Effect of Emulsified Water on Asphaltene Instability in Crude Oils

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Effect of Emulsified Water on Asphaltene Instability in Crude Oils Mohammad Tavakkoli, Andrew Chen, Chi-An Sung, Kelly M. Kidder, Je Jin Lee, Saeed M. Alhassan, and Francisco M. Vargas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02180 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 11, 2016

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Effect of Emulsified Water on Asphaltene Instability in Crude Oils Mohammad Tavakkoli1, Andrew Chen1, Chi-An Sung1, Kelly M. Kidder1, Je Jin Lee1, Saeed M. Alhassan2, and Francisco M. Vargas1,* 1. Department of Chemical and Biomolecular Engineering, Rice University, Houston, USA 2. Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE Authors Email Address: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], and [email protected] Corresponding Author: email - [email protected], Phone - +1 (713) 348-2384 Abstract Understanding asphaltene precipitation and subsequent deposition during oil production is of great importance for the oil industry nowadays because of the potential risk associated to this heavy fraction in plugging wellbores and production equipment. Although water is commonly present in the produced fluids, because of instrument limitations and inadequate techniques, it is usually separated from the oil prior to any experimental analysis. Therefore, the effect of water on asphaltene stability and deposition tendency is not completely understood and the information available in the open literature is scarce. In this work, the effect of emulsified water on asphaltene instability in crude oil systems is investigated. Three crude oils and one bitumen sample were used in this study. The crude oils had API gravities ranging from 26 to 40 °API, and asphaltene content between 1.2 to 13 wt%. Model oils were also prepared with asphaltenes extracted from these crudes. A total of nine systems were investigated with and without the presence of emulsified water. It was found that for the crude oils from the Middle East and 1 ACS Paragon Plus Environment

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Canada and their corresponding model oils, the addition of water neither did have a significant effect on the detection of asphaltene precipitation nor the amount of precipitated asphaltenes. However, the stability of asphaltenes in the crude oil from the Gulf of Mexico and the model oils from the Athabasca bitumen (containing n-C5 and n-C7 asphaltenes) was significantly affected by the presence of water. The experimental evidence suggests that some asphaltenes are more prone to interact with water at the oil−water interface. This work provides a simple technique to screen whether water has an effect on asphaltene stability for a given crude oil at ambient pressure and different temperatures. With this study we aim to contribute to a better understanding of the interaction of water and asphaltenes in crude oil systems, which will eventually lead to the development of cost-effective strategies for the mitigation of this flow assurance problem. Keywords: Emulsified Water, Asphaltene Instability, Indirect Method. 1. Introduction Asphaltenes are a polydisperse mixture of the heaviest and most polarizable fraction of the crude oil1. They are defined according to their solubility properties as being soluble in aromatic solvents, but insoluble in light paraffin solvents. Asphaltenes are well-known for their tendency to precipitate and deposit during oil production because of changes in pressure, temperature, and composition. For studying asphaltene precipitation in the laboratory, usually the water is first removed from the crude oil sample, resulting in crude oil samples with no or very little water (typically around 0.5 wt%) being studied2. However, while producing the oil formation water also co-produces with the oil. In some cases, the water used for the oil recovery also co-produces with the oil. Therefore, water-in-oil emulsion formation is in many cases unavoidable during oil production from the reservoir.

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Several studies have shown that asphaltenes contribute to the stabilization of water-in-oil emulsions, because asphaltenes can form a viscoelastic interfacial layer around an emulsion3–5. The stabilization of emulsions can be attributed to the self-association of asphaltenes and the formation of a 3D network within organic films, which slows the coalescence of emulsified water droplets6. The thickness of the layer of asphaltenes at the water/oil interface increases with the concentration of asphaltenes in the system6,7. Moreover, previous research has shown that the stabilization of water-in-oil emulsions is related to the onset of precipitation8. Sztukowski et al.9 showed that near the onset of precipitation, asphaltenes have a strong role in stabilizing the emulsions. After the onset, a more rigid layer forms around the emulsion which is less effective in stabilizing the emulsion9. Before the onset, the layer of asphaltenes is flexible and contributes to the stabilization of water in oil emulsions9. There is strong evidence that only a fraction of asphaltenes contributes to the stabilization of water-in-oil emulsions. A major topic of interest is how the asphaltenes that are attracted to the interface and contribute to the stabilization of the emulsion differ from the total asphaltene content of the oil. This information could clarify the effects water has on overall asphaltene precipitation. Previous research suggests that asphaltenes attracted to the interface have a lower hydrogen-to-carbon atomic ratio therefore have a lower aromaticity2,10,11. Asphaltenes at the interface have also been shown to exhibit a higher polarity, stemming from a higher oxygen-tocarbon atomic ratio and a higher concentration of heavy metals. A higher polarity suggests increased attractive interactions between polar water molecules and the asphaltene2,11,12. Khadim et al.13 showed that asphaltenes at the interface exhibit less alkyl side chain lengths and reduced branching. Multiple sources have predicted that the asphaltenes that precipitate first (heavy fraction 3 ACS Paragon Plus Environment

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of asphaltenes) are likely the asphaltenes found on the interface14,15. In general, it is believed that asphaltenes which precipitate first have a higher heteroatom content, and asphaltenes with a higher heteroatom content are also more surface-active2. Therefore, asphaltenes that precipitate first are more likely to be found on the interface. There is disagreement with respect to the size of asphaltenes associated with the interface. Spiecker et al.15 asserted that asphaltenes with higher molecular weight and aggregate size are more likely to be found at the interface. However, this assertion has been contradicted by Sztukowski et al.7. They stated that large asphaltene aggregates with molar mass over 10,000 g/mol do not adsorb on the interface7. Sztukowski et al.7 speculated that cross-linked networks formed by smaller asphaltenes on the interface prevent the adsorption of larger asphaltenes. This could also be due to a slower diffusion rate of large asphaltenes to the interface or because the large asphaltene’s structure inhibits their ability to attach to the interface7. The focus of the works mentioned above was on investigating the interactions at the oilwater interface, not on the effects water has on overall asphaltene precipitation. Although the effect of solubilized water on asphaltene precipitation from crude oil systems has been already investigated by several researchers16–19, the effect of emulsified water on asphaltene instability has not yet been fully understood. Tharanivasan et al.2 used a centrifugation method to investigate the effect of emulsified water on asphaltene precipitation from crude oil systems. However, analyzing the deposited materials after centrifugation is a tedious and lengthy procedure, and because the amount of sediment is not very large, the experimental results might be prone to significant experimental error. Tharanivasan et al.2 concluded that the presence of emulsified water had no noticeable effect on the instability of asphaltenes in a crude oil above the onset of asphaltene precipitation. They noted that the adsorbed asphaltenes at the water/oil


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interface are separated when the water is settled during centrifugation and they reported these results as the yield below the precipitation onset2. However, from this work it is not clear whether the water emulsions have any effect on other components of the crude oil, such as resins, or if this is a general behavior observed in asphaltenes and/or crude oils from different sources. The difficulties in the investigation of emulsified water effect on asphaltene instability using the traditional techniques for determination of asphaltene precipitation could be a reason for having limited experimental data in this area. Most of these techniques, such as microscopy20–22, spectroscopy23–25, conductivity method26,27, refractive index measurements28, and interfacial tension method29, cannot be readily applied for measuring the amount of asphaltene precipitation. In addition, emulsified water may disturb the measurements and cause other difficulties; for instance, direct spectroscopy, even in the near-infrared (NIR) region, cannot be used because the signal is saturated in the presence of water droplets. In this work, a recently developed technique called “Indirect Method” is used for both detection and quantification of asphaltene precipitation30 in the presence of water-in-oil emulsions. The term “indirect” refers to an indirect detection of the precipitation of asphaltenes by measuring the absorbance of the supernatant fluid after centrifugation of oil/n-alkane mixtures. Because in this method all the water (along with any asphaltenes that might be adsorbed at the water/oil interface) will be separated during the centrifugation step, the NIR absorbance of the supernatant liquid can be readily obtained, in the same way it is done for an oil sample without emulsified water. In the present work, the indirect method was used to study the effect of emulsified water on the stability of asphaltenes in different crude oil samples. To isolate the effect of emulsified 5 ACS Paragon Plus Environment

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water on only the asphaltene fractions, model oils were prepared dissolving the extracted asphaltenes from different sources in toluene. Also, the effect of water on the behavior of the corresponding maltene fractions (i.e. the remaining hydrocarbon mixture after removing asphaltenes from the crude oil) is also studied in this work. The indirect method is performed at two different temperatures, 23°C and 70°C, to investigate the effect of the emulsified water on the stability of asphaltenes as a function of temperature. Because the precipitation of asphaltenes depends on both thermodynamic and kinetic phenomena, and to avoid confusion, the term “detection of asphaltene precipitation” will be used instead of “onset of asphaltene precipitation” in this work. The term “detection of asphaltene precipitation” represents the minimum amount of a precipitating agent, such as a normal alkane, that causes detectable precipitation of asphaltenes, in specific experimental conditions. 2. Experimental Procedure 2.1. Water in oil emulsion preparation Experiments were performed using three different crude oils: Crude oil CN from the Gulf of Mexico, crude oil SE, which is a Canadian crude oil, and crude oil S4 from the Middle East. Table 1 shows the properties of these crude oils at ambient conditions. The crude oils were first centrifuged to remove any sediments and sand particles or water present in the system prior to use in the experiments. The water content was then measured by a Metrohm Karl Fischer titration apparatus model 870 KF Titrino plus. Molecular weights reported in Table 1 were measured using Cryoscope Cryette WRTM which determines the molecular weight based on the change in the freezing point of a solvent, such as benzene, as the crude oil sample is added. For


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SARA (Saturates, Aromatics, Resins, and Asphaltenes) fractionation of the samples, the standard test method ASTM D2007-9831 was used and the amount of each fraction was determined. The centrifuged samples was used and water in oil emulsion was prepared by slowly adding the desired amount of deionized water to the sample, while the mixture was homogenized with a SCILOGEX D-160 homogenizer at approximately 20,000 rpm for 10 minutes. Deionized water was obtained from a Millipore Direct–Q3 water purification system. Table 1. Properties of the crude oils CN, SE, and S4 at 1 atm and 23 °C. Property

Crude Oil CN

Crude Oil SE

Crude Oil S4

Density (g/cc)

0.899

0.893

0.826

Molecular Weight (g/mol)

278.92

220.41

176.05

Viscosity (cP)

55.01

33.79

5.36

Water Content (wt%)

0.076

0.183

0.031

Saturates (wt%)

31.90

55.27

69.60

Aromatics (wt%)

25.10

15.25

22.02

Resins (wt%)

29.90

19.45

7.17

n-C5 Asphaltenes (wt%)

13.10

10.03

1.21

After the homogenization process was completed, the sample was observed under the microscope to confirm the presence of the emulsion and note the size distribution of the water droplets. The microscope used in this study was an AmScope model B340 with a 400× of combined magnification, coupled with an AmScope MU300 digital camera. 2.2. Indirect Method In this work, a recently developed technique called “Indirect Method” was used for determination of asphaltene precipitation30. This method, which is a combination of gravimetric and spectroscopic techniques, has been proposed for the detection and quantification of asphaltene precipitation in dead oil samples. The term “indirect” refers to an indirect detection of 7 ACS Paragon Plus Environment

