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Study on the Effect of Dispersed and Aggregated Asphaltene on Wax Crystallization, Gelation, and Flow Behavior of Crude Oil Yun Lei, Shanpeng Han, Jinjun Zhang,* Youquan Bao, Zhiwei Yao, and Ya’nan Xu National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing 102249, China S Supporting Information *

ABSTRACT: Asphaltene can exist in both the dispersed state and the aggregated state in crude oil. Because of the changes in crude oil composition, pressure, or temperature, the asphaltene transition from dispersed asphaltene to aggregated asphaltene will occur and then influence the wax crystallization, gelation, and flow behavior of crude oil. In this paper, the asphaltene transition was realized by mixing two different crude oils for different times. The aggregated asphaltene was characterized by the optical microscopy and centrifugation-based separation method. The effects of asphaltene transition on wax crystallization, gelation, and flow behavior of crude oil were investigated by differential scanning calorimetry and rheological measurements. The results show that the aggregated asphaltene can serve as a crystal nucleus for wax molecules, promoting the wax precipitation, weakening the strength of the network of wax crystals, and delaying the gelation process of crude oil. On the other hand, the dispersed asphaltene can serve as the connecting point between wax crystals, accelerating the gelation of crude oil, and increasing the gel strength. The viscosity measurements below the wax appearance temperature show that the viscosity of crude oil increases because of the interaction between aggregated asphaltene and wax.

1. INTRODUCTION Crude oil is a complex mixture primarily consisting of paraffins, aromatics, naphthenes, asphaltenes, and resins. From the perspective of flow assurance, the precipitation and deposition of waxes and asphaltene during the production, transportation, and processing of petroleum fluids are serious problems in the petroleum industry. When the temperature is higher than the WAT (wax appearance temperature), the wax molecules can be dissolved in crude oil. The crude oil behaves as a Newtonian fluid. When the oil temperature is lower than the WAT, the wax molecules precipitate. With temperature decreasing, the amount of precipitated wax increases. The precipitated wax crystals grow larger and suspend in crude oil, causing the transition of crude oil from Newtonian fluid to non-Newtonian fluid and exhibiting complex behaviors, such as pseudoplasticity,1 thixotropy,2,3 and viscoelasticity.4,5 When the amount of the precipitated wax reaches 2−3 wt %, a spongy network structure of wax crystals forms, turning the crude oil into a gel.6,7 Asphaltene is the heaviest component in crude oil, defined as the fraction that can precipitate in light n-alkanes (e.g., nheptane) but completely dissolve in toluene.8 In most cases, asphaltenes are dispersed as nanosized particles rather than dissolved in crude oil.9−13 A stable equilibrium between the dispersed asphaltene and the other components exists in crude oil.14 When the equilibrium is broken, the dispersed asphaltene can aggregate to become aggregated asphaltene with a bigger size and density.15−19 Nowadays, the role of asphaltene during wax crystallization and gelation of crude oil has not been comprehensively explained. Kriz et al.20 reported a dramatic increase in WAT and yield stress of model oil when the concentration of asphaltene was decreased from 0.02 to 0.01 wt %. Chanda et al.21 found that the asphaltene improved the flow © 2014 American Chemical Society

ability of waxy crude oils, and the dispersion degree of asphaltene was identified as an important factor. Garcia et al.22 also proposed that the flocculated asphaltene can serve as nucleation sites for wax crystallization, increasing the WAT in reconstituted oil. Tinsley23,24 reported that the addition of asphaltene at low concentrations decreased both the WAT and the gelation temperature of model waxy oil. When the concentration of asphaltene became higher, the WAT was increased and yield stress was greatly reduced. However, the previous studies mostly take the asphaltene as a single state. The roles of the dispersed and aggregated asphaltene on wax crystallization, gelation, and flow behavior of crude oil have been few separately investigated. In the petroleum industry, mixing crude oils from different blocks or oil fields is very common for the convenience in transportation. If the mixed crude oils are not compatible with each other, mixing can upset the stability of asphaltene in crude oil.25−29 Once the stable equilibrium is broken, the dispersed asphaltene may join together to become the aggregated asphaltene. This transition can influence the wax crystallization, gelation, and flow behaviors of crude oil. In comparison, the previous research methods, such as adding asphaltene or precipitant into a crude oil or model oil, cannot adequately simulate the real asphaltene transition that occurred in crude oil. Therefore, it is of practical importance to investigate the asphaltene transition from the dispersed asphaltene to the aggregated asphaltene that occurred during mixing incompatible crude oils. Received: November 16, 2013 Revised: March 17, 2014 Published: March 20, 2014 2314

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The purposes of this work are twofold. The first is to investigate the asphaltene transition from dispersed asphaltene to aggregated asphaltene by mixing two different crude oils. Then, the kinetics of asphaltene transition is investigated by measuring the amount of the aggregated asphaltene at different time periods after mixing two different crude oils. The second is to separately characterize the effects of the dispersed and aggregated asphaltene on wax crystallization, gelation, and flow behavior of crude oil. First, by measuring the changes in the wax precipitation, gelation temperature, storage modulus, and viscosity of a crude oil mixture, the role of the aggregated asphaltene is investigated. Second, the aggregated asphaltene is removed using the centrifugation-based separation method, and the dispersed asphaltene is retained in the remaining oil. By measuring the gelation temperature and storage modulus of the remaining oil, the effects of the dispersed asphaltene on gelation of crude oil are obtained.

measurements, the shear and thermal histories of crude oils were removed by heating the samples at 80 °C for 2 h, then cooling them quiescently, and holding them at ambient temperature for 48 h before measurements.1 2.2. Experimental Procedures. Figure 1 shows the schematic diagram of the experimental procedures. A crude oil mixture (900 g) was prepared at 1.01 × 105 Pa and 40 °C by adding BJ crude oil into TH crude oil at the mass ratio of BJ/TH = 4:1. For the kinetic experiments of the asphaltene transition, the time when the two crude oils were mixed together for 20 min was set as the beginning time, i.e., t = 0 h. The WAT of the crude oil mixture at t = 0 h was measured to be 19.60 °C. At t = 0 h, the crude oil mixture was subpackaged into four sealed bottles. To investigate the kinetics of the asphaltene transition, the oil samples in these sealed bottles were continuously stirred using magnetic stirrers at the temperature of 40 °C controlled by a water bath. After different time periods of 1, 12, 24, and 48 h, the crude oil mixture was taken out from the sealed bottle for further measurements. 2.2.1. Quantification of the Transition from the Dispersed Asphaltene to the Aggregated Asphaltene. The asphaltene transition from the dispersed asphaltene to the aggregated asphaltene was quantified using the centrifugation-based separation technique. Following are the major procedures: (1) After a certain duration time, the crude oil mixture was centrifuged at 40 °C and 11 000 rpm (relative centrifugal force of 12 844 g) for 30 min. (2) The remaining liquid oil was retained for the following experiments, and the centrifugal cake was washed using approximately 20 mL of n-heptane, and then centrifuged for another 15 min at 40 °C and 11 000 rpm. The washing and centrifuging procedures were repeated several times until the supernatant became almost colorless. (3) The washed centrifugal cake, i.e., the aggregated asphaltene, was dried in a −5.00 × 104 Pa vacuum oven at 120 °C for 2 days until no significant changes in weight. The amount of the aggregated asphaltene was reported as mass fraction. For the purpose of ensuring the accuracy of experimental data, another two repeated experiments were conducted. The reported results were the average values of the three measurements. The three repeated measurements show that the experimental error is lower than 0.02 wt %. 2.2.2. WAT and Precipitated Wax Measurements. The wax crystallization that occurred in the crude oil mixture was performed using a DSC apparatus (differential scanning calorimeter, TA Q20). The DSC cell is purged using nitrogen at a rate of 50 mL/min and cooled using a compressor refrigerator cooling accessory (RCS 40/ 90). The calibration of temperature and heat flow of the equipment was carried out by measuring the melting point temperature of highpurity metal indium (the melting point of 156.61 °C; the heat of melting of 28.42 J/g). For the DSC measurements of wax crystallization, an aluminum crucible was used to seal around 4−8 mg of crude oil to assemble the specimen. Experiments were carried

2. EXPERIMENTAL SECTION 2.1. Crude Oils. Two chemical-free crude oils, BJ crude oil and TH crude oil, are obtained from a long-distance crude oil pipeline, Wulumuqi-Shanshan pipeline in Northwest China. Before the crude oils were allowed to enter the pipeline, the dewatering and solid removals were conducted. The composition (SARA analysis, ASTM D 2007-80), density, WAT, and viscosity of the BJ and TH oil samples are shown in Table 1. For a better repeatability of rheological

Table 1. Properties of Crude Oils Used in This Work density @ 20 °C (g·cm−3) WAT (°C) viscosity (mPa·s) 30 °C 40 °C SARA analysis (wt %)a saturate aromatic resin nC7-Asphaltene

BJ crude oil

TH crude oil

0.859 19.43

0.844 23.27

124.44 71.25

24.18 17.26

87.0 10.2 0.5 2.3

82.9 15.3 0.7 1.1

a

The measurement error of asphaltene content is within 5%, and the measurement errors of saturate, aromatic, and resin contents are within 10%.

Figure 1. Schematic diagram of the experimental procedure. 2315

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out by heating the crude oil to 80 °C and keeping it at the temperature for 1 min, and then cooling it to −20 °C at the two cooling rates of 5 and 1.5 °C/min. The measurement was repeated three times for each crude oil sample. The WAT was determined as the onset of the exothermal peak in the cooling process corresponding to the liquid−solid phase transition. The total heat released during the cooling process is proportional to the area between the baseline and the exothermal peak, and it was calculated from the integration of the heat flow curve. Some researchers30−33 have recommended the enthalpy of wax precipitation to be 210 J/g for crude oils with unknown wax compositions. In this work, the value of 210 J/g for wax precipitation enthalpy was applied, and the amount of precipitated wax at a certain temperature can be calculated by dividing the released heat by 210 J/g. In addition, the cumulative concentration of precipitated wax from the WAT to a certain temperature can be obtained. The experimental error of these repeated measurements of precipitated wax concentration is lower than 4% relative. 2.2.3. Wax Crystal Measurements. The optical microscopy was used to observe the change in the morphology of wax crystals caused by the asphaltene transition from dispersed asphaltene to aggregated asphaltene. The experimental procedure is as follows: A drop of the crude oil mixture was first transferred onto a preheated microscope slide at 40 °C. The oil sample was quickly spread using a cell scraper on the glass slide, and then covered using another glass slide. Using the Linkam PE60 Peltier thermal stage, the slide covered oil sample was subsequently cooled to 15 °C to get the wax precipitated. The wax crystals were observed using a Nikon OPTIPHOT2-POL polarizing microscope equipped with a 10× objective lens and a 10× eyepiece and photographed with a CoolSNAP 3.3 M digital charge-coupled device (CCD) color camera (Roper Scientific, Inc., Sarasota, FL) connected to a computer. The software ImageJ (from Wayne Rasband, at the Research Services Branch, National Institute of Mental Health, Rockville, MD) was used to analyze the digital photos and extract the average area of wax crystals. In the digital photos, the wax crystals show a significantly different color from the background of liquid crude oil. The border of wax crystals can be captured using ImageJ. The boxing-counting method is used for extracting the average area of wax crystals. For more details about the determination of the average area, one may refer to ref 34 by Gao et al. In this work, 15 high-quality micrographs (2048 × 1536 pixels) were obtained for each sample to ensure the precision and accuracy of the measurements. The second repeated experiment was also done. Hence, 30 average areas of wax crystals were obtained in 30 images. The average value of these 30 ones was used to represent the average area of wax crystals. 2.2.4. Gelation Temperature and Storage Modulus Measurements. To characterize the gelation process of the crude oil mixture, the gelation temperature and storage modulus were measured using a HAAKE RS-150H rheometer equipped with the coaxial cylinder sensor system Z41Ti. The experimental procedures are as follows: (1) preheat the test segment of the rheometer to 40 °C for 5 min; (2) load the specimen (∼12 mL) into the rheometer; (3) keep the specimen isothermally for 15 min; (4) cool the specimen at a rate of 0.5 °C/min; and (5) shear the specimen at a strain of 0.0005 ± 0.0001 and a scanning frequency of 0.5 Hz during the cooling procedure. In this work, the storage modulus (G′) and loss modulus (G″) were recorded during the cooling procedure, and the gelation temperature is taken as the temperature at which G′ crosses above G″. For ensuring the accuracy of experimental data, another two repeated experiments were conducted by using the same procedure. The reported results were the average values of the three experiments. The accuracy of these measurements of gelation temperature was calculated to be about ±0.5 °C. 2.2.5. Viscosity Measurements. The viscosity of the crude oil mixture was measured by an Anton Paar RheolabQC rheometer equipped with a C-LTD80/QC temperature control unit and a concentric cylinder sensor. The experimental procedures are as follows: (1) preheat the test segment of the rheometer to 40 °C for 5 min; (2) load the specimen (15−20 mL) into the rheometer; (3)

