Experimental Study on the Strength of Original Samples of Wax

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Experimental Study on the Strength of Original Samples of Wax deposits from Pipelines in the Field Miao Li, Mengran Sun, Yingda Lu, and Jinjun Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02396 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Experimental Study on the Strength of Original Samples of Wax deposits from Pipelines in the Field Miao Li, Mengran Sun, Yingda Lu, Jinjun Zhang* National Engineering Laboratory for Pipeline Safety/ MOE Key Laboratory of Petroleum

Engineering/ Beijing Key Laboratory of Urban Oil & Gas Distribution Technology, China University of Petroleum (Beijing), Beijing 102249, China. E-mail: [email protected]

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ABSTRACT:

The yield stress of the wax deposit, characterizing its mechanical strength, provides critical design basis for pigging. The deposits naturally formed in a pipeline (hereafter, "natural wax deposits") and those artificially generated from model wax-oil mixtures (hereafter, "model wax deposits") usually present different yield stress due to structural variations. Investigations on the distinctive yielding characteristics between natural and model deposits are limited in the literature. In this research, we present a comprehensive comparative mechanical and structural analysis of natural and model wax deposits, based on which representative laboratory tests can be designed to guide pigging operations. A rheometer with the vane geometry was enhanced to preserve the microstructure of the deposit sample collected from the field prior to the yielding test. Field wax deposits from different radial positions of the pipe were analyzed. It was discovered that the yield stress of the natural wax deposits increase exponentially with solid paraffin content. Moreover, the deposit layer closer to the center has lower solid paraffin content and lower resulting yield stress than the layer in the vicinity of the inner pipe wall. The original sample of natural wax deposits (called "original sample" for short following) was heated until completely melted and cooled again for the reformed solid sample similar to the model wax deposits in common use. The tested yield stress for the newly formed deposits can be 5-13 times that of the original sample at the same temperature due to the compact microstructure. Consequently, the required pressure to remove the wax deposits in the pipeline could be relatively high


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estimated based on the yield stress of model wax deposits. On the other hand, the natural wax deposits and model wax deposits formed on the cold finger or in the flow loop are more alike in structure. So model deposits obtained in these ways should be used in the studies relative to pig motion, rather than the wax-oil gel which is currently very popular.

Key words: Waxy crude oil, natural wax deposits, yield stress, vane, pigging

1. Introduction

Wax deposition is a frequently encountered issue during the transportation of waxy crude oils. The wax deposit can reduce the effective area for oil flow. If not handled properly, the wax deposit can be too thick and hard and makes it impossible to be removed by pigging. So it’s necessary to understand the nature of the wax deposits and study the deposition process. Many theoretical and experimental studies have been conducted to understand the physics of wax deposition1-6 and to predict the deposit growth rate and thickness.7-10 The models are based on heat and mass transfer mechanisms in the bulk flow as well as the internal diffusion mechanism. However, there is little work on the structural characteristics and strength of wax deposits. If the wax deposit is too hard, pigging would be difficult or even the pig would be stuck.

Strength here refers to the maximum stress that the material can stand before breaking, characterized by yield stress in this paper. Breakdown of wax deposits occurs if the applied pressure exceeds the yield stress. A number of models have been 3 ACS Paragon Plus Environment

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proposed to describe the breakdown of wax-oil gels.11-14 A structural parameter, defined by the number of ‘links’ of the solids crystals in the gel structure, is used to describe the breakdown process. On the other hand, different kinds of rheological experiments were conducted to study the yielding process of wax-oil gels.15-18 The linear-to-nonlinear viscoelastic transition is characterized in the experiments. These experiments are frequently carried out by loading liquid-state samples into the rheometer, so they are improper for testing solid-state wax deposit. Since the original structure of the sample would break down during sample loading due to its thixotropic characteristics. In order to overcome this problem, conventional test methods in soil mechanics, including the compressing test,19 the direct shear test,20 and the vane method,21 were applied to test the strength of wax deposit. Both compressing and direct shear test are easy to operate and provide accurate testing results of yield strength. However, in lack of effective measures for insulation, they are not suitable if different temperatures are required in experiments, especially for wax deposits that are sensitive to temperature. Taking this problem into account, an apparatus based on vane method was designed in our work. A vane is a cylindrical shaft that centered by 4-8 thin blades at the end. When a vane is used, proved by numerical simulation,22 yielding occurs approximately along a localized cylindrical surface over which the shear stress distributes uniformly. The maximum torque Tm relates to the yield stress τy as following:


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Tm 

 D3 H (

2

1  ) y D 3

Where, H and D are the height and the diameter of the blades respectively. Vane was firstly used by Nguyen and Boger to test the yield stress of mud.23 By comparing with the results of capillary method, they confirmed that the vane method is simple and accurate for measuring the yield stress directly. Yoshimura et al.24 compared three techniques involved concentric-cylinder, parallel disk and vane by measuring yield stress of oil-in-water emulsions, and they considered vane as a method of high precision. The studies above demonstrate that the results of vane method are comparable with other techniques measuring yield stress. So vane was widely used to test yield stress in food, cement or other industries.25-28 As for testing the yield stress of the natural wax deposits, vane method is advantageous because the original structure of sample wouldn’t be destroyed during sample loading.29 In addition, when vane is applied to test yield stress, wall slip is avoided as the yield surface is in the material itself.30 It is relatively important to measure the wax deposit with yield stress of great value. Apart from testing methods, materials used in previous researches about deposits are also different. Three kinds of experiment materials have showed up actually, including deposits of model-oil gels,19,31,32 model deposits formed on cold finger33 or in flow loop,34 and natural wax deposits sampled from the field.20,21 They have major difference in the structures because of their different oil compositions, thermal and shear histories, which have great effects on the morphology of wax crystals.35-40 Wang 5 ACS Paragon Plus Environment

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et al.20 and Bai et al.21 observed the microscopic characteristics of wax crystals in wax deposits obtained from field pipelines. Their results both show that wax deposits are greatly different with crude oil in the size of wax crystals. On the other hand, a number of studies have confirmed that yield stress of model oils is greatly influenced by the morphology of wax crystals.35,36 Therefore, yield stress of natural wax deposits could be different from model wax deposits formed in various ways. Few works, however, are found on this topic. The main purpose of this research is to understand the mechanical and structural characteristics of natural wax deposits from production field, and compare them with the model wax deposits prepared in laboratory. Before testing the yield stress, an apparatus based on vane method was designed. The feasibility of testing the yield stress of original sample by the vane method was investigated. Firstly, we analyzed how yield stress of natural wax deposits is affected by temperature. We also analyzed the natural wax deposits at different radial positions in pipe, which can be meaningful to wax deposition theories. Secondly, we heated the natural wax deposits to melt, and studied the mechanical differences between the original sample and the reformed one. A number of experiments on the thermal characteristics and microscopic characteristics of wax deposits, like DSC and polarizing microscope, were also taken to explain the results.

2. EXPERIMENTAL SECTION

2.1. Experiment apparatus. 2.1.1. Introduction to experiment apparatus. A


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measuring apparatus based on vane method was designed to determine the yield stress of wax deposit samples. Beside the temperature control and the way that the vane is inserted, this apparatus is similar to the one designed by Briggs et al.41 to test the yield stress of ice creams.

The design parameters of the vane measuring system are critical to the accuracy of the test results. Over the years, mainly two kinds of efforts were made regarding to this aspect. On one hand, the effect of sample cylinder size on results was considered. In Boger et al.’s42 opinion, wall slip can be eliminated only when the vane was inserted in an ‘infinite’ cylinder, but no quantitative conclusion to ‘infinite’ was achieved. Anthony et al.43 provided answers to this question through experiments. A cup-to-vane diameter ratio of 3 was recommended for avoiding wall slip in their research, and this ratio was found to be material dependent. On the other hand, the depth that vane was inserted in the sample was studied. The resistances by the sample below and above the vane were taken into consideration, or the results could be overestimated.44 Joseph et al.45 tested the yield stress of cement by two techniques: the vane method and the direct shear method. Their work indicates that the vane method provides a larger measured yield stress than the direct shear method, and the difference becomes greater with the depth of the vane in the sample. In summary, the foregoing studies indicate that the size of the sample holder and the vane depth inserted in the sample are significant to the measurements, forming an important basis for designing the experimental apparatus in the present work. Incorporating their conclusions into the designs should add the accuracy level of the results. 7 ACS Paragon Plus Environment

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Our apparatus consists of five parts (Figure 1): (1) A sample holder. Wall slip might happen if the force that wax deposits attach to the wall is smaller than the strength of deposit structure. So the size of sample holder will affect the yielding process of wax deposits. To avoid wall slip between the sample and the sample holder, parameters of sample holder should meet the criterion following: H/D2.0,23 where H and D are the height and the diameter of the blades respectively, and DT is the diameter of the inscribed circle of sample holder. The cross section of the sample holder is designed to be square to avoid wall slip on the wall surface. The sample holder, eventually, was designed to be a 4.5cm × 4.5cm (inside dimensions) aluminum shell with walls of 4 cm in height. It is surrounded by a 2 cm thick water jacket to maintain the temperature of the wax deposits sample. (2) Adjustable base. The sample holder is attached to the adjustable base firmly so that they can only move in the vertical direction precisely. The adjustable range of the height is 0 to 20 cm. (3) A vane. There are six blades attached in the vane. It has a height of 1.6 cm and a diameter of 2.2 cm. (4) Anton Paar Rheolab QC rheometer. It is a torque sensor, actually, that can be used for stress controlled experiment. The range of the torque is 0.25~75mN·m. (5) F32-ME programmable water bath. It can operate in the temperature range of 5 °C to 80 °C.

2.1.2. Feasibility of experiment apparatus. Prior to testing the sample of natural wax deposits, the apparatus was applied to Daqing waxy crude oil at different temperatures to test its reliability. The waxy crude oil was placed in water bath at 80 °C for 2 h to eliminate its shear and thermal histories and then placed at the room 8 ACS Paragon Plus Environment

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temperature (35 °C) in shade for 48 h. Before tested, the waxy crude oil was loaded at 80 °C, cooled (cooling rate 1 °C/min) to the test temperature, and subsequently maintained at this temperature for 1 h. The method used for testing the yield stress is the same with the wax deposits which would be introduced in detail below. The stress loading rate is 4 Pa/min.

Figure 2 presents the results of the yield stress of the waxy crude oil at several temperatures utilizing three geometries: vane, coaxial cylinder with grooves inside, coaxial cylinder with smooth wall. With decreasing temperature, the structural strength of the gelled oil increases, and the distinctions among the results for each method also become more pronounced. These results shows that the vane geometry is more effective for preventing wall slip effectively when testing wax-oil gels.

In order to verify whether the sample’s structure would be destroyed during the process of vane inserting, the yield stresses of waxy crude oil gels formed before and after the vane was inserted were tested and compared. The structural strength of the gelled oil at lower temperature was closer to that of the original sample, so test temperatures were taken much lower than gel point. Figure 3 indicates that there is no great difference between the results of the two methods at different temperatures, which means most of the structure would be preserved when vane was inserted. Based on these results, it was concluded that the apparatus is reliable enough and can serve the purpose of the present work.

2.2. Experimental materials. The wax deposit samples to be tested were collected 9 ACS Paragon Plus Environment

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from two field pipelines (referred as Pipeline 1 and Pipeline 2 following) before pigging. The basic operation parameters for these two pipelines are provided in the Supporting Information. To maintain the structural integrity, the wax deposits that attached to the pipe wall were obtained by directly cutting. The collected wax deposits were further divided into the outer layer sample and the inner layer sample basis on their radial positions in the pipeline. The inner layer refers to the deposit closer to the pipe wall while the outer layer refers to the deposit layer closer to the pipe center. Aside from the wax deposits, the mixture of removed deposits particles and oil (called ‘mixture’ following) in front of the pig after pigging operation in pipeline 2 was also obtained. We use radial position together with an Arabic number to distinguish different samples. For example, ‘outer 1’ represents the out-layer deposit collected from pipeline 1. In Table 1, the compositions of the deposits and the bulk oil were summarized and their physical parameters of the deposits were summarized in Table 2. Table 1 indicates differences between the compositions of these wax deposit samples. Because the wax deposit in inner layer has a higher wax content, its WAT is also higher than the outer-layer wax deposit in Table 2.

2.3. Experiment methods. 2.3.1. Test temperature. The test temperatures were decided by estimating the temperature of wax deposits in field pipelines. A compound cylinder model was introduced to simulate heat transfer from the oil in the pipeline to the soil in the field. A detailed description of the calculation process could be found in the Supporting Information. According to the calculation, three temperatures of wax deposits in field including the highest, the lowest and an average are taken as the test 10 ACS Paragon Plus Environment

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temperatures. They are 20 °C, 30 °C and 40 °C. 2.3.2. DSC test. A TA2000/MDSC2910 DSC apparatus was applied to test the WAT and precipitated curve of the original samples from field pipelines. The range of temperature tested is from 80 °C to -20 °C and the cooling rate is maintained as 5 °C/min.

2.3.3. Optical microscopy. A polarizing microscope (Nikon OPTIPHOT2-POL) was applied to microscopically observe the morphology of wax crystals in the deposits. Wax deposits were smeared onto the glass slide and were kept on the thermal stage at the testing temperature for 30 min before observing. Wax crystals observed through the microscope above were taken as 2-D images by a camera and were transferred to the computer. The images were analyzed by the image processing software Image J. The morphological parameters of wax crystals, including average area, aspect ratio and boundary fractal dimension are obtained to characterize the size of wax crystals, the shape of wax crystals and the complexity of wax crystals, respectively. Wax crystals with higher values of these three parameters tend to be larger, more rod-like and more complex in structure.46 In order to get a statistically meaningful result, more than 30 images were analyzed at each testing temperature, and the average results are presented.

2.3.4. Yield stress measurements. The wax deposit to be tested was loaded into the sample holder preheated to the desired temperature and kept for 1h. An electronic thermometer was used to test the temperature at the middle of the sample. When the 11 ACS Paragon Plus Environment

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desired temperature was reached, the vane was inserted into the sample by raising the height of the sample holder. The depth that the vane was inserted was based on the study by Anderman et al.:47 z1=1/2H, z2=H. Where z1 is the distance from the upper end of blades to the surface of wax deposit; z2 is the distance from the lower end of blades to the bottom of wax deposit and H is the height of blade.

Shear stress sweep was used to measure the yield stress of wax deposits, in which the shear stress increased linearly with a rate of 30 Pa/min and the responded strain was recorded. Figure 4 exhibits a typical stress-strain curve plotted on logarithmic coordinates, from which the yield stress of the original sample could be determined. As the stress increases linearly, the strain first increases rapidly, and then a slower increase in the strain is expected until a dramatic rise appears. The result shows that yielding of natural wax deposits occurs by an initial elastic response, followed by a viscoelastic creep and a final fracture. The stress, identified as yielding point in Figure 4 is regarded as the yield stress of the original sample.

After the above measurements, the original sample was heated to 80 °C until it completely melted, and was cooled quiescently (with cooling rate 1 °C/min) to the same temperature. The yield stress of the reformed deposits was tested, using the same protocol as was used for the original sample. Given that the sample might experience some expansion and contraction when temperature changed, the vane was inserted into the sample once again after cooling for the test of yield stress for the reformed wax deposits. 12 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION

3.1. The yield stress of natural wax deposits. When wax removal is operated, the pig produces a stress distribution within the wax deposit. To determine whether and where the failure occurs, the applied local stress should be compared with the yield stress of wax deposits. However, the yield stress of wax deposits varies greatly in the radial direction of the pipeline (Figure 5) due to two factors: temperature gradient and concentration gradient of solid wax. Thus, in this section yield stress of natural wax deposits at different temperatures and at different radial positions are studied.

3.1.1. The yield stress of natural wax deposits at different temperatures. Experimental strain curves in Figure 6 show the mechanical behavior of natural wax deposits being subjected to linearly increasing stress at different temperatures. When temperature drops, the curve moves to the lower right while the yield point moves to the upper right, both of which indicate an increase in the structural strength of the wax deposit at lower temperatures. This observation, though previously reported in the tests using deposit samples formed in the lab, is the first report for field deposit samples.

The wax content in the wax deposit is one of the factors that contribute to the structural strength’s dependence on temperature. Figure 7 compares the precipitation curves of the wax deposit samples collected from the two field pipelines. For comparison, the precipitation curves of the bulk oil and oil/wax mixture collected in front of the pig are also included. It shows that the wax appearance temperature (WAT) 13 ACS Paragon Plus Environment

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of the wax deposit is higher than that of the bulk crude oil, so is the solid wax content. Unlike the bulk oil, the wax precipitation for the deposit sample goes through two-stage mode when temperature decreases: a rapid increase in the temperature range of 50 °C-70 °C followed by a gradual and almost linear in the temperature range of 0 °C-40 °C. The results probably attributed to wax molecules with carbon numbers larger than the critical carbon number48 diffusing into the wax deposits. The wax deposit mainly consists of normal alkenes with higher carbon numbers. The wax molecules with higher carbon number would precipitate first and faster compared to the wax molecules with lower carbon number when temperature decreases, leading to the two-stage precipitation shown in Figure 7. According to the rough calculation, the temperature of wax deposits used in this study was formed around 40 °C in the pipelines. The samples were transported and handled in summer which has a temperature of 35 °C. The temperature fluctuation is small. It can be calculated from Figure 7 that about 80% of the dissolved wax have been precipitated at 40 °C and only about 1% of the wax in wax deposits would be precipitated when temperature decreases from 40 to 35 °C. Therefore, though the properties of wax deposits might be slightly altered during the transportation and handling process, we believe that the conclusions observed in the lab are representative in the field.

Although the precipitated solid wax increases almost linearly in the temperature range of 20 °C to 40 °C, the yield stress increases nonlinearly. For example, as the temperature decreases from 40 °C to 30 °C, the precipitated solid wax content of Outer 1 deposit increases by 2.25% and its yield stress increases by 58.2%. As the 14 ACS Paragon Plus Environment

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temperature further decreases from 30 °C to 20 °C, its solid wax content increases by 2.75% while the yield stress increases by 138.0%. It means the same amount of wax precipitation may cause greater structural enhancement (approximately twice as much, valued by yield stress) at lower temperature range. In Figure 8, the yield stress of the wax deposit as a function of the precipitated solid wax content is plotted. An exponential correlation is found between the yield stress (τ) and solid wax concentration (w %): τ=aebw, which is consistent with results of wax-oil gels.49,50

The two-stage growing of wax crystals when temperature decreases is also demonstrated in Figure 8. Parameters of wax crystals in the original sample of the outer-layer deposit of pipeline 1 at different temperatures are listed in Table 3. Area of wax crystals increases and aspect ratio of wax crystals decrease when temperature decreases. In stage 1, wax molecules precipitate and form crystals that entrap liquid oil inside. In stage 2, precipitated wax molecules would tend to grow on the wax crystals that already exist rather than forming new crystals. Note that these two stages do not come successively. They always happen simultaneously during cooling, with stage 1 dominating at high temperature and stage 2 at low temperature. At lower temperature, more complex wax structures lead to a more rapid increase in yield stress. Therefore, when the temperature of bulk oil flow in pipeline drops, the strength of wax deposits attached to pipeline increases rapidly. It would be wiser to operate pigging in summer rather than in winter if it’s warmer in summer.

3.1.2. The yield stress of natural wax deposits at different radial positions. The 15 ACS Paragon Plus Environment

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strain-stress curves of the wax deposits at different radial positions are compared in Figure 9. Clearly, the structural strength of the wax deposits from different radial positions is greatly different.

Differences between wax deposits of different radial positions could be figured out on the basis of the wax deposition theory. Generally, the formation of wax deposits go through five steps:3 (1) wax molecules settle on the cold surface and form a gel layer; (2) wax molecules above the critical carbon number (CCN) diffuse into the condensate layer from the bulk fluid; (3) wax molecules continue to precipitate at the surface of gel layer, leading to an increase of the deposit thickness; (4) wax molecules go through the liquid phase of deposit and precipitate inside; (5) light components with carbon numbers smaller than the CCN counter diffuse out of the gel layer. It can be concluded from step (3) to (5) that there is a solid wax content gradient in the radial direction of the wax deposit which drives wax molecules inside (Figure 7). Also, the carbon number of wax molecules increases in the radial direction of wax deposit. The carbon number distribution (mass fraction of wax molecules with different carbon numbers) of wax molecules in original samples was tested, and the results of wax deposits of different radial positions in pipelines are presented in Table 4.

Table 4 indicates that there is a difference in carbon number distribution between the deposits at different radial positions of the pipe. For example, the average carbon number of the outer-layer deposit of pipeline 1 is about C25 and the average carbon number of the inner-layer deposit of pipeline 1 is about C40. It is also obvious that 16 ACS Paragon Plus Environment

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they are different in paraffin polydispersity. The carbon number distribution is of great significance to analyze the structures of wax crystals.51 More than 30 microscopic photos of wax crystals (Figure 10) were taken by the polarizing microscope to observe the morphology and micro structure at each temperature. Parameters of wax crystals in the original samples at temperature of 30 °C are listed in Table 5.

It can be concluded from Table 5 that wax molecules with smaller carbon numbers form rod-like wax crystals of smaller size and simpler structure. Wax molecules with larger carbon numbers form plate-like wax crystals of larger size and more complex structure. As wax crystals in the outer-layer deposit turn to be more rod-like, the sharp crystal edges of which would hinder the crystal-crystal ‘‘anchoring’’ interaction, which leads to a weak structure of the network.52 Therefore, the inner-layer deposit has a higher yield stress than the outer-layer deposit at the same temperature. The yield stresses of wax deposits at different radial positions are presented in Figure 11. It can be easily inferred that yield stress of the wax deposits increases in the radial direction from the center to the wall. When pigs are utilized to remove wax deposits from the pipeline, it is necessary to know the exact position where failure occurs in the wax deposits, as pigging is effective only when the failure occurs near the pipe wall. And according to the experiment results, there is a great chance that the outer-layer deposit may yield prior to the inner layer because the radial distribution of the yield stress is non-uniform.

It can also be concluded from Figure 11 that the difference in yield stress between 17 ACS Paragon Plus Environment

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wax deposits at different radial positions are enlarged when temperature decreases. The inner-layer deposit has higher wax contents, so the yield stress of the inner-layer deposit has a larger increasing rate as temperature decreases. When temperature decreases from 40 °C to 30 °C, solid wax concentrations of the outer-layer deposit and the inner-layer deposit increase almost equally, but the increasing rates of yield stress are much different: 64.7% for the outer 2 and 180.6% for the inner 2. If temperature of the bulk flow in pipeline drops due to operation requirements, the yield stress gradient in wax deposits along the radial direction becomes larger. The outer-layer deposit would be easier to break down which may reduce the pigging efficiency.

3.2. Comparison of natural wax deposits and model wax deposits. To illustrate the structural differences between natural wax deposits and model wax deposits with the same composition, the original samples of pipeline 1 were heated until completely melted and then cooled again in the way that used in previous researches.19,31,32 Yield stresses were tested and microscopic pictures were taken for both the original sample and the reformed sample. The results for the yield stress are shown in Table 6. It can be seen that the yield stress increases by 5~13 times after the original sample has gone through the thermo-recycling process. Differences in both macroscopic and microscopic structures may be used as explanations to this result.

3.2.1. Macroscopic observation. Figure 12 compares the pictures of the original samples, the reformed wax sample and the wax deposits formed in a cold finger test. 18 ACS Paragon Plus Environment

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It is obvious that the original sample has a loose structure with high porosity and a rough surface, whereas the reformed wax sample has a compact structure and a smooth surface. The wax deposit formed in the cold finger test (wax content 20%) also has a loose structure. The yield stress measured by vane method could be calculated as:53 0 =

2M 0 H 1 1 (  )  D3 D 3

where σ0 is the yield stress, M0 is the peak torque. The correlation above indicates that yield stress increases as the torque acting on vane increases. And the torque increases with the creep angle that the vane has rotated in the wax deposits.54 Creep angle could be higher in the sample with looser structure under the same stress due to its high porosity. Natural wax deposits with looser structures are easier to yield. Therefore, the reformed wax sample has a higher yield stress than the original sample.

3.2.2. Microscopic observation. Aside from the macroscopic appearance, the microscopic structures of these wax deposits are also different, since they underwent different thermal and shear histories. The original samples were formed under the shear flow of the oil, while the reformed sample formed during quiescently cooling. The network of wax crystals that forms under quiescent condition is mainly constructed by continuous lamellas, while more individual discs would be observed in the wax deposits which formed under flow because of shearing disturbance.40 The morphology of wax crystals of the original and the reformed sample at 30 °C were studied (Figure 13) and the parameters of wax crystals are listed in Table 7. Wax crystals of the reformed sample turn out to be larger, more plate-like and more 19 ACS Paragon Plus Environment

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complex. The ‘‘Anchoring’’ interactions between crystals in the reformed wax sample are much stronger. Consequently, the reformed sample has a higher yield stress than the original sample, which is consistent with Venkatesan’s work.38 The average size of wax crystals in original samples is within the range of 30~40 μm2 and the aspect ratio is within the range of 2~3, which are consistent with Soedarmo’s work33,34 that studied wax deposits formed on the cold finger and in the flow loop.

3.2.3. Oilfield application. The results of the rheometric studies have some implications for the field operations of crude oil pipelines. A correlation between the pressure for wax removal and yield stress of wax deposits has been obtained.31,32 So it’s important to test the yield stress of wax deposits in field. On the other hand, when wax deposits break into particles during pigging, the size of particles mainly depends on the yield stress. The size of particles affects the viscosity and pressure drop of slurry greatly.55 According to the experiment results above, pressures required to remove wax deposits are over estimated based on the experimental yield stress when model wax deposits are used. Apart from pigging operation, melting wax deposits by heating is also considered as an effective method to remove wax deposits.56 If the melting wax are not removed effectively, the remaining deposits would form a much stronger structure as temperature falls, which would do harm to the next pigging operation.

4. CONCLUSIONS

We designed an apparatus with vane geometry to test the yield behavior of wax 20 ACS Paragon Plus Environment

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deposits collected from the different radial position of field pipeline. The results clearly demonstrate the feasibility of the designed apparatus as an effective way for the measurement of the yield stress of wax deposits. The yield stress of natural wax deposits increases exponentially with the concentration of solid wax when temperature decreases. The average carbon number and the yield stress of the natural wax deposits increases along the radial direction (from the center to the inner surface of the pipe), while the gradients would become larger at lower temperatures. Both of them help understand the distribution of yield stress of wax deposits along the radial direction in field pipeline, based on which a few suggestions are given for pigging operation.

Average area, aspect ratio and boundary fractal dimension of wax crystals are chosen to describe the structure of wax crystals in the natural wax deposits. These parameters for the natural wax deposits are within the ranges of 30~40 μm2, 2~3, 1.10~1.30, respectively, which are consistent with the results of wax deposits that form on the cold finger and in the flow loop.

The yield stress of natural wax deposit increases by 5-13 times after the deposit being heated and cooled to the same temperature. This finding indicates that previous estimation of the pressure required to remove wax deposits using reformed sample might be overrated.

AUTHOR INFORMATION Corresponding Author 21 ACS Paragon Plus Environment

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*To whom correspondence should be addressed. Tel: +86-10-8973-4627. Fax: +86-10-8973-4627. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Support from the National Natural Science Foundation of China (Grant No. 51534007, 51134006) is greatly acknowledged. ASSOCIATED CONTENT Supporting information Operation parameters of the field pipeline; Calculation to estimate the temperature of wax deposits in field.

REFERENCES (1) Svendsen, J. A. AIChE Journal 1993, 39 (8), 1377-1388. (2) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. AIChE Journal 2000, 46 (5), 1059-1074. (3) Venkatesan, R. The deposition and rheology of organic gels. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 2004. (4) Paso, K. Paraffin gelation kinetics. Ph.D. Thesis, University of Michigan, 2005. (5) Huang, Z.; Lee, H. S.; Senra, M.; Fogler H. AICHE J. 2011, 57 (11), 2955-2964. (6) Sarica, C.; Panacharoensawad, E. Energy Fuels 2012, 26 (7), 3968-3978. (7) Lu, Y.; Huang, Z.; Hoffmann, R.; Amundsen, L.; Fogler, H. S. Energy Fuels 2012, 26 (7), 4091-4097.


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(8) Zheng, S; Zhang, F, Huang Z; Fogler, H. S. Energy Fuels 2013, 27 (12), 7379-7288. (9) Zheng, S; Fogler, H. S.; Akbari, A. H. AICHE J. 2017, 63 (9), 4201-4213. (10) Singh, A.; Panacharoensawad, E.; Sarica, C. Energy Fuels 2017, 31 (3), 2457-2478. (11) Chang, C.; Boger, D. V.; Nguyen, Q. D. Ind. Eng. Chem. Res. 1998, 37 (4), 1551-1559. (12) Stokes, J. R.; Telford, J. H. J. Non-Newtonian Fluid Mech. 2004, 124, 137-146. (13) de Souza Mendes, P. R. Soft Matter 2011, 7 (6), 2471-2483. (14) Teng, H.; Zhang, J. Ind. Eng. Chem. Res. 2013, 52 (23), 8079-8089. (15) Malkin, A.; Kulichikhin, V.; Ilyin, S. Rheo. Acta 2017, 56 (3) 177-188. (16) Dinkgreve, M.; Paredes, M.; Denn, M.; Bonn, D. J. Non-Newtonian Fluid Mech. 2016, 238, 233-241. (17) Fraggedakis, D.; Dimakopoulos, Y.; Tsamopoulos, J. J. Non-Newtonian Fluid Mech. 2017, 145, 1-14. (18) Fernandes, R. R.; Andrade, D. E. V.; Franco, A. T.; Negrão, C. O. R. Journal of Rheology 2017, 61, 893-903. (19) Mendes, P. R. S.; Braga, A. M. B.; Azevedo, L. F. A.; Corra, K. S. Journal of Energy Resources Technology 1999, 121, 167-171. (20) Wang, Z. Mechanical response characteristics of the paraffin deposit in pipelines. PhD. Thesis, China University of Petroleum, EastChina, 2008 (in Chinese). (21) Bai, C.; Zhang, J. Energy Fuels 2013, 27, 752-759. (22) Yan, J.; James, A. E. J. Non-Newtonian Fluid Mech. 1997, 70 (3), 237-253. (23) Nguyen, Q. D.; Boger, D. V. Journal of Rheology 1985, 29, 335-347. (24) Yoshimura, A. S.; Prud’homme, R. K.; Princen, H. M.; Kiss, A. D. Journal of Rheology 1987, 31, 699-710. (25) Jervis, S.; Campbell, R.; Wojciechowski, K. L.; Foegeding, E. A.; Drake, M. A.; Barbano, D. M. Journal of Dairy Science 2012, 95 (6), 2848-2862. (26) Bey, H. B.; Baumann, R.; Schmitz, M.; Radler, M.; Radler, M.; Roussel, N. Cement and concrete research 2015, 76, 98-106. (27) Massoussi, N.; Keita, E.; Roussel, N. Cement and concrete research 2017, 95, 108-116. (28) Assaad, J. J.; Harb, J. Maalouf, Y. Journal of Non-Newtonian Fluid Mechanics 2016, 230, 31-42. (29) Stokes, J. R.; Telford, J. H. J. Non-Newtonian Fluid Mech. 2004, 124, 137-146. (30) Saak, A. W.; Jennings, H. M.; Shah, S. P. Cement and concrete research 2001, 31 (2), 205-212.


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(31) Wang, Q.; Sarica, C. Transactions of ASME 2005, 127 (9), 302-309. (32) Wang, Q.; Sarica, C.; Volk, M. Journal of Energy Resources Technology 2008, 130 (9), 043001-043005. (33) Soedarmo, A. A.; Daraboina, N.; Lee, H. S.; Sarica, C. Energy Fuels 2016, 30 (2), 954-961. (34) Soedarmo, A. A.; Daraboina, N.; Sarica, C. Energy Fuels 2016 30 (4), 2674-2686. (35) Imai, T.; Nakamura, K.; Shibata, M. Colloids Surf. A 2001, 194, 233−237. (36) Guo, X.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K.; Adamson, D. H.; Fetters, L. J. Energy Fuels 2004, 18, 930−937. (37) Tinsley, J. F.; Jahnke, J. P.; Dettman, H. D.; Prud’home, R. K. Energy Fuels 2009, 23, 2056−2064. (38) Venkatesan, R.; Nagarajan, N. R.; Paso, K.; Yi, Y.-B.; Sastry, A. M.; Fogler, H. S. Chem. Eng. Sci. 2005, 60, 3587−3598. (39) Visintin, R. F. G.; Lapasin, R.; Vignati, E.; D’Antona, P.; Lockhart, T. P. Langmuir 2005, 21, 6240−6249. (40) Kané, M.; Djabourov, M.; Volle, J. L.; Lechaire, J. P.; Frebourg, G. Fuel 2003, 82, 127−135. (41) Briggs, J. L.; Steffe, J. F.; Ustunol, Z. J. Dairy Science 1996, 79, 527-631. (42) Leong, Y. K.; Scales, P. J.; Healy, T. W.; Boger, D. V. J. AM Ceram Soc. 1995, 78, 2209-2213. (43) Anthony, D. S.; Kumar, A.; Kusuma, T. E.; Scales, P. J.; Tindley, A.; Biggs, S.; Buscall, R. Rheology Acta 2015, 54, 337-352. (44) Laskar, A. I.; Bhattacharjee, R. Construction and Building Materials 2011, 25, 3443-3449. (45) Joseph, J. A.; Jacques, H.; Yara, M. J. Non-Newtonian Fluid Mech. 2014, 214, 17-18. (46) Wettimuny, R.; Penumadu, D. Particle & Particle Systems Characterization 2003, 20 (1), 18-24. (47) Anderman, N. J.; Meeten, G. H.; Sherwood, J. D. J. Non-Newtonian Fluid Mech. 1991, 39 (3), 291-310. (48) Singh, P.; Youyen, A.; Fogler, H. S. AIChE Journal 2001, 47 (9), 2111-2124. (49) Aiyejina, A. International Journal of Multiphase Flow 2011, 37, 671-694. (50) Oh, K.; Deo, M. Energy Fuels 2009, 23 (3), 1289-1293. (51) Bai, C.; Zhang, J. Ind. Eng. Chem. Res. 2013, 52 (7), 2732-2739. (52) Paso, K.; Senra, M.; Yi, Y.; Sastry, A. M.; Fogler, H. S. Ind. Eng. Chem. Res. 2005, 44 (18), 7242-7254. (53) Barnes, H. A.; Nguyen, D. J. Non-Newtonian Fluid Mech. 2001, 98, 1-14. (54) Petra, V. L.; Boger, D. V. J. Non-Newtonian Fluid Mech. 1996, 63, 235-261.


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Figures:

Figure 1. Illustration of the experiment apparatus based on rheometer with vane geometry.

Figure 2. Yield stresses of waxy crude oil (gelation temperature is 37 °C) at different temperatures tested by three measuring systems: vane, grooved coaxial cylinder, smooth coaxial cylinder.


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Figure 3. Comparing yield stresses of waxy crude oil (gelation temperature is 31 °C) formed both before and after vane is inserted at different temperatures.

Figure 4. A typical strain-stress curve of wax deposits when shear stress increases linearly.


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Figure 5. Illustration of two gradients in wax deposits in the field pipeline.

Figure 6. Tested strain-stress curves of the original samples of natural wax deposits from the outer layer of pipeline 1 at different temperatures.


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Figure 7. Solid wax precipitation curves of the wax deposits and of the bulk crude oil at different temperatures tested by DSC.

Stage 2 Stage 1

Figure 8. Yield stress of wax deposits as a function of solid wax concentration together with two phases of wax crystals in wax deposits.


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Figure 9. Tested flow curves of wax deposits at different radial positions of pipelines under temperature of 30 °C.

Figure 10. Images of wax crystals in original samples taken at 30 °C: (a) outer 1; (b) inner 1; (c) outer 2; (d) inner 2.


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Figure 11. Yield stress of natural wax deposits at different radial positions in the field pipelines (Yield stress of the inner-layer deposit in pipeline 1 at 20 °C is absent for material is not enough).

.

Figure 12. Photographs of wax deposits: (a) original sample of outer 1 in field; (b) reformed sample of outer 1; (c) original sample of inner 1 in field; (d) wax deposit on the cold finger.


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Figure 13. Images of wax crystals in deposits of pipeline 1 taken at 30 °C: (a) original sample of outer 1; (b) reformed sample of outer 1; (c) original sample of inner 1; (d) reformed sample of inner 1.

Tables:

Table 1. Compositions of Wax deposits and waxy crude oil in field pipeline Sample

Compositions (wt%) Asphaltenes

Resins

Wax

Outer 1 Inner 1 Bulk oil flow 1

2.51 2.54 2.22

9.65 9.62 7.92

38.60 39.47 24.53

Outer 2 Inner 2 Mixture Bulk oil flow 2

2.34 2.42 2.30 2.15

9.78 9.63 8.36 7.83

39.32 40.57 28.30 25.21


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Table 2. Critical temperatures for wax deposits in outer and inner layers Physical parameters Sample

WAT (°C)

Wax precipitation peak (°C)

Wax complete melting Temperature (°C)

Outer 1 Inner 1 Bulk oil flow 1

68.7 70.2 43

64.6 65.1 25

77.1 78.5 /

Outer 2 Inner 2 Mixture Bulk oil flow

72.0 73.9 62.2 44

64.1 66.8 55.3 26

76.3 77.2 65.2 /

Table 3. Parameters of wax crystals in the outer 1 at different temperatures Area of wax crystal Total

Temperature

count

(μm2)

Aspect ratio

Boundary box fractal

Average

Variance

Average

Variance

dimension

40 °C

3013

30.1

25.42

2.8

0.96

1.03

35 °C

3121

31.2

23.21

2.7

0.85

1.05

30 °C

3216

32.2

26.50

2.6

0.92

1.11

25 °C

3234

34.5

19.63

2.3

0.73

1.17

20 °C

3245

36.2

18.52

2.0

0.89

1.20

Table 4. Carbon number distribution in wax deposits of different radial positions in pipelines Carbon

Heavy oil

Distillate oil

Kerosene

Diesel

Gasoline

(C25-C90)

(C19-C25)

(C14-C18)

(C12-C14)

(C9-C12)

5.78

50.89

19.98

5.29

13.99

3.21

0.67

6.31

53.18

23.26

3.56

10.78

2.22

Outer 2

1.10

4.66

35.32

25.06

8.33

20.63

4.89

Inner 2

0.89

4.87

47.98

20.44

6.25

15.94

3.63

C90

Outer 1

0.87

Inner 1

number


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Table 5. Parameters of wax crystals in the original samples of wax deposits obtained from different radial positions (30 °C) Area of wax crystal Sample

(μm )

Total count

Boundary

Aspect ratio

2

box fractal

Average

Variance

Average

Variance

dimension

Outer 1

3216

32.2

26.50

2.6

0.92

1.11

Inner 1

4130

35.3

28.54

2.2

0.83

1.25

Outer 2

3342

31.5

24.32

2.9

0.72

1.10

Inner 2

4263

36.7

13.57

2.4

0.53

1.26

Table 6. Measured yield stress for the original sample of wax deposits in pipeline 1 and reformed sample T

τ

Sample Original

(°C)

(MPa)

19.6

4.77

Outer

T

ratioa

τ

(°C)

(MPa)

29.1

2.00

13.01 Reformed

19.8

61.98

Original

/

/

Inner /

/

τ

(°C)

(MPa)

39.0

1.03

39.4

6.69

39.1

1.50

39.4

7.35

ratio

13.39 29.5

26.80

29.2

2.37

/ Reformed

T ratio

6.49

12.83 29.8

30.40

4.91

a is the ratio of the yield stress of the reformed wax sample to that of the original sample of natural wax deposits.


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Table 7. Microscopic parameters of wax crystals in the original sample of wax deposits and the reformed sample together with model wax deposits that form on cold finger (30 °C) Area of wax crystal Sample

Original outer 1 Original inner 1 Reformed outer 1 Reformed inner 1

Total count

(μm ) 2

Aspect ratio

Boundary box fractal

Average

Variance

Average

Variance

dimension

3216

32.2

26.50

2.6

0.92

1.11

4130

35.3

28.54

2.2

0.83

1.25

2710

40.4

20.32

1.8

0.42

1.20

3293

46.2

19.26

1.6

0.45

1.58

/

20~40

/

2~3

/

/

Deposits on cold finger21


Experimental Study on the Strength of Original Samples of Wax

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