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the precipitation of asphaltenes by measuring the absorbance of the supernatant fluid after centrifugation of oil/n-alkane mixtures. More details of the indirect method procedure have been provided in the Appendix of this paper. The indirect method was used in this study since it is more sensitive than other techniques that based on microscopy or direct spectroscopy. Also, it can be applied to detect asphaltene precipitation and quantify the amount that precipitates. By having a proper calibration curve, the absorbance of the supernatant fluid after the centrifugation step can be readily and accurately related to the amount of precipitated asphaltene30. Also, as part of this technique, all the water can be separated by centrifugation and therefore the NIR absorbance of the supernatant liquid can be readily obtained, in the same way it is done for an oil sample without emulsified water. Asphaltene precipitation from the crude oils CN, SE, and S4 upon addition of normal heptane was studied with and without the addition of water. The samples were prepared at ambient conditions and were aged for one day. Aging time is the time allotted between sample preparation and centrifugation. After one day, the samples were centrifuged and the absorbance of the supernatant liquid was measured at different wavelengths in the range 700-1300 nm, using a Shimadzu UV-3600 Spectrophotometer. The centrifugation step is necessary to remove and settle the asphaltene aggregates that are 100 nm and larger. Stokes’s law is used for determination of proper centrifugation speed and time based on the size and the density of the particle, and the density and viscosity of the liquid30,32. For more details on how to find the proper centrifugation speed and time, refer to Tavakkoli et al.30. The test tubes were centrifuged for 25 min at 10,000 rpm (12,740 relative centrifuge force) using a temperature controlled XIANGZHI benchtop centrifuge model XZ-10. This heated centrifuge can operate from room temperature up to the 70oC. Therefore, the effect of emulsified water on asphaltene stability can


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be studied at temperatures between ambient and 70oC. In this work, for the experiments conducted at 70°C, the samples were kept inside a Jeio Tech oven model OV-12 at 70°C for one day before the centrifugation step. After centrifugation at 70°C, the supernatant was taken and diluted with toluene and the absorbance value was measured at 70°C and wavelength of 1100 nm, using the Shimadzu UV-3600 Spectrophotometer equipped with a “TC1 Temperature Controller” and a “t2×2 Dual Temperature-Controlled Cuvette Holder” provided by Quantum Northwest. Actually, keeping the temperature at 70°C is critical during aging of the sample and also during the centrifugation step. However, it was found that the NIR absorbance measurement could also be done at room temperature since all the unstable asphaltenes and water emulsion have been removed at the centrifugation step, which will be discussed later in the results. Every experiment presented in this study was repeated at least twice to ensure the repeatability of the experimental results. The average value of the results obtained is plotted in each figure, and the corresponding standard deviation (ASD) is reported in the figure caption. 2.3. Model Oil Preparation Asphaltenes were extracted from different crude oils used in this study by addition of npentane (n-C5) as the asphaltene precipitating agent. These asphaltenes are called n-C5 asphaltenes. To perform the asphaltene extraction, one volume of the oil sample was added to forty volumes of n-pentane. The mixture was allowed to age for 2 days in a dark and cool area. After aging, the solution was vacuum-filtered using a 0.2 μm nylon membrane filter. The filter cake was then refluxed in a Soxhlet apparatus using n-pentane to further purify the precipitated asphaltenes. The system was run at the solvent boiling temperature until the run-down effluent was colorless. The Soxhlet apparatus was then allowed to cool down before the n-pentane was


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exchanged with toluene to proceed to the dissolution of asphaltenes. The system was again run until the run-down effluent was colorless. The solution of asphaltenes in toluene was transferred to a pre-weighted beaker. The beaker was then left on a hotplate under an explosion proof fume hood at 90°C for 1 day to evaporate the toluene. After constant weight was attained, the mass of collected asphaltenes was determined by subtracting the mass of the beaker. To divide the extracted n-C5 asphaltenes into two sub-fractions based on their solubility, n-C5 asphaltenes were run in the Soxhlet apparatus using n-heptane as the washing agent. The system was run until the run-down n-heptane was colorless. The asphaltene sub-fraction which is dissolved in n-heptane is called n-C5-7 asphaltenes (i.e. asphaltenes which are insoluble in n-C5, but soluble in n-C7). The remaining asphaltenes on the filter paper are called n-C7 asphaltenes. nC7 asphaltenes could also be extracted by addition of n-heptane to the crude oil sample at the first step of the extraction procedure followed by the washing step in the Soxhlet setup using nheptane. 200 mL of a solution containing 2.0 wt% n-C5 asphaltenes in toluene was prepared and then heated at 90°C and under ultrasonication at 40 kHz for 30 min. For the asphaltenes extracted from bitumen A1 from Athabasca region in Canada, two solutions, each 200 ml, were prepared. One solution contained 0.5 wt% n-C5-7 asphaltenes in toluene and the other one was made of 0.5 wt% n-C7 asphaltenes in toluene. To make sure that there were no undissolved asphaltene particles in the mixture, the solution was then centrifuged at 10,000 rpm for 15 min. No sediment was found after centrifugation. Also, the solution was observed under the microscope with a 400× combined magnification and no particles were detected.


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3. Results and Discussion 3.1. Ambient Conditions Experiments 3.1.1. Crude oil CN Figure 1 shows the results of the indirect method for the crude oil CN diluted with nheptane and aged for one day at 23°C and at ambient pressure, with and without presence of emulsified water. It can be found from Figure 1 that the emulsified water does not have a significant effect on the detection of asphaltene precipitation and the amount precipitated after the detection point. However, the absorbance of the supernatant liquid before the precipitation detection decreases when the amount of emulsified water is increased in the system. We infer that before the detection of precipitation, some of the asphaltenes get adsorbed at the oil/water interface. Then, the water in oil emulsion is broken by centrifugation and the asphaltenes adsorbed at the interface are removed along with the water. Therefore, in the presence of emulsified water, the supernatant liquid after the centrifugation step has less asphaltenes and therefore shows a lower absorbance value compared to the system without water. This result is in line with the results presented by Tharanivasan et al.2. They showed that for dilution ratios below the detection of precipitation, asphaltene yield for the samples with emulsified water is greater than the asphaltene yield for the samples without emulsified water2. Several studies have shown asphaltenes affinity to the oil/water interface and how they contribute to the stabilization of water-in-oil emulsions3–5. Tchoukov et al.6 suggested that the stabilization of emulsions can be due to the self-association of asphaltenes and the formation of a 3D network within organic films. This 3D network will then slow down the coalescence of emulsified water droplets6. During the centrifugation step in the indirect method, the water in oil emulsion is broken, and


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hence, this 3D network is disrupted. Asphaltenes adsorbed at the interface are then removed by centrifugation and this will result in a decrease in the absorbance of the supernatant liquid. The more water that is added to the system, the higher the interfacial area between oil and water and the more asphaltenes are attracted to the interface. Therefore, the absorbance value shows a higher drop. This drop in the absorbance value in the presence of emulsified water continues close to the detection of asphaltene precipitation and even after the detection point. However, as more asphaltenes precipitate out from the system at higher concentration of n-heptane, there is no significant change between the absorbance value of the sample with and without the presence of emulsified water. The amount of precipitated asphaltenes can be readily correlated to the visible or NIR absorbance of the supernatant fluid using a proper calibration curve. For more information, refer to Tavakkoli et al.30.

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Absorbance @ 1100 nm

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12 10 8 6 4

No Water

30 vol% emulsified water

2


0 0

20

40

60

80

100

n-C7 vol%

Figure 1. Results of the indirect method for the crude oil CN diluted with n-heptane and aged for 1 day at 23 oC and at ambient pressure, with and without presence of emulsified water. The ASD is 4.30, 6.82, and 6.19%, for no water, 30 vol% water, and 60 vol% water, respectively.


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It is important to clarify that the absorbance values shown in Figure 1 are not experimentally measured. The spectrophotometer cannot go above 6 for the absorbance value. However, during the indirect method, the crude oil sample is diluted with n-alkane at the first stage and with toluene at the second stage. After the centrifugation step, the absorbance of the supernatant liquid plus toluene is measured by the spectrophotometer. The absorbance of the original sample is then back-calculated by removing the effect of dilution and this calculated absorbance could be much higher than the instrument limit based on how dark the crude sample is. This is one of the advantages of the indirect method over direct techniques. Indirect method can be used for very dark crude oils without any problem in finding the absorbance value30. The procedure for post-processing the raw spectroscopy data of the indirect method to correct the dilution effect can be found in the supporting information of this paper, section A. To find the shift in the detection of asphaltene precipitation for the mixtures containing water, the first deviation from the horizontal trend was identified for each case, as presented in Figure 1. Tavakkoli et al.33,34 showed that the intersection between the two trend lines that are drawn before and after the detection point represents the volume percent of n-heptane at the detection of asphaltene precipitation. Figure 2 depicts the precipitation detection point for the crude oil CN, with and without added water. As it can be seen from Figure 2, the shift in the detection of asphaltene precipitation is very small.


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16

31.17 vol%

14


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31.76 vol%

12 10 8 6 4 No Water

2


0 0

20

40 60 n-C7 vol%

80

100

Figure 2. Detection of asphaltene precipitation for the crude oil CN, with and without the presence of emulsified water. One can conjecture from Figure 1 that the most insoluble asphaltenes are also highly attracted to the oil/water interface. It can be seen from Figure 1 that there is almost no difference between the results of the indirect method after the precipitation detection point for the samples with and without presence of emulsified water. Therefore, there are no more asphaltenes with high affinity for the oil/water interface in the solution after the detection of asphaltene precipitation and at high volume percent of added n-heptane. They have precipitated out of the solution at that point and have been removed from the supernatant liquid by centrifugation. Since the asphaltenes that precipitate first are the most unstable asphaltenes, one can conclude that the asphaltenes with high affinity for the oil/water interface are the most insoluble asphaltenes in the system. At high concentrations of n-heptane, the most stable asphaltenes remain in the solution. These asphaltenes are less attracted to the oil/water interface and, therefore, are less affected by the presence of water. 14 ACS Paragon Plus Environment

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3.1.2. Model oil system made of asphaltenes from crude oil CN To confirm that the changes in the asphaltene precipitation curve in presence of water are due to presence of asphaltenes in the system, n-C5 asphaltenes were extracted from the crude oil CN and were dissolved in toluene to prepare a model oil. A concentration of 2 wt% asphaltene in toluene was chosen. Figure 3 shows the results of the indirect method for the prepared model oil diluted with n-heptane and aged for one day at 23 oC and at ambient pressure, with and without emulsified water. The same results as the real crude oil CN were also found for the model oil with and without the presence of emulsified water. The emulsified water neither does have a significant effect on the detection of asphaltene precipitation nor on the amount precipitated after the detection point. However, the experimental results suggest that asphaltenes are adsorbed at the oil/water interface, especially at concentrations of n-heptane lower than the amount required to precipitate asphaltenes from the model oil.

5


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4

3

2

1

No water 30 vol% emulsified water

0 0

20

40 60 n-C7 vol%

80

100

Figure 3. Results of the indirect method for the model oil prepared using the asphaltenes extracted from the crude oil CN. The model oil was diluted with n-heptane and aged for 1 day at


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23 oC and at ambient pressure, with and without presence of emulsified water. The ASD is 2.37, and 3.55%, for no water, and 30 vol% water, respectively. Once asphaltenes are removed from a crude oil, the remaining liquid is called “Maltenes” (i.e. deasphaltened crude oil). The effect of emulsified water on the maltene fraction of the crude oil CN was also investigated. The maltene fraction was mixed with water for one day and the emulsion was then centrifuged to remove the water. The absorbance of the supernatant liquid was measured and it was the same as the absorbance value of pure maltene before mixing with water. Thus, the results obtained from the model oil prepared using the asphaltenes extracted from the crude oil CN as well as the results for the effect of water on the maltene fraction of the oil CN indicate that the changes to the asphaltene precipitation curve in the presence of emulsified water are due to the interaction between asphaltenes in the crude oil and the emulsified water. Other components of the crude oil do not seem to interact with the emulsified water as strongly as the asphaltenes. To verify that the amount of water dissolved in the supernatant liquid after centrifugation was not significant, the crude oil CN, pure toluene, and pure heptane were mixed with water for one day. Then, the emulsions were centrifuged and the absorbance values of the supernatant fluids were measured. The absorbance values of the supernatant liquids were unaffected by the addition of water. The conclusion was that the amount of water present in the supernatant liquid was not sufficient to affect the visible and NIR spectroscopic measurements. Furthermore, the water content of the samples was measured by a Karl Fischer titration apparatus, and the results are reported in Table 2. It can be seen that the water content of the supernatant liquid after the centrifugation step is almost the same as the liquid when excess water was not added. Therefore, the indirect method is a reliable technique to study the effect of emulsified water on asphaltene 16 ACS Paragon Plus Environment

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instability in both real crude oil and model oil systems. More details on the results of the absorbance measurements for the blank systems are available in the supporting information of this paper, section B. Table 2. Water content of the crude oil CN, pure toluene, and pure n-heptane before mixing with water, and also after removing the emulsified water by centrifugation at 1 atm and 23 °C. Water content

Water content of supernatant liquid,

in original sample, wt%

after centrifugation of the emulsion, wt%

Crude oil CN

0.0760

0.0560

Pure toluene

0.0254

0.0147

Pure n-heptane

0.0140

0.0115

Sample

3.1.3. Crude oils SE and S4 The indirect method was also used to study the effect of emulsified water on asphaltene instability for the crude oils SE and S4; their properties are available in Table 1. Figure 4 shows the results of the indirect method for the crude oils SE and S4 diluted with n-heptane and aged for one day at 23°C and at ambient pressure, with and without the presence of emulsified water. As it can be seen from Figure 4, the emulsified water has no effect on the asphaltene stability in these crude oils. The precipitation curves, with and without the presence of emulsified water, are almost identical. Even before the precipitation detection, water had no effect on the results of the indirect method. This analysis suggests that the amount of asphaltenes with high affinity for the oil/water interface in the crude oils SE and S4 is less abundant compared to the crude oil CN.


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0.3


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80

0

100

0

20

(a)

40 60 n-C7 vol%

80

100

(b)

Figure 4. Results of the indirect method for the crude oils (a) SE; and (b) S4, diluted with nheptane and aged for 1 day at 23°C and at ambient pressure, with and without presence of emulsified water. The ASD is 3.69, 6.19, 1.30, and 1.71% for no water and 30 vol% water for the crude oil SE, and for no water and 30 vol% water for the crude oil S4, respectively. Model oil systems were also prepared using n-C5 asphaltenes extracted from the crude oil SE and S4, and the effect of emulsified water on the model oils was investigated in the same way as it was done for the actual crude oils. For both model oil systems, it was found that the two precipitation curves with and without presence of emulsified water are almost identical. 3.1.4. Model oil system made of asphaltenes from bitumen A1 Based on the results obtained for the three crude oils CN, SE, and S4, it can be inferred that the amount of asphaltenes with high affinity for the oil/water interface in crude oils SE and S4 are less abundant than crude oil CN. Crude oil CN is heavier compared to the crudes SE and S4 and it contains more asphaltenes according to Table 1. Also, based on the results obtained for 18 ACS Paragon Plus Environment

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the crude oil CN and previously discussed in section 3.1.1, one may conclude that the most insoluble asphaltenes are highly attracted to the oil/water interface. To validate this hypothesis, bitumen A1 from Athabasca region in Canada, which contains 22 wt% n-C5 asphaltene, was chosen. The bitumen was very viscous with a very low mobility and it was not feasible to prepare a homogenous emulsion of bitumen and water. Therefore, to study the effect of emulsified water on the asphaltenes in the bitumen A1, asphaltenes were extracted from bitumen and dissolved in toluene to prepare a model oil. 200 ml of a solution with 0.5 wt% n-C5 asphaltenes in toluene was prepared. The extracted n-C5 asphaltenes from bitumen A1 were not as soluble in toluene as the asphaltenes extracted from crude oil CN, SE, and S4. Therefore, a concentration of 0.5 wt% was selected to have a stable solution. Figure 5 depicts the effect of emulsified water on the model oil prepared from the bitumen asphaltenes. It can be seen that the absorbance values decrease in the presence of water both before and after the detection of asphaltene precipitation. Based on this observation, one might conclude that the asphaltenes of bitumen A1 have high affinity for the oil/water interface. According to Figure 5 the asphaltenes in the model oil are getting less stable once emulsified water is added to the system. Figure 6 presents the precipitation detection points obtained using the indirect method experimental data for the model oil prepared with the n-C5 asphaltenes extracted from the bitumen, with and without presence of water. In the presence of emulsified water, the detection of asphaltene precipitation significantly shifts to the lower amount of added n-heptane which means the system is getting more unstable. Also, the amount of asphaltenes precipitated after the detection of precipitation increases when water is present in the system.


Energy & Fuels


1.2 1 0.8

0.6 0.4 0.2

No Water 30 vol% emulsified water

0 0

20

40 60 n-C7 vol%

80

100

Figure 5. Results of the indirect method for the model oil prepared with the asphaltenes extracted from the bitumen A1. The model oil was diluted with n-heptane and aged for 1 day at 23°C and at ambient pressure, with and without presence of emulsified water. The ASD is 2.18, and 2.92%, for no water, and 30 vol% water, respectively.

1.2 36.08 vol%


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24.51 vol%

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Figure 6. Detection of asphaltene precipitation for the model oil prepared using the asphaltenes extracted from the bitumen A1, with and without the presence of emulsified water. To confirm that the most insoluble asphaltenes contain more asphaltenes with high affinity for the oil/water interface than the most soluble asphaltenes, n-C5-7 and n-C7 asphaltenes were extracted from bitumen A1. Two model oil systems, each 200 ml, containing 0.5 wt% of asphaltenes in toluene were prepared using the extracted n-C5-7 and n-C7 asphaltenes. n-C5-7 asphaltene is a relatively light sub-fraction of n-C5 asphaltene which is soluble in n-heptane, but insoluble in n-pentane. n-C7 asphaltene is a heavier sub-fraction which is insoluble in n-pentane, n-hexane and n-heptane, but soluble in toluene as an aromatic solvent. The results obtained for the effect of emulsified water on these two asphaltene sub-fractions are presented in Figure 7. As it was expected, water has no effect on the light asphaltenes, i.e. n-C5-7 asphaltenes. However, the n-C7 asphaltenes have a high affinity for the oil/water interface, and therefore, the absorbance values of the supernatant drop significantly in the presence of emulsified water. It can be inferred that the n-C5-7 asphaltenes, which are the most soluble asphaltenes, are less attracted to the oil/water interface, but n-C7 asphaltenes in the bitumen A1, which are much more insoluble compared to the n-C5-7 asphaltenes, have the highest affinity for the oil/water interface. It should be mentioned that n-pentane was used to precipitate asphaltenes out of the model oils in Figure 7 since n-C5-7 asphaltenes are soluble in n-heptane but not in n-pentane. nC7 asphaltenes can be precipitated using both n-pentane and n-heptane.


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0.08 0.06 0.04 No Water

0.02

1

0.8 0.6 0.4 0.2

No Water

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30 vol% of emulsified water

0

0

20

40 60 n-C5 vol%

80

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100

0

20

(a)

40 60 n-C5 vol%

80

100

(b)

Figure 7. Results of the indirect method for the model oils prepared using the asphaltenes extracted from the bitumen A1: (a) n-C5-7 asphaltenes; (b) n-C7 asphaltenes. The model oils were diluted with n-pentane and aged for 1 day at 23°C and at ambient pressure, with and without presence of emulsified water. The ASD is 2.66, 3.01, 3.12, and 1.95% for no water and 30 vol% water for n-C5-7 asphaltenes, and for no water and 30 vol% water for n-C7 asphaltenes, respectively. 3.2. High Temperature Experiments 3.2.1. Crude oils CN, SE, and S4 The effect of emulsified water on asphaltene instability was studied at 70°C and ambient pressure for the crude oils CN, SE, and S4. As it was mentioned earlier in section 2.2, keeping the temperature at 70°C is critical during aging of the sample and also during the centrifugation step. It was found that the NIR absorbance measurement could be done either at 70°C or at room


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temperature. The results of the indirect method at 70°C for the crude oil CN are shown in Figure 8. The NIR absorbance values at 23 and 70°C are compared in Figure 8(a). It can be seen from Figure 8(a) that there is a difference between the absorbance values measured at 23°C and the ones measured at 70°C. This difference is greater for lower concentrations of n-heptane. To find out the reason for this difference, the absorbance values of pure toluene and pure n-heptane were first measured at different temperatures from 20°C to 70°C. The absorbance values were exactly the same at the different temperatures and, therefore, temperature does not have an effect on the absorbance values for pure toluene and pure n-heptane. Actually, this is the reason why the absorbance values at 23°C and at 70°C plotted in Figure 8(a) are very similar when the volume percent of n-heptane is high. When the concentration of n-heptane in the mixture is high, the concentration of asphaltenes in the supernatant liquid is very low, and the absorbance values are mostly determined by the n-heptane, which according to these results are independent of the temperature. Thus, the difference between the absorbance values measured at 23°C and the ones measured at 70°C for lower concentrations of n-heptane (and therefore higher concentrations of asphaltenes) is caused by the volume expansion of the mixture.


Energy & Fuels

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14

14


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12

10 8 6 4 Abs. @ 23 C

2

Abs. @ 70 C

Page 24 of 37

12 10 8 6 4 Abs. @ 23 C

2

Abs. @ 70 C after density correction

0

0 0

20

40 60 n-C7 vol%

80

100

0

20

(a)

40 60 n-C7 vol%

80

100

(b)

Figure 8. Results of the indirect method for the crude oil CN diluted with n-heptane and aged for 1 day at 70 oC. (a) Comparison of the absorbance values measured at 23 and 70 oC; (b) Comparison of the absorbance values measured at 23 and 70 oC after correction for the density changes. Based on the indirect method procedure, the supernatant liquid is diluted after the centrifugation step with pure toluene30. The effect of dilution can be removed in a later step and by post-processing of the data30. For the crude oil CN, 0.5 ml of the supernatant liquid was diluted with 3 ml of pure toluene and then the absorbance value was measured. Therefore, by considering the density of toluene at different temperatures, the true dilution ratio can be found and used for data post-processing. Figure 8(b) presents the absorbance values at 23°C and at 70°C after correction for the dilution effect due to the density changes. It can be seen that the precipitation curves are almost identical. Based on this observation, all the absorbance measurements for the high temperature experiments were performed at room temperature.


Page 25 of 37

Figure 9 shows the results of the indirect method for the crude oil CN diluted with nheptane and aged for one day at 70°C and ambient pressure, with and without emulsified water. It can be seen that the results obtained at 70°C for the effect of emulsified water is the same as the results found at 23°C. The emulsified water does not have a significant effect on the detection of asphaltene precipitation nor on the amount precipitated after the detection point. However, the experimental results suggest that asphaltenes are adsorbed at the oil/water interface, especially at a concentration of n-heptane that is lower than the amount at the detection of precipitation.

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12 10 8 6 4 No Water, T=70 C

2

30 vol% emulsified water, T=70 C 0 0

20

40 60 n-C7 vol%

80

100

Figure 9. Results of the indirect method for the crude oil CN diluted with n-heptane and aged for 1 day at 70 oC and ambient pressure, with and without presence of emulsified water. The ASD is 2.98, and 4.02%, for no water, and 30 vol% water, respectively. Figure 10 compares the results of the indirect method applied to the crude oil CN with and without the presence of emulsified water at two different temperatures: 23 and 70°C. It can be observed that once the emulsified water is added to the system, the drop in the absorbance 25 ACS Paragon Plus Environment

Energy & Fuels

value is higher at 70°C compared to the drop at 23°C. It is necessary to mention here that a greater drop in the absorbance value at higher temperature is not due to the presence of more soluble water in the supernatant (i.e. organic phase) at higher temperature. It was found that the solubility of water in the organic phase is not drastically affected by increasing the temperature from 23 to 70°C, using the Karl Fischer titration apparatus. The soluble water content changed from 0.0560 wt% to 0.0885 wt% by changing the temperature from 23 to 70°C, and the corresponding NIR absorbance values were almost identical. However, at a higher temperature, both density and viscosity of the crude oil decrease and, therefore, the mobility of asphaltenes in the system increases. In a less dense and less viscous system, asphaltenes can move more quickly to the oil/water interface and equilibrium conditions will be reached faster. Change of the adsorption isotherm, and therefore, change of the total amount that can be adsorbed at equilibrium at a higher temperature should be also considered here.

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12 10 8 6 No Water, T=23 C 30 vol% emulsified water, T=23 C No Water, T=70 C 30 vol% emulsified water, T=70 C

4

2 0 0

20

40 60 n-C7 vol%

80

100

Figure 10. Results of the indirect method for the crude oil CN diluted with n-heptane and aged for 1 day at two different temperatures: 23 and 70°C, at ambient pressure, with and without presence of emulsified water. 26 ACS Paragon Plus Environment

Page 27 of 37

In Figure 10, one can also compare the results of the indirect method for the crude oil CN without presence of emulsified water at 23 and 70°C. The absorbance value of the supernatant liquid after the detection of asphaltene precipitation is higher for the experiments performed at 70°C compared to the experiments at 23°C. Therefore, more asphaltenes are available in the supernatant liquid and less precipitation occurs at 70°C. At 70°C, the oil is a better solvent for asphaltenes and the precipitated amount after the detection of precipitation is less than at 23°C. 3.2.2. Model oil system made of asphaltenes from crude oil CN The indirect method was also performed at 70°C for the model oil with 2 wt% n-C5 asphaltenes extracted from the real crude oil CN, with and without presence of emulsified water. Figure 11 compares the results of the indirect method at two different temperatures: 23 and 70°C.

5


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4

3

2 No Water, T=23 C 30 vol% emulsified water, T=23 C No Water, T=70 C 30 vol% emulsified water, T=70 C

1

0 0

20

40 60 n-C7 vol%

80

100

Figure 11. Results of the indirect method for the model oil prepared using the asphaltenes extracted from the crude oil CN. The model oil was diluted with n-heptane and aged for 1 day at two different temperatures: 23 and 70°C, at ambient pressure, with and without presence of


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emulsified water. The ASD is 2.37, 3.55, 3.29, and 4.82% for no water at 23°C, 30 vol% water at 23°C, no water at 70°C, and 30 vol% water at 70°C, respectively. It can be observed in Figure 11 that once the emulsified water is added to the system, the drop in the absorbance value at 70°C is almost the same as the drop observed at 23°C. Since the asphaltene concentration is low for the prepared model oil, the density and viscosity of the model oil are very similar to pure toluene and are much lower than the density and viscosity of the real crude CN. Therefore, the changes in density and viscosity of the model oil are not as significant as the changes for the real crude oil CN as the temperature increases and finally the mobility of asphaltenes in the model oil does not drastically increase by increasing the temperature from 23°C to 70°C. Figure 11 also compares the indirect method results for the model oil without presence of emulsified water at temperatures 23 and 70°C. It can be seen that the absorbance value of the supernatant liquid after the detection of precipitation is higher for the experiments performed at 70°C compared to the experiments at 23°C. This is because more asphaltenes are present in the supernatant liquid and therefore less precipitation occurs at 70°C. 3.2.3. Crude oils SE and S4 High temperature experiments were also performed for the crude oils SE and S4 and the results are shown in the Figure 12. Similar to the results obtained at 23°C, the emulsified water has no effect on the asphaltene stability in these crude oils. At 70°C the precipitation curves, with and without presence of emulsified water, are almost identical. Based on these results, it can be inferred that the amount of asphaltenes with high affinity for the oil/water interface in the crude oils SE and S4 is not as significant as in the crude oil CN and the bitumen A1.


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12 10

8 6 4 No Water, T=70 C

2

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0.3

30 vol% emulsified water, T=70 C

30 vol% emulsified water, T=70 C

0

0

0

20

40 60 n-C7 vol%

80

100

0

(a)

20

40 60 n-C7 vol%

80

100

(b)

Figure 12. Results of the indirect method for the crude oils (a) SE; and (b) S4, diluted with nheptane and aged for 1 day at 70 oC and at ambient pressure, with and without presence of emulsified water. The ASD is 1.57, 3.84, 0.94, and 1.33% for no water and 30 vol% water for the crude oil SE, and for no water and 30 vol% water for the crude oil S4, respectively. 3.3. Effect of Brine on Asphaltene Instability It should be mentioned here that the work presented in this paper is a laboratory work and is not directly extendable to field operations. In the field, the operation pressure and temperature are much higher than the ambient conditions. Also, to prepare water in oil emulsions in the previous sections, deionized water was used for simplification. However, in an oil reservoir, the formation water found as emulsion in oil is not deionized. Brine with salt concentration of 180,000-220,000 ppm may impose a deviation from the results obtained in the previous sections. The investigation of the effect of brine, with differing concentrations and salts, on asphaltene


Energy & Fuels

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instability is an extensive work which will be required in the future for further understanding of asphaltene instability under reservoir-like conditions. In this work, only a simple, preliminary case was performed in order to gain some intuition on the effect of brine on asphaltene instability. Crude oil CN was chosen and mixed with a brine solution with a concentration of 200,000 ppm pure NaCl. Figure 13 compares the results of the indirect method for the crude oil CN mixed with n-heptane in the presence of emulsified DI water and emulsified brine. It can be seen that the absorbance of the supernatant liquid before the detection of asphaltene precipitation is similar to the system without emulsified water. This means that in the presence of brine, affinity of asphaltenes to the oil/brine interface decreases. This is in line with some of the available research in the literature. Moeini et al.35 suggested that the presence of brine reduces asphaltenes’ ability to decreases the interfacial tension of an emulsion due to the “salting-out effect”, i.e. when the amount of cations near the interface increases, the water molecules which have been interacting with both ions and organic compounds by dipole–ion and dipole–dipole interactions, respectively, are no longer able to support the charges and polarity of both ions and asphaltenes36. The presence of brine reduces the ionization of the asphaltenes causing them to become more soluble in the oil phase35. This results in a decrease in asphaltene concentration at the interface. Also, Price et al.37 proposed the same phenomenon that due to the salting-out effect, hydrocarbon fractions dissolve less in the aqueous solution with the increase of salinity content. However, contradictory results are also available in the literature. Serrano-Saldaña et al.38 and Alotaibi et al.39 concluded that the interfacial tension of n-C12/brine interface decreases with increasing concentration of brine, because ions from sodium chloride prefer to localize at or 30 ACS Paragon Plus Environment

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around the n-C12/brine interface. This might be the reason for decrease in the absorbance of the supernatant liquid after the detection of asphaltene precipitation in the presence of brine compared to the DI water, as shown by Figure 13. The absorbance drops sharply after the precipitation detection and closer to the detection point, which means more asphaltenes have been removed by the centrifugation in the presence of brine. Based on the results shown in Figure 13, there is a need for extensive research to gain a better understanding on the effect of brine on asphaltene instability in crude oil systems, which will be done as a future work.

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12 10 8 6 4

No Water 30 vol% emulsified DI water

2

30 vol% emulsified brine 0 0

20

40

60

80

100

n-C7 vol%

Figure 13. Results of the indirect method for the crude oil CN diluted with n-heptane and aged for 1 day at 23 oC and at ambient pressure, without emulsified water, with emulsified DI water, and with emulsified brine. Brine solution was prepared using pure NaCl with concentration of 200,000 ppm. The ASD is 4.30, 6.82, and 2.15%, for no water, 30 vol% emulsified DI water, and 30 vol% emulsified brine, respectively.


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4. Conclusion In this work, the effect of emulsified water on the instability of asphaltenes was studied for both real crude oils and model oil systems. It was found that the emulsified water affects the systems differently. For the crude oils SE and S4, the precipitation curves were almost identical with and without the presence of emulsified water in the oil. However, the experimental results for crude oil CN suggested that asphaltenes were adsorbed at the oil/water interface, especially at lower concentrations of n-heptane, i.e. before the detection of asphaltene precipitation. However, water neither did seem to affect the detection of precipitation nor the amount of precipitated asphaltenes in crude oil CN. The same experiments were performed on the model oils with asphaltenes extracted from real crude oils, with and without the presence of emulsified water. The effect of water on the stability of asphaltenes in the real oil and the corresponding model oil was consistent. Also, the effect of water on the maltene fraction (i.e. deasphaltened crude oil) obtained from crude CN was investigated and it was found that the compounds present in the maltene fraction do not interact with the water as strongly as the asphaltenes. The indirect method was then performed at 23°C and 70°C and it was found that the effect of the emulsified water on the adsorption of asphaltenes at the oil/water interface increases by increasing temperature. Experiments were also performed on asphaltenes extracted from bitumen A1 from Athabasca region and it was found that the asphaltenes are highly attracted to the oil/water interface, both before and after the detection of asphaltene precipitation. The results obtained for the model oil made of bitumen asphaltenes showed that the detection of precipitation significantly shifts to the lower amount of n-heptane causing a more unstable system. Asphaltenes extracted from bitumen were divided into two parts based on their solubility; n-C5-7 sub-fraction and n-C7 sub-fraction. It was found that emulsified water has no effect on n-C5-7 32 ACS Paragon Plus Environment

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fraction of bitumen asphaltenes. However, the less soluble sub-fraction, i.e. n-C7 asphaltenes, seems to have a high affinity for the oil/water interface. Acknowledgments The authors are grateful for the generous support of Abu Dhabi National Oil Company (ADNOC) R&D Oil Subcommittee. Fruitful discussions with Renato Evangelista are also acknowledged. Appendix: Indirect Method Procedure To see the effect of water on asphaltene precipitation, first the oil/water emulsion was prepared as explained in the section 2.1. Then, samples with different ratios of the precipitant, such as n-heptane, and the oil/water emulsion were prepared, ranging from pure emulsion to 90 vol% of the precipitant. The test tubes were then shaken vigorously and allowed to stand undisturbed for the specified aging time. Aging time, which is the time allotted between sample preparation and centrifugation, was 24 hours for all the experiments performed in this paper. Then, the test tubes were centrifuged for 25 min at 10,000 rpm which corresponds to 12,740 relative centrifuge force (RCF) in the temperature controlled XIANGZHI benchtop centrifuge model XZ-10. The centrifugation step is necessary to remove and settle the asphaltene aggregates that are 100 nm and larger. For more details on how to find the proper centrifugation speed and time, refer to Tavakkoli et al.30 After the centrifugation step, a total of 1 mL of the supernatant liquid was taken and diluted with 4 mL of toluene. Then, the absorbance at different wavelengths in the range 700-1300 nm was measured by an UV-3600 Shimadzu spectrophotometer, using air as the blank. It is necessary to mention that the chosen ratio for mixing supernatant liquid and toluene depends on how dark a crude oil sample is. For a dark 33 ACS Paragon Plus Environment

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crude oil with a very high asphaltene concentration, a higher dilution ratio can be used to prevent saturation of the absorbance signals. Tavakkoli et al.30 showed that identical results are obtained at different dilution ratios after eliminating the dilution effect from the absorbance values. Figure A1 shows the schematic for the indirect method procedure used to study the effect of water on asphaltene precipitation.

Figure A1. Schematic of the indirect method procedure


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References (1)

Tavakkoli, M.; Panuganti, S. R.; Taghikhani, V.; Pishvaie, M. R.; Chapman, W. G. Fuel 2014, 117, 206–217.

(2)

Tharanivasan, A. K.; Yarranton, H. W.; Taylor, S. D. Energy Fuels 2012, 29, 6869–6875.

(3)

Yarranton, H. W.; Hussein, H.; Masliyah, J. H. J. Colloid Interface Sci. 2000, 228 (1), 52– 63.

(4)

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