cool the specimen to the measured temperature at a rate of 0.5 °C/ min; (4) keep the specimen isothermally for 25 min; and (5) shear the specimen with shear rates in the range of 20−250 s−1 for 15 min at each shear rate until equilibrium. In this work, the flow curves are plotted as apparent viscosity vs shear rate. Another two repeated experiments were conducted under the same procedure conditions. The experimental error for these three repeated measurements is lower than 5% relative. The reported results were the average value of the three experiments.

3. RESULTS AND DISCUSSION 3.1. Quantifying the Asphaltene Transition from Dispersed Asphaltene to Aggregated Asphaltene. The asphaltene transition from the dispersed asphaltene to the aggregated asphaltene in crude oil is a kinetic phenomenon.35 In this work, the asphaltene transition was characterized using the microscopy observation and the centrifugation-based separation technique. Figures 2 and 3 show the microscopic

Figure 2. Micrographs of the asphaltene (black points) at 40 °C for crude oil mixtures at different duration times.

images and asphaltene sizes in crude oil mixtures at 1, 12, 24, and 48 h after BJ and TH crude oils were mixed. Note that the

Figure 3. Size distributions of the asphaltene particle in crude oil mixtures at different duration times. 2316

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microscopic observation was taken at 40 °C (20.4 °C higher than WAT) to avoid the precipitated wax existing in the observation area, and the asphaltene size was obtained from asphaltene images using the Nano Measurer 1.2 software. The details about the asphaltene size are shown in the Supporting Information. For the purpose of ensuring the measurement accuracy of asphaltene size, 15 high-quality micrographs (2048 × 1536 pixels) were captured for each sample. As can be seen in Figures 2 and 3, at t = 1 h, few asphaltene particles can be captured, with the largest size close to 1.80 μm and an average size of 0.70 μm. At t = 12 h, the largest asphaltene is close to 3.00 μm, and the average size increases to 1.00 μm. From 12 to 24 h, the number of detectable particles increases with an increase in the particle size as well. At t = 24 h, the average size increases to 1.25 μm, and the largest size increases to 4.20 μm. Finally, at t = 48 h, the number of detectable particles has a very slight increase with a slight increase in the particle size as well. Centrifugation-based separation experiments provide further insight into the kinetics of asphaltene transition. Figure 4 shows

Figure 5. Amount of the dispersed asphaltene as a function of duration time for crude oil mixture.

Figure 6. Variation of the WATs for crude oil mixtures at different duration times.

among these four WATs is 0.16 °C. Similarly, the results at the cooling rate of 1.5 °C/min show a similar change trend, and the maximum difference for WAT measurements is 0.94 °C. It can be concluded from these results that the phase transition point (i.e., WAT) of wax molecules is not affected by the asphaltene transition. The cumulative concentrations of precipitated wax in the crude oil mixture at various duration times were obtained by integrating the heat flow curve from the WAT to −20 °C. Figure 7 shows the results at the cooling rates of 5 and 1.5 °C/

Figure 4. Amount of the aggregated asphaltene as a function of duration time for crude oil mixture.

the amount of the aggregated asphaltene at different duration times. As seen in Figure 4, at t = 0 h, the amount of aggregated asphaltene is close to 0.02 wt %. At 1 h after mixing BJ and TH crude oils, the amount of the aggregated asphaltene is 0.14 wt %. After a time duration of 24 h, the amount of the aggregated asphaltene increases to 0.24 wt %. The amount of the aggregated asphaltene maintains at a plateau of 0.24 wt % after 24 h. According to the ASTM D2007-80, the total asphaltene content is 2.05 wt % in the crude oil mixture. The amount of the dispersed asphaltene in the crude oil mixture at different times can be obtained by subtracting the amount of the aggregated asphaltene from the total asphaltene content. As shown in Figure 5, the amount of the dispersed asphaltene decreases from 2.03 to 1.81 wt % with time. 3.2. Effect of the Asphaltene Transition on the Phase Behaviors of Wax Molecules in Crude Oil Mixture. Corresponding to the asphaltene transition from dispersed asphaltene to aggregated asphaltene, the changes in the phase behaviors of wax molecules in crude oil mixtures were analyzed using the DSC measurements. Figure 6 shows the WATs of crude oil mixtures at the cooling rates of 5 and 1.5 °C/min. For the cooling rate of 5 °C/min shown in Figure 6, the WATs of the crude oil mixture at various duration times are 20.22, 20.17, 20.32, and 20.33 °C, respectively. The maximum difference

Figure 7. Variation of the cumulative concentration of precipitated wax (WAT to −20 °C) for crude oil mixtures at different duration times. 2317

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min. For the cooling rate of 5 °C/min shown in Figure 7, at t = 1 h, the cumulative concentration of precipitated wax is 3.24 wt %. At t = 12 h, t = 24 h, t = 48 h, the cumulative concentrations of precipitated wax increase to 3.94, 4.45, and 4.51 wt %, corresponding to the amount of the aggregated asphaltene increases from 0.14 to 0.20, 0.24, and 0.24 wt %, respectively. At the cooling rate of 1.5 °C/min, the cumulative concentration of precipitated wax also increases when the amount of the aggregated asphaltene increases. In other words, the asphaltene transition plays the major role of affecting the amount of the precipitated wax, and the effect of the cooling rate can be ignored. This can be explained from two possible aspects: First, the aggregated asphaltene can serve as a crystal nucleus for saturated wax molecules. When the temperature is below the WAT, the saturated wax molecules can easily precipitate from the crude oil mixture, increasing the amount of the precipitated wax. This has been indirectly demonstrated by the change in the average area of wax crystals shown in the latter part. Second, as proposed by Kriz and Andersen,20 the saturated wax molecules can exist in a “sub-cooled” state because of the spatial interference of the dispersed asphaltene. At the same amount, the dispersed asphaltene can occupy more space due to larger specific surface area than the aggregated asphaltene, resulting in having a stronger spatial interference for the crystallization of saturated wax molecules. When the asphaltene aggregation occurs, the amount of the dispersed asphaltene decreases. The influence of the spatial interference on wax precipitation is weakened. These supersaturated wax molecules can combine with each other to produce more wax crystals. 3.3. Effect of the Aggregated and Dispersed Asphaltene on Gelation Temperature and Storage Modulus of Crude Oil Mixture. When the temperature is lower than the WAT, more waxes precipitate in crude oil with temperature decreasing. When wax crystals interact to form a volume-spanning network, it can entrap the remaining liquid oil. When a network of wax crystals is formed, the gelation of waxy crude oil occurs. During the gelation process, the dispersed and aggregated asphaltene can be incorporated in the wax network, affecting the strength of the wax network.20,36−38 The gelation temperature and storage modulus can be changed consequently. Case 1. Only the Dispersed Asphaltene Shown in the Network of Wax Crystals. The effect of the dispersed asphaltene on the network strength of wax crystals was investigated by characterizing the gelation process of the remaining oil, where the aggregated asphaltene was removed from the crude oil mixture. Take the remaining oils at t = 1 h and t = 24 h as examples. In this case, the amount of the dispersed asphaltene in the remaining oil decreases from 1.91 to 1.81 wt %. As shown in Figure 8a, the gelation temperature decreases from 13.1 to 11.2 °C. The storage modulus at 10 °C also decreases from 38.62 to 2.13 Pa, as seen in Figure 9a. In other words, the capacity of wrapping the remaining liquid oil for the wax network is improved because of the dispersed asphaltene. It can be inferred that, if the asphaltenes are welldispersed in the waxy crude oil, when wax crystallization occurs, the dispersed asphaltenes serve as the connecting sites, helping the network of wax crystals occupy the total volume of waxy crude oil. The remaining liquid oil is subsequently entrained by the network. Case 2. Both the Dispersed and the Aggregated Asphaltene Shown in the Network of Wax Crystals. In this case, the gelation process of the crude oil mixture at t = 24 h

Figure 8. Variation of the gelation temperature for crude oil mixtures at different duration times: (a) with the aggregated asphaltene removed; (b) with the aggregated asphaltene.

Figure 9. Variation of the storage modulus G′ versus temperature for crude oil mixtures at different duration times: (a) with the aggregated asphaltene removed; (b) with the aggregated asphaltene.

was compared with the one at t = 1 h. In section 3.2, it has been found that, from t = 1 h to t = 24 h, the amount of the aggregated asphaltene increases from 0.14 to 0.24 wt %, and the amount of the dispersed asphaltene decreases from 1.91 to 1.81 wt %. Correspondingly, the amount of the precipitated wax from the WAT to −20 °C also increases from 3.24 to 4.45 wt %. In other words, for the crude oil mixture at t = 24 h, the 2318

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formation of a wax network potentially becomes easier because more wax precipitates. However, the gelation temperature and storage modulus decrease from t = 1 h to t = 24 h instead. For the gelation temperature in Figure 8b, it decreases from 11.4 to 9.6 °C. For the storage modulus in Figure 9b, it also decreases from 12.46 to 0.58 Pa at 10 °C. According to the results in “Case 1”, the decrease in the amount of the dispersed asphaltene can weaken the capacity of wrapping the remaining liquid oil for the network of wax crystals. In “Case 2”, compared with the results of “Case 1” at the same duration times, it provides a clue that the increase in the amount of the aggregated asphaltene can weaken the strength of the wax network. With decreasing the temperature, wax molecules tend to precipitate on the surface of the aggregated asphaltene (weak points) instead of precipitation on the surface of the wax crystals (strong points) to build a proper wax network. The aggregated asphaltene cannot tightly connect with wax crystals. Once the shear stress is applied, the network structure can break at the joints between the aggregated asphaltene and wax crystals. In summary, both the dispersed asphaltene and the aggregated asphaltene cooperate with wax crystals to form a network. The decrease in the amount of the dispersed asphaltene and the increase in the amount of the aggregated asphaltene can both weaken the network strength of wax crystals. Consequently, the gelation temperature and storage modulus become much lower. 3.4. Effect of the Aggregated Asphaltene on the Viscosity of Crude Oil Mixture. For the suspensions, the liquid viscosity can be changed by the volume concentration of the suspended phase. In this work, both the aggregated asphaltene and the wax crystals exist as the suspended phase. Figure 10 shows the apparent viscosities of crude oil mixtures at three different temperatures: T1 = 25 °C (higher than WAT), T2 = 12 °C, and T3 = 5 °C. It can be seen that, at T1 = 25 °C, a constant apparent viscosity is seen for crude oil mixtures at different amounts of the aggregated asphaltene. This constant apparent viscosity indicates that the tiny changes of the volume concentration of the suspended phase (aggregated asphaltene) have little influence on the liquid viscosity. When the temperature is lower than the WAT, the wax molecules precipitate, and the crude oil gradually transforms from Newtonian fluid to non-Newtonian fluid. At T2 = 12 °C, as shown in Figure 10b, the apparent viscosity at 50 s−1 is increased from 468.28 mPa·s at t = 1 h to 628.93 mPa·s at t = 24 h. At T3 = 5 °C, as shown in Figure 10c, the increased extent becomes larger, and the apparent viscosity of the mixed crude oil at t = 24 h (0.24 wt % aggregated asphaltene) is 37% larger than that of the mixed crude oil at t = 1 h (0.14 wt % aggregated asphaltene). As proposed by Li and Zhang,39 when the wax precipitation occurs, wax crystal clusters can entrap a significant amount of the liquid oil, leading to the increase in the apparent viscosity of waxy crude oil. In this work, because of the asphaltene transition in the crude oil mixture, the apparent viscosity is affected from two aspects: First, when the amount of the aggregated asphaltene increases, the amount of the precipitated wax from the WAT to −20 °C in the crude oil mixture increases from 3.24 to 4.45 wt %. At the same temperature, the amount of the suspended wax crystals increases. Second, the wax tends to form larger clusters with the aggregated asphaltene. These larger clusters can entrap more remaining liquid oils, increasing the apparent viscosity. These larger

Figure 10. Variation of the apparent viscosity versus temperature for crude oil mixtures at different duration times.

clusters are proved using the microscopic observation. Figures 11 and 12 show the microscopy images and average areas of wax crystals at various duration times for crude oil mixtures at

Figure 11. Micrographs of the wax crystals (white points) at 15 °C for crude oil mixtures at different duration times. 2319

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Article

ASSOCIATED CONTENT

* Supporting Information S

The main steps for determining the asphaltene size by the Nano Measurer 1.2 software. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-8973 4627. Fax: 86-10-8973 4627. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 12. Variation of the average area of wax crystals at 15 °C for crude oil mixtures before and after the aggregated asphaltene removed.

ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 51134006 and 51204194) and the Science Foundation of China University of Petroleum Beijing (Grant Nos. LLYJ201155 and KYJJ0406) for financial support.

15 °C, respectively. As seen in Figures 11 and 12, the average area of wax crystals increases from 69.3 to 86 μm2 when the amount of the aggregated asphaltene increases from 0.14 to 0.24 wt %. When the aggregated asphaltene is removed, the number of visible wax crystals is small, and the average areas of wax crystals are remarkably smaller.

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4. CONCLUSION The effect of the cooperation of asphaltene and wax on the flow assurance of waxy crude oil is an unknown area. In this work, the asphaltene transition from the dispersed asphaltene to the aggregated asphaltene is realized by mixing two crude oils for different duration times. The amount of the aggregated asphaltene gradually increases with the duration time and then reaches a plateau after 48 h. By measuring the phase behavior of wax molecules, gelation process, and viscosity of the crude oil with and without the aggregated asphaltene, the effects of the aggregated asphaltene and dispersed asphaltene on the wax crystallization, gelation, and flow behavior of crude oil are obtained. The asphaltene transition results in the increase in the amount of the aggregated asphaltene and the decrease in the amount of the dispersed asphaltene. The aggregated asphaltene can serve as a crystal nucleus for wax precipitation. For waxes, it is very easy to crystallize on the aggregated asphaltene and produce an unorganized asphaltene−wax composite rather than a proper wax network because they cannot be fully and tightly incorporated with the aggregated asphaltene. In addition, the dispersed asphaltene is well-dispersed in the crude oil mixture. For waxes, these dispersed asphaltenes can connect smaller wax crystals, helping to form a proper wax network to occupy the total volume of waxy crude oil. The decrease in the amount of the dispersed asphaltene also weakens the influence of the spatial interference on the wax precipitation, driving more wax molecules to precipitate. Both the decrease in the dispersed asphaltene and the increase in the aggregated asphaltene can weaken the strength of the wax crystal network, depressing the gelation temperature and storage modulus of crude oil gel. Additionally, because of the cooperation of the aggregated asphaltene and wax, more solid particles suspend in crude oil, and the viscosity of crude oil increases at temperatures below the WAT. 2320

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Energy & Fuels

Article

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