Investigating the Effect of Salinity on the Behavior of Asphaltene

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"Investigating the Effect of Salinity on the Behavior of Asphaltene Precipitation in the Presence of Emulsified Water" Faryar Shojaati, Seyed Hamed Mousavi, Masoud Riazi, Farshid Torabi, and Mohammad Osat Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03331 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Industrial & Engineering Chemistry Research

Investigating the Effect of Salinity on the Behavior of Asphaltene Precipitation in the Presence of Emulsified Water Faryar Shojaati1, 2, Seyed Hamed Mousavi1, Masoud Riazi2*, Farshid Torabi3, Mohammad Osat1, 2 1

2

Separation Processes & Nanotechnology Lab, Faculty of Caspian, College of Engineering, University of Tehran, Tehran, Iran

Enhance Oil Recovery Research Center, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran

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Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Canada

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Corresponding Author Email: [email protected]

Abstract Asphaltene precipitation and deposition is one of the main problems in petroleum industries which has attracted the attention of many scholars. Precipitation and deposition of asphaltenes can lead to many problems in oil reservoirs such as plugging the pores of the reservoir’s rocks and changing the wettability of the rocks from water-wet to oil-wet. This ultimately causes a reduction or put an end to production from reservoirs. Therefore, understanding the factors affecting the formation of asphaltene precipitation can help us to avoid these drawbacks. Several factors included pressure, temperature and composition changes have been studied in the literature. The effects of these parameters on the stability of asphaltenes are almost clear. However, the effects of water emulsions, which are formed during the water-based enhanced oil recovery (EOR) methods such as smart water and low salinity water flooding, on the instability of asphaltenes are still unknown and blurred. In this study, the effects of several synthetic brines which were prepared by different salts in a wide range of concentrations were investigated to understand the mechanism of ions on the instability of asphaltenes. It was found that the divalent cations have more effects on the instability of asphaltenes compared to monovalent cations due to the chelate formation. Furthermore, the presence of divalent anions in the system can hinder the effect of cations on the instability of asphaltene. Keywords: asphaltene precipitation; asphaltene onset point; water/oil emulsion; indirect methods;

1. Introduction During oil production from reservoirs, water is also co-produced with oil. The origin of this water can be different. Part of it is related to underground water that has been trapped in reservoirs and the rest is attributed to water that has been used in water-based EOR processes such as steam injection or smart water. This water can be either in free or emulsion phases.1-4 Presence of water in the system may lead to mechanisms that yield asphaltene precipitation and deposition within the reservoir.4-6 Asphaltenes are known as the heaviest fraction of the crude oils. Their structures are very complicated and composed of different components such as aromatics and heteroatoms (i.e. S, O, and N).5, 7, 8 According to common define basis of their solubility, they are components that are dissolved in aromatics such as toluene but are insoluble in normal paraffin such as n-heptane.9, 10 Asphaltenes are famous for their propensity to precipitate and deposit during oil production from reservoirs due to temperature, pressure or composition alteration. Reduction of permeability and blockage of reservoir’s rock pore spaces, formation damage, and wettability alteration of reservoir’s rock toward oil-wet that leads to reduction of oil productivity, are the main problems of asphaltene deposition during oil production.7, 8, 11 For the reasons that have been mentioned, it seems essential to investigate the extent to which water affects the behavior of asphaltene precipitation. Due to limited works that have been done, there is no precise information on this matter. In 2001, Anderson et al. investigated the effect of water on the solubility behavior of asphaltenes in toluene using calorimetric titration. The results showed that the solubility of asphaltene in toluene significantly decreased in the presence of trace water.12 In 2002, Murgich et al. applied molecular mechanics and micro calorimetric methods to investigate the effect of water molecules on asphaltene aggregation. They used two model oils from Athabasca sand 1 ACS Paragon Plus Environment


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oil to probe the interaction between asphaltenes and water molecules. According to the result, it was determined that water molecules can form bridging H bonds between heteroatoms of asphaltene molecules and lead to increase in asphaltenes aggregation.13 In 2009, Tan et al. investigated the effect of water on the stability and aggregation behavior of the two asphaltene model oils by using H-NMR spectroscopy titration. The results showed that the existence of water in the system leads to more asphaltene aggregation. They expressed that hydrogen bonds between water molecules and pyridyl nitrogens were responsible for the stronger association of asphaltenes in the presence of water.6 Aslan et al. in 2014 investigated the effect of water on deposition, aggregation size, and viscosity of asphaltenes in a flow line. They utilized dynamic light scattering (DLS) and differential interference contrast (DIC) microscopy to measure the size of the aggregates. The results revealed that water delays the deposition of asphaltenes due to hydrogen bonds between the water molecules and heteroatoms in asphaltenes which in turn leads to decreasing in asphaltene deposition within the pipe surface. Furthermore, they declared that the size of aggregates is independent of water content. In addition, based on the results, they told that the oil sample viscosity decreased as water concentration increased. However, after 2000 ppm of water concentration, the viscosity of the oil sample increased with water concentration.4 Hu et al. in 2015 probed the effect of water on asphaltene deposition in a transparent packed-bed micro-reactor that was made of quartz. The experimental results illustrated that little water in the system help asphaltene dissolve better in toluene. However, once water content exceeded 3.18 wt%, the solubility decreased and led to more asphaltene deposition. They justified this phenomenon in the way that soluble water molecules decreases asphaltene aggregation as long as water content in the system is low; but, at higher content above 3.18 wt%, emulsion formation facilitates asphaltene deposition on the pore throat surfaces in the porous media of quartz micro-reactor.5 Studies conducted in the literature are mainly focused on investigation of free water on asphaltene’s precipitation or deposition behavior. But, as expressed before, water is present in the system in the form of emulsions. A few studies have been done to investigating the effect of water emulsions on asphaltene precipitation and deposition behavior. Tharanivasan et al. in 2012 investigated the effect of emulsified water on asphaltene precipitation from crude oils by using centrifugation method. They compared asphaltene precipitation before and after adding water to the oil to make a deductions about the effects of emulsified water on asphaltene’s solubility behavior. The results showed that presence of emulsified water above the onset point of precipitation had no significant effect on the precipitation of asphaltenes. Furthermore, under the onset point of precipitation, asphaltenes molecules gathering at the interface of water/oil exit from the system during the centrifugation. They reported these as the yield below the onset point.3 In 2016, Tavakkoli et al. studied the effect of emulsified water on the stability of asphaltenes of three crude oil and one bitumen sample with a pioneer and modern technique named indirect methods. Several samples including crