressed. [email protected]. (1) Labes-Carrier, C.; Ronningsen, H. P.; Kolnes, J.; Leporcher, E. Wax Deposition in North Sea Gas Condensate and Oil Systems: Comparison between Operational Experience and Model Prediction. Presented at 2002 SPE Annual Technical Conference and Exhibition, San Antonio, TX, September 29-October 2, 2002, SPE Paper No. SPE 77573.

fundamental models and/or improvement in existing models. The second objective was to gain a better understanding of the operation of our coldfinger apparatus. Coldfinger testing is used extensively for evaluating paraffin inhibitors for mitigating flow-line deposition. Coldfinger testing has also been used for empirically predicting flow-line deposition,2 although these applications are less common. Coldfingers provide a simple, yet reliable, method to simulate flow-line deposition, particularly for screening inhibitors. Equally important, only small sample sizes are required. Although the coldfingers provide a reasonable simulation of flow-line deposition, they certainly are not a direct representation of flow lines. With different geometries and flow fields, coldfingers can have different heat transfer and shear regimes than flow-line conditions. For some flow-line conditions, these regimes can be significantly different than in the coldfingers. Also, high-pressure multiphase testing (with gas phases) is not possible in most coldfinger devices. Hence, understanding all the various factors that affect coldfinger testing is important, particularly with respect to empiri(2) Weispfennig, K. Advancement in Paraffin Testing Methodology. Presented at the 2001 SPE International Symposium on Oilfield Chemistry, Houston, TX, February 13-16, 2001, SPE Paper No. SPE 64997.

10.1021/ef049784i CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005

Effects of Shear and Temperature on Wax Deposition

cally predicting flow-line deposition from coldfinger data. The discrepancy of having lower shear regimes in the coldfingers is viewed as not being a major concern in the selection of inhibitors for crude oils, because subsequent studies have indicated that inhibitor performance improves with increasing shear. Background Introduction Wax Problems in Petroleum Production. The petroleum industry has addressed wax problems almost since its inception. The main problems that wax can cause include deposition in flow lines or dramatic viscosity increases or complete “gelling” of crude oil. Whether an operation will experience wax problems is dependent on the specifics of the crude oil chemical composition and the production operation conditions. Many operations do not experience any wax problems whatsoever; yet, others have experienced problems ranging from severe production losses to complete plugging and abandonment of wells and flow lines. In some fields, millions of dollars have been lost because of wax problems. Excluding specific process designs or procedures for wax management, the thermal conditions are the single most influential process parameters in a production operation, in regard to whether wax deposition will occur. The solubility of higher-molecular-weight waxes becomes limited as low temperatures are encountered. For this reason, cold environments are especially prone to experiencing problems, because wax will precipitate and deposit from crude oils that do not necessarily have particularly high wax concentrations. Cold environments include deepwater subsea developments and arctic region operations. In deepwater operations, concerns are especially prevalent, because of the extreme high cost of the developments and remediation operations. Wax Deposition Mechanism. Wax deposition occurs in petroleum production when flow-line surface temperatures are below the crude oil wax appearance temperature (WAT) and a temperature differential exists between the crude oil and the colder deposition surface. The consensus in the scientific literature is that wax deposition occurs from a combination of molecular diffusion and shear dispersion mechanisms, with molecular diffusion being the predominant contribution.3-6 Shear dispersion is believed to be potentially important only when concentrations of precipitated wax in the bulk are high,3,4 which occurs in temperatures significantly below the WAT. There is even some evidence that (3) Bern, P. A.; Withers, V. R.; Cairns, R. J. R. Wax Deposition in Crude Oil Pipelines. Presented at European Offshore Petroleum Conference and Exhibition, London, England, October 21-24, 1980, Paper No. EUR 206. (4) Burger, E. D.; Perkins, T. K.; Striegler, J. H. Studies of Wax Deposition in the Trans Alaska Pipeline. SPE J. Pet. Technol. 1981, (June), 1075. (5) Brown, T. S.; Niesen, V. G.; Erickson, D. D. Measurement and Prediction of the Kinetics of Paraffin Deposition. Presented at the 68th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Houston, TX, October 3-6, 1993, SPE Paper No. SPE 26548. (6) Creek, J. L.; Matzain, B.; Apte, M.; Brill, J. P.; Volk, M.; Delle Case, E.; Lund, H. Mechanism for Wax Deposition. Presented at the Second International Conference on Petroleum Phase Behavior and Fouling, held in conjunction with the 1999 AIChE Spring National Meeting, March 15-18, 1999, Paper No. 53A.

Energy & Fuels, Vol. 19, No. 4, 2005 1377

deposition from shear dispersion might not occur without another mechanism (such as molecular diffusion), because experimental investigations performed at zero heat flux have yielded no deposition.4,5 The diffusion mechanism at the pipe surface is responsible for the transport of solute wax material. Existing heat-transfer gradients with temperatures below the solubility limits of wax in crude oil (i.e., WAT), in combination with the thermodynamic relationship between the temperature and the solute wax concentration, are believed to be of major importance in establishing a radial solute concentration gradient. Although wax precipitation within the main flow regime is not believed to participate significantly in the deposition process, surface roughness of the pipe wall provide ample nucleation sites for crystallization. Turbulence, as well as kinetic effects, are additional factors that may influence this concentration gradient. Other potential mechanisms (such as Brownian diffusion and gravity settling) are not believed to be significant.4,6 A review of wax deposition mechanisms is given by Matzain7 and discussed by Creek et al.6 for single-phase flow. Matzain and co-workers7,8 have discussed modeling wax deposition in multiphase flow. Simulators. To assess the potential severity of any given wax-related operational problem, several different avenues are pursued. Some people heavily rely on the predictive capabilities of wax deposition simulators. These deposition simulators are typically either an integral part or modular component to production system simulators that provide detailed thermal and hydraulic assessments of all associated production equipment. OLGA 2000, which is one of the state-ofthe-art production system simulators, offers a modular add-on wax deposition simulator. The University of Tulsa offers a multiphase paraffin deposition simulator to their Consortium members. Its internal calculation routines are heavily based on their own research efforts within this area. To calculate the thermodynamic driving force for deposition, most (if not all) of the available simulators make use of so-called thermodynamic lookup tables, which provide the necessary solubility data for solid-liquid-gas systems. Although these look-up tables are sometimes an integral part of the deposition simulator software, they often can be integrated via separate software packages (e.g., PVTSim). To calculate other parameters (such as diffusion coefficients, pipe roughness, shear effects, etc.), most of the available simulators incorporate empirical correlations or userdefined input values. The empirical correlations often are not universally applicable, because their model parameters have been derived from specific experimental setups and conditions or the best-available (literature) data. Experimental Investigations. Experimental investigations are often undertaken to supplement simulation work for assessing wax deposition. Experimental work is also performed to enhance the understanding of the deposition process for refining existing models or devis(7) Matzain, A. Single Phase Liquid Paraffin Deposition Modeling, M.S. Thesis, The University of Tulsa, Tulsa, OK, 1996. (8) Matzain, A.; Creek, J. L.; Apte, M.; Zhang, H.-Q.; Volk, M.; Redus, C. L.; Brill, J. P. Multiphase Flow Wax Deposition Modeling. Presented at the Engineering Technology Conference on Energy, Houston, TX, February 5-7, 2001, Paper No. ETCE 2001-17114.

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Figure 1. Schematic diagram of the coldfinger apparatus.

ing new models. Flow-loop tests provide the best direct representation of production systems. However, sample and manpower requirements often are significant, thereby limiting the use of flow-loop testing as a routine assessment method. Instead, the use of small-scale systems is preferred by some investigators. These systems (e.g., coldfinger, rotating cylinders, etc.) allow a multitude of deposition tests to be run with only small sample volumes and small time requirements. Although small-scale bench-top experimental devices are sometimes operating in different flow fields and may experience depletion effects if sample volumes are inadequate, the deposition data derived from such tests not only yield qualitative insight into the deposition process, but (if all physical processes are adequately accounted for) can also be used to provide quantitative flow-line deposition predictions. Experimental Description Apparatus and Procedure. A schematic of the coldfinger devices used in this study is shown in Figure 1. Each device consists of two coldfingers connected to a circulating water bath. The circulating water bath controls the temperature of each coldfinger at a regulated flow rate that is determined with a flowmeter. Thermocouples inside the coldfingers also allow monitoring of the inlet and outlet water temperatures to the coldfinger. A modular design allows the interchangeability of two different coldfinger geometries: a small 1.59-cm-outerdiameter (OD) finger and a large 3.34-cm-OD finger. The actual coldfinger is centered within a glass jar assembly filled with crude oil. The coldfinger/jar assembly is placed inside a second water bath, which is used to control the crude oil temperature. The crude oil temperature is regulated at the inside wall of the jar. The annular gap between the jar wall and coldfinger is ∼1.03 cm for the small coldfinger and 0.64 cm for the large coldfinger. A speed-controlled magnetic stirring bar in the bottom of the coldfinger jar provides stirring, which creates a helical decaying rotating flow field. This flow field influences both the shear stress field and the rate of heat transfer at the coldfinger surface. Nominal operating speeds of the stirrer are in the range of 500-1000 rpm. The flow field along the coldfinger is dependent on the geometry of the coldfinger and jar assembly, the crude oil density, the crude oil viscosity, and the stirring speed. The crude oil loading can also affect the flow field, particularly with respect to vertical decay in the flow field that occurs in highloading tests. For comparative inhibitor testing, the vertical decay in the flow field is not a concern, because testing is relative where conditions are the same for each sample, with

Jennings and Weispfennig the exception of the inhibitor treatment. However, in examining the effects of changing stirring conditions (shear) on deposits, one must be cognizant of its occurrence. In performing tests, crude oil is first conditioned above the WAT to solubilize any precipitated wax completely. The crude oil is then charged into the coldfinger jar in the annular space around the coldfinger. The jar is then placed into the preheated water bath. The stirring is started and allowed to continue for a predetermined time. After this time, the coldfinger is removed from the water bath and jar. Surface oil is rinsed off with cold methyl ethyl ketone (MEK). Visual assessments are made of the physical characteristics of the deposits. Photographs are taken for documentation. The deposit is then scrapped from the finger, weighed, and saved for potential analysis. The crude oil used in this study was a medium-gravity Gulf of Mexico crude oil with a measured WAT of 117 °F. It was supplied by the Tulsa University Paraffin Deposition Project. The crude oil temperature was held slightly below the WAT at 105 °F in all tests and was conditioned at 150 °F prior to testing. Typically, duplicates of each condition were performed, with wax deposit analyses being performed on only one of the duplicate samples. Analyses of Wax Deposits. The total content of wax deposits consists of both wax species physically depositing and crude oil entrained within the deposit matrix. The amount of entrained oil present is dependent on the specifics of each system. Shear forces on the deposit surface and the temperature differential, providing the thermodynamic driving force for deposition, are the two most important parameters involved in determining the entrained oil concentration. Aging, which is defined as the result of the diffusive transport of wax and oil from the deposit, may also play a role in extended test durations. Generally, for uninhibited systems, the “hardness” of the wax deposit is highly related to the wax and entrained oil concentrations in the deposit. To determine the wax content of the coldfinger deposits, inhouse high-temperature gas chromatography (HTGC) analyses were performed on the deposits. In the HTGC analysis method that has been used, quantification of n-alkane peaks is accomplished using a valley-to-valley integration procedure. Unfortunately, with most real petroleum fluids, some baseline shift occurs, because of unresolved species in the chromatogram analysis. The lack of complete resolution results in potential under-accounting of n-alkanes in the HTGC analysis.9 Also, non-n-alkane waxes are not taken into consideration, which, for some crude oils, can be a significant portion of the wax. Hence, the HTGC analyses are viewed more as a measure of minimum wax concentration. For the crude oil used in this study, the waxes were predominantly n-alkanes, verified by nuclear magnetic resonance (NMR) analyses on wax isolated from a wet extraction method. For the purpose of making relative comparisons of various wax deposits from the same crude oil, analysis errors from unresolved components are approximately comparable in each of the various samples. Hence, they do not bias the conclusions of overall trends occurring. An example of the deposit HTGC analyses is shown in Figure 2. It is from a coldfinger deposition experiment with a temperature differential (∆T) between the crude oil and coldfinger surface of 15 °F. The C10+, C20+, and C30+ wax compositions were measured to be ∼34.7, ∼32.2, and ∼30.5 wt % for this deposit. The carbon number chain length of the n-alkane peaks are labeled in the chromatogram. The second hump, which covers chain lengths of approximately C30-C70+, represents the majority of the “physically depositing” wax (9) Jennings, D. W.; Hager, H.; Weispfennig, K. Wax in Crude Oil: Analytical Methods and Their Uses for Flow Assurance. In Proceedings of the Chemistry in the Oil Industry VIII Symposium, Manchester, U.K., November 3-5, 2003; pp 59-69.



Figure 2. Example of a high-temperature gas chromatography (HTGC) chromatogram of the coldfinger deposits. species in this deposit. The majority of the other species are from entrained crude oil contained within the deposit. Computational Fluid Dynamic Modeling of the Flow Field. Because of the intrinsic difficulty of determining the shear stress levels at the coldfinger surface, computational fluid dynamics (CFD) modeling was performed. For this purpose, it was assumed that the flow field could be adequately modeled as being axisymmetric. A solid body rotation at the bottom of the computational domain (representing the coldfinger stirring bars) constituted the source for the fluid momentum in the modeling. Natural convection influences due to the presence of the temperature field were neglected. The flow field was assumed to fall within the turbulent flow regime. Rotating, turbulent flow fields constitute complex flow fields that still pose open questions, in regard to the exact representation of the various transport terms, thereby rendering an exact numerical computation difficult. The present investigation was supported by the use of a k- turbulent model in conjunction with a Boussinesq approximation for the Reynolds stress. This model implementation, although certainly not the best model for streamline curvature flows, was chosen because it is compact, robust, and requires a small amount of computation time. It was considered sufficiently accurate for a magnitude estimate of surface shear stresses. Figure 3 shows a typical stream function for a stirring rate of 500 rpm with an L-shaped computational domain (representing the annular gap and space below the coldfinger). It can be observed that, at this rotation rate, the entire fluid domain is in motion. Turbulent viscosity calculations indicated a 20-fold increase, with respect to the molecular viscosity. All calculations were performed assuming a constant coldfinger surface temperature equal to the temperature setting of the circulating water bath. The surface temperature of the opposite side of the domain (coldfinger jar wall) was dictated by the set point temperature for the crude oil. The calculations were performed for fluid equivalents, i.e., all relevant thermophysical parameters were used for simulation purposes, yet no deposition was permitted to occur. From the CFD calculations, the surface shear stress seemed to be on the order of 1-2 Pa for the coldfinger test runs. Although this level ranges in the lower range of applicable shear stresses typically encountered in pipelines and flow lines, the consequences of changing shear on wax deposition is clearly evident even within the shear range achievable in coldfinger operation. This is demonstrated in the results shown in this paper below.

Results and Discussion Numerous coldfinger tests were performed under different conditions to evaluate the effects of temperature and shear on the deposition process. Tests were also conducted at different loadings when it became

Figure 3. Example of flow-field domain from computational fluid dynamics (CFD).

apparent that the amount of crude oil used in the coldfinger tests could affect the flow fields, which, in turn, can affect deposition. Results of all the testing performed are given in Table 1. The tests ranged from 2 h to 16 h in duration. Again, in all tests, the crude oil temperature was maintained at 105 °F. The coldfinger temperatures were

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Table 1. Results of Coldfinger Experimentsa C30+ wax (wt %)

C20+ wax (wt %)

C20+ wax density (g/cm2)

∆T ) 45 °F 0.05508 0.04777 0.06243 0.07076

2.19 2.19 2.70 2.70

3.88 3.88 4.40 4.40

0.002137 0.001853 0.002747 0.003114

16 16 16 16

∆T ) 25 °F 0.01796 0.01653 0.02458 0.02813

na 5.59 6.33 na

na 8.04 8.75 na

na 0.001329 0.002151 na

60.6 61.2 60.4 60.8

16 16 16 16

0.00990 0.01131 0.01527 0.01447

na 9.01 10.77 na

na 11.43 12.98 na

na 0.001292 0.001983 na

1000 1000 1000 1000

58.9 61.3 61.7 61.3

16 16 16 16

0.00523 0.00534 0.00975 0.01126

16.21 na 17.27 na

17.88 na 19.17 na

0.000934 na 0.001869 na

2L 4L 5S 6S

1000 1000 1000 1000

62.8 60.9 63.5 64.6

8 8 8 8

0.00679 0.00547 0.00980 0.01063

na na na na

na na na na

na na na na

2L 4L 5S 6S

1000 1000 1000 1000

62.5 62.4 62.0 63.5

4 4 4 4

0.00567 0.00482 0.00857 0.00881

na na na na

na na na na

na na na na

2L 4L 5S 6S

1000 1000 1000 1000

61.6 62.0 62.4 61.3

2 2 2 2

0.00449 0.00417 0.00800 0.00754

na na na na

na na na na

na na na na

1L 2L 7S 8S

500 500 500 500

59.6 59.5 59.9 58.9

16 16 16 16

∆T ) 15 °F 0.00689 0.00594 0.00673 0.00734

5.65 5.83 9.54 na

7.48 7.59 11.18 na

0.000516 0.000450 0.000752 na

1L 2L 5S 6S

500 500 500 500

61.4 60.7 60.3 59.3

8 8 8 8

0.00680 0.00536 0.00606 0.00615

4.81 4.57 8.37 na

6.41 6.21 9.84 na

0.000436 0.000333 0.000597 na

3L 4L 7S 8S

500 500 500 500

60.0 60.6 60.8 60.6

4 4 4 4

0.00653 0.00457 0.00407 0.00456

3.73 3.91 5.15 na

5.38 5.61 6.77 na

0.000351 0.000256 0.000276 na

1L 2L 5S 6S

500 500 500 500

59.8 60.8 61.0 61.1

2 2 2 2

0.00299 0.00234 0.00380 0.00277

3.65 3.56 9.35 2.26

5.40 5.12 10.90 3.99

0.000161 0.000120 0.000415 0.000111

3L 5S 6S

750 750 750

59.1 60.1 61.5

17 17 17

0.00268 0.00422 0.00375

10.29 na 14.33

11.42 na 15.20

0.000306 na 0.000570

2L 4L 5S 6S 7S

750 750 750 750 750

61.2 61.7 61.3 61.1 59.9

16 16 16 16 16

0.00329 0.00345 0.00423 0.00434 0.00388

18.89 na 24.31 na na

21.41 na 26.25 na na

0.000705 na 0.001110 na na

3L 4L 5S 6S

750 750 750 750

61.2 61.6 62.1 61.6

8 8 8 8

0.00226 0.00174 0.00303 0.00203

11.26 na na 21.88

12.36 na na 22.62

0.000279 na na 0.000460

1L 2L 7L 8L

750 750 750 750

61.5 61.3 60.2 62.1

4 4 4 4

0.00201 0.00160 0.00203 0.00222

na 9.63 16.47 na

na 11.12 17.47 na

na 0.000178 0.000355 na

1L 2L 7L 8L

750 750 750 750

60.1 63.5 62.2 60.4

2 2 2 2

0.00149 0.00138 0.00178 0.00161

na 8.75 12.91 na

na 10.26 13.56 na

na 0.000141 0.000242 na

2L 4L 5S 6S

1000 1000 1000 1000

60.7 62.5 62.2 61.7

16 16 16 16

0.00176 0.00163 0.00328 0.00367

21.79 na 23.99 na

23.25 na 25.24 na

0.000408 na 0.000828 na

finger

stirrer speed (rpm)

loading weight (g)

nominal run time (h)

1Lb 2Lb 7Sb 8Sb

500 500 500 500

58.9 60.3 60.4 60.0

16 16 16 16

2L 4L 5S 6S

500 500 500 500

61.4 62.0 60.8 60.6

2L 4L 5S 6S

750 750 750 750

2L 4L 5S 6S

deposit density (g/cm2)



Table 1 (Continued) stirrer speed (rpm)

loading weight (g)

2L 4L 5S 6S

1000 1000 1000 1000

61.7 60.7 60.7 60.6

8 8 8 8

2L 4L 5S 6S

1000 1000 1000 1000

60.5 61.3 61.7 63.0

2L 4L 7S 8S

1000 1000 1000 1000

2L 4L 5S 6S

finger

nominal run time (h)

deposit density (g/cm2)

C30+ wax (wt %)

C20+ wax (wt %)

C20+ wax density (g/cm2)

∆T ) 15 °F 0.00136 0.00118 0.00311 0.00264

21.45 na 19.41 na

22.81 na 20.89 na

0.000310 na 0.000650 na

4 1/2 4 1/2 4 1/2 4 1/2

0.00093 0.00115 0.00202 0.00179

14.42 na na 20.52

15.42 na na 21.28

0.000143 na na 0.000380

62.9 63.4 61.0 60.6

2 2 2 1/2 2 1/2

0.00090 0.00093 0.00189 0.00124

19.95 na 17.39 na

21.48 na 19.17 na

0.000192 na 0.000362 na

500 500 500 500

61.0 61.3 61.1 61.8

16 16 16 16

∆T ) 8 °F 0.00126 0.00121 0.00201 0.00185

na 7.89 16.25 na

na 11.36 19.05 na

na 0.000138 0.000383 na

2L 4L 5S 6S

500 500 500 500

60.6 61.9 61.7 62.4

8 8 8 8

0.00104 0.00072 0.00160 0.00183

na 8.08 13.10 na

na 11.50 15.45 na

na 0.000083 0.000248 na

2L 4L 5S 6S

500 500 500 500

59.8 60.5 61.2 60.4

4 1/2 4 1/2 4 1/2 4 1/2

0.00059 0.00044 0.00140 0.00153

na 12.45 15.41 na

na 16.12 17.93 na

na 0.000071 0.000251 na

2L 4L 5S 6S

500 500 500 500

61.2 61.8 61.1 60.8

2 2 2 2

0.00027 0.00022 0.00104 0.00084

na na na na

na na na na

na na na na

2L 4L 5S 6S

750 750 750 750

60.8 59.5 61.6 61.3

16 16 16 16

0.00081 0.00065 0.00119 0.00099

na 20.12 26.26 na

na 22.85 28.30 na

na 0.000149 0.000338 na

2L 4L 5S 6S

1000 1000 1000 1000

61.8 61.5 61.5 62.6

16 16 16 16

0.00080 0.00067 0.00131 0.00089

na 17.22 25.65 na

na 20.03 27.76 na

na 0.000135 0.000364 na

2L 4L 5S 6S

750 750 750 750

52.2 50.8 55.6 50.2

16 16 16 16

∆T ) 15 °F 0.00226 0.00187 0.00377 0.00383

na na na na

na na na na

na na na na

2L 4L 5S 6S 2L 4L

750 750 750 750 750 750

50.5 50.6 50.6 50.6 50.7 50.6

16 16 16 16 16 1/2 16

0.00209 0.00165 0.00350 0.00331 0.00229 0.00198

25.13 30.59 30.50 30.09 22.35 na

26.97 32.26 32.20 31.74 24.45 na

0.000565 0.000533 0.001128 0.001050 0.000559 na

2L 4L 5S 6S

500 500 500 500

50.5 50.6 50.7 50.6

16 16 16 16

0.00503 0.00474 0.00689 0.00660

9.26 na 15.57 na

11.93 na 17.86 na

0.000600 na 0.001230 na

2L 4L 5S 6S

1000 1000 1000 1000

50.6 50.6 50.6 50.7

16 16 16 16

0.00147 0.00147 0.00275 0.00320

33.27 na 37.68 na

34.62 na 38.79 na

0.000507 na 0.001068 na

2L 4L 5S 6S

750 750 750 750

42.4 42.6 41.8 43.1

16 16 16 16

0.00170 0.00156 0.00381 0.00355

na 38.00 27.04 na

na 39.30 28.75 na

na 0.000614 0.001097 na

2L-total 2L-bottom 2L-middle 2L-top

750 750 750 750

60.2 60.2 60.2 60.2

16 16 16 16

0.00286c 0.00145 0.00163 0.00452

18.19 (avg) 32.07 27.38 10.42

20.43 (avg) 33.46 29.34 13.51

0.000533 0.000484 0.000479 0.000611

a na ) not analyzed. b The C 10+ and C20+ wax analyses were from combined samples; sample 1L with sample 2L and sample 7S with sample 8S. c Includes weight of residual unscrapped deposit wiped from finger. Residual deposit not taken into account in the individual section samples.

adjusted to give temperature differential (∆T) values between the crude oil and coldfinger surface of 25, 15,

and 8 °F. A couple of selected tests were also performed at ∆T ) 45 °F.

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WAT measurements were performed on retained crude oil after some 16-h deposition tests, to ascertain the extent of depletion of waxes that occurs during the test duration. Approximately 5-10 °F suppression of WATs were observed. On this basis, it was concluded that the extent of wax depletion that was occurring in the tests was minimal and did not affect the study objectives. Effect of Temperature on Deposition. As mentioned previously, for wax deposition to occur, a temperature differential (∆T) must exist between the bulk crude oil (or crude oil WAT) and the colder deposition surface. Because the ∆T value is increased, the thermodynamic driving force for deposition increases and more wax will deposit. The “total” deposit includes both the wax species that are physically precipitating and depositing and the crude oil that is entrained into the wax deposit. Panels a and b in Figure 4 show the increase in the amount of deposition as the ∆T is increased for 16-h coldfinger runs at stirring speeds of 500 rpm. Shown are both the amounts of total deposit and the C20+ n-alkane wax portion of the deposit (shown as mass/ surface area of deposit). As seen in Figure 4a and 4b, both the total deposition and C20+ wax deposition increase almost linearly. However, the rate at which each increases is different. This is illustrated in Figure 4c, which shows a decrease in C20+ wax concentration with ∆T for the coldfinger runs. Lower crude oil viscosities at the lower temperature coldfinger surfaces are believed to be a principal reason for the decreasing C20+ wax concentrations at higher ∆T values. The faster rate of wax deposition (at the larger ∆T values) may also contribute to the deposit being able to incorporate more crude oil. Because wax concentration is generally related to hardness, softer deposits occur at higher ∆T values for a fixed shear. Trends similar to those shown in Figure 4a-4c exist in the coldfinger data given in Table 1 for other stirring speeds and time durations. Another change in the deposits occurring with ∆T is a shift in the mean carbon number of the wax that is physically depositing. Lower ∆T runs had higher mean carbon numbers of the physically depositing wax. Lee-Tuffnell10 reported similar results for coaxial shear-cell and flow-loop tube experiments performed for examining shear, temperature, and live-end effects on deposition. He reported that “the overall weight of the deposit increased and became mechanically easier to remove” as the ∆T value increased for experiments performed at equal shear rates. The deposits also were reported to consistently contain more trapped oil with increasing ∆T values, although specific wax concentration data were not shown. n-Alkane wax peak distributions were also reported to be shifted to higher carbon numbers with lowering ∆T values. Examples of deposits formed at different ∆T values are shown in Figure 5a and 5b. Pictures of deposits at ∆T ) 8 °F and 25 °F are shown for 16-h coldfinger runs with stirring at 750 rpm for the large and small (10) Lee-Tuffnell, C. A Laboratory Study of the Effects on Wax Deposition of Shear, Temperature and Live End Addition to Dead Crude Oils. Presented at the Symposium on Controlling Hydrates, Waxes, and Asphaltenes, Aberdeen, Scotland, September 16-17, 1996.


Figure 4. (a) Effect of temperature differential (∆T) on (a) the amount of total deposit, (b) the amount of C20+ wax portion of the total deposit, and (c) the C20+ wax concentration of the total deposit. Results for each are taken from data from 16-h coldfinger runs after stirring at 500 rpm.

coldfinger geometries. The deposits from the ∆T ) 25 °F runs are thicker, as well as “wetter” and significantly darker in appearance, because of higher concentrations of entrained crude oil. Entrained crude oil gives wax deposits their black to brownish color. Without any entrained crude oil, a deposit with solely wax would be white. Note that C20+ wax compositions are used in the comparison shown. C20+ wax was chosen because it includes all the physically depositing n-alkane waxes in the different ∆T experiments that have been performed. However, it does include some wax from the entrained crude oil portion of the deposit, particularly in the C30- range for the lower ∆T values, as illustrated in Figure 2. However, comparison trends shown here



Figure 5. Examples of the change in deposit appearance with varying ∆T values: (a) 16-h large coldfinger runs after stirring at 750 rpm and (b) 16-h small coldfinger runs after stirring at 750 rpm.

and elsewhere in the paper are no different, whether C30+ species or C20+ species are examined. Effect of Shear on Deposition. Increasing the amount of shear generally decreases the total wax deposition amount and increases the wax concentration of deposits. Panels a and b in Figure 6 illustrate these effects with results from 16-h coldfinger test runs at various stirrer speeds at ∆T ) 15 °F and 50 g loading. In Figure 6a, the amount of total deposition is shown to decrease as stirring speeds are increased. The decrease is drastic from 500 rpm to 750 rpm. The change from 750 rpm to 1000 rpm is less pronounced and almost seems to suggest that an asymptotic limit is being approached. In Figure 6b, the C20+ wax concentration can be observed to increase by a factor 2-3 with increasing stirrer speed, depending on the coldfinger geometry. Interestingly, the actual wax density was observed to remain almost constant with changing stirrer speeds in the coldfinger testing. This is illustrated in Figure 6c with the C20+ wax deposit densities for the ∆T ) 15

°F runs presented in Figure 6a and 6b. Results from other tests at different ∆T values show the same trend of a relatively constant wax density. Hence, it seems that the predominant effect of increasing shear stress is the removal of entrained oil. Note that increasing the stirring rate not only increases the shear stress at the deposit surface, but also increases the heat transfer from the coldfinger. Hence, because shear stress is not the only “variable” that is changing, it is not possible to unambiguously attribute all the changes to be solely from shear-stress effects. However, decreases in the amount of total deposition and entrained crude oil in the deposits must be related to shear stress here, because the heat transfer increases, rather than decreases, as the stirring rate increases. Increased heat transfer favors increasing both wax deposition and entrained oil concentrations, similar to the changes observed with increasing ∆T, as discussed previously. Examples of the changes in the deposit characteristics are shown in Figure 7a and 7b. Pictures of 500- and

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Figure 6. Effect of shear on (a) the amount of the total deposit, (b) C20+ wax concentration in total deposits, and (c) the amount of C20+ wax portion in the total deposit. Each used data from 16-h coldfinger runs at ∆T ) 15 °F and 50 g loading.

1000-rpm deposits are shown for the large and small coldfinger geometry runs at ∆T ) 15 °F. The higher wax concentration deposits at 1000 rpm can be seen to have a drier, harder, and lighter appearance. Other investigators have reported observing similar results on the effect of shear. Bern et al.3 reported a decrease in deposition tendency with increasing shear in a laboratory coaxial shearing cell study with North Sea Forties crude oil. The authors viewed the decrease in deposition as evidence that shear dispersion was not contributing to deposition in their study. No mention of the effect on the deposit character (entrained oil content, softness, hardness, etc.) was made in this early 1980 study. However, Brown et al.5 specifically indicated reduced deposition that was occurring from increased


shear, being due to a decrease in “the amount of oil trapped in the deposits, reducing the total (paraffin + oil) rate of deposition”. The conclusions were based on mini-flow-loop results from a variety of crude oils. They also reported that deposits formed at high shear rates were “hard and brittle”, whereas deposits formed at low shear rates were “soft and pliable”. However, actual wax concentrations analyses were not presented to quantify the changes in the deposits. The effect of increasing shear, decreasing the amount of trapped oil in the deposit, was also reported in two subsequent coaxial shear cell studies by Lee-Tuffnell10 and Dawson.11 The effect of shear on deposition has been examined in two other flow-loop studies. Lund12 studied the effect of shear on deposition for single-liquid-phase flow in a large flow-loop assembly. Only one crude oil was used in the study. Results showed a decrease in total deposition with increasing flow rate/shear, with apparent asymptotic behavior (within experiment precision) at the higher flow rates that had been examined. The composition of wax in the deposits was measured and showed a decreasing trend in the entrained crude oil composition up to the region where total deposition seemed to stabilize. Venkatesan13 also examined shear effects on deposition in a flow loop. This study was conducted in a smaller flow loop and used a model wax solution instead of actual crude oil. A reduction in total deposition with increasing wax compositions was reported with increasing shear under laminar and turbulent flow conditions. In addition to an observation of reduced deposition, Venkatesan proposed an empirical mathematical expression for deposition reduction due to shear. The other available shear reduction term proposed in the literature has been reported by Matzain et al.8 for single and multiphase deposition modeling. In this work, the total deposit growth reduction by shear is linked to a tuning parameter related to the flow regime, flow velocity, crude physical properties, and deposit thickness. No apparent phenomenological relation is evident in the shear reduction expression. Effect of Loading on Coldfinger Results. A consequence of the bottom stirring in the coldfinger apparatus is a vertical decaying flow field where shear stresses exerted at the top of the finger can be, under certain conditions, significantly less than the bottom portions of the finger. As the shear stress varies, the deposit character varies accordingly. The amount of decay is dependent on the coldfinger/jar assembly, stirring, crude oil physical properties, and crude oil loading. The decay is more significant in the large coldfinger assemblies, because these contain a narrower annular gap between the finger and jar than the small coldfinger assemblies. An example of the variations that can be seen in deposits on the large coldfingers is shown in Figure 8. Here, analyses were made on equal bottom, middle, and top sections of a 16-h deposit at 750 rpm and ∆T ) 15 °F with ∼60 g crude oil loading. Note that (11) Dawson, S. G. B. Wax Deposition Modeling. Presented at the Symposium on Controlling Hydrates, Waxes, and Asphaltenes, Aberdeen, Scotland, September 16-17, 1996. (12) Lund, H. J. Investigation of Paraffin Deposition during Single Phase Liquid Flow in Pipelines, M.S. Thesis, The University of Tulsa, Tulsa, OK, 1998. (13) Venkatesan, R. The Deposition and Rheology of Organic Gels, Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 2004.



Figure 7. Examples of change in deposit appearance with varying shear: (a) 16-h large coldfinger runs at ∆T ) 15 °F and 50 g loading and (b) 16-h small coldfinger runs at ∆T ) 15 °F and 50 g loading.

Figure 8. Variations within the deposit, relative to coldfinger location. Results from 16-h large coldfinger run at ∆T ) 15 °F and 60 g loading.

the bottom and middle sections are similar in total deposit weight density and C20+ wax concentration. However, the top one-third section is significantly

different, because of the lower shear field present. It is a softer deposit containing a greater amount of entrained crude oil. The higher entrained oil concentration in the top section influences the average deposit properties to a greater extent than the bottom and middle sections, because of its greater weight. As indicated in Figure 8, if the loading is kept below ∼40 g (two-thirds of 60 g), then only minor effects in the decaying flow field are present in the large coldfinger. However, performing experiments at lower loading does result in a tradeoff of generating less deposit. The effect of loading between 40 and 60 g is illustrated in Figure 9a and 9b, showing total deposit weight densities and wax concentrations for a variety of 16-h runs at different loadings at 750 rpm and ∆T ) 15 °F. Results from large and small coldfingers are shown. In the figures, the effect of loading can be seen to be much less prevalent in the small coldfingers, because both deposit density and wax concentration only vary slightly with changing loading. The effect of loading in the large fingers is clearly more substantial.

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Figure 9. Effect of loading on (a) the amount of coldfinger deposits and (b) C20+ wax concentrations in coldfinger deposits. Results for each are taken from data from 16-h coldfinger runs at ∆T ) 15 °F and after stirring at 750 rpm.

Conclusions A series of coldfinger experiments was performed with a medium-gravity Gulf of Mexico crude oil to examine the effects of changing shear and temperature on the wax deposition process. In this study, increasing shear was observed to decrease the amount of total deposition, primarily through a reduction in the amount of entrained crude oil contained in the deposits. However, the amount of actual wax in the deposits was observed to be relatively constant, for the range of variation in shear that has been examined. The concentration of


wax, however, increased as shear increased, because of the reduction in entrained crude oil. The effect of increasing temperature differential (∆T) (between bulk oil and cold surface) led to an expected increase in total deposition. The amount of actual wax and entrained crude oil both increased. However, the concentration of entrained crude oil in the deposits was observed to increase at a greater rate. Lower crude oil viscosities at the lower-temperature coldfinger surfaces are believed to be one of the principal reasons behind the higher entrained oil concentrations. The faster rate of wax depositing (at the larger ∆T values) may also contribute to the deposit being able to incorporate more crude oil. Presently, no simulation method exists for truly predictive modeling of wax deposition. Particularly, the ability to adequately describe the phenomena causing the deposition and formation of the nature of the deposit is lacking. The nature of the deposit formed is dependent on the environment of shear and temperature; therefore, these factors should be considered in any simulation method developed for modeling wax deposition. The other notable conclusion of the study was that the design and operation of the coldfingers themselves can influence deposition results. In our coldfingers, it was observed that the amount of sample loading could influence results because of variation in the shear stress along the coldfinger length, which is caused by the vertical decaying flow field provided by the bottom stirring. Higher shear stress at the bottom of the coldfingers (closer to the stirrer) yielded deposition with higher wax concentrations than deposition near the top of the coldfingers. The change in deposition along the fingers versus loading was more substantial in the large coldfinger geometry than the smaller geometry, because of a narrower annular gap between the finger and the coldfinger jar. Acknowledgment. The authors thank Helen Hager of the Baker Petrolite Analytical Department for performing numerous high-temperature gas chromatography analyses of wax deposits. The Tulsa University Paraffin Deposition Project is thanked for providing the crude oil used in the study. EF049784I

Effects of Shear and Temperature on Wax Deposition - American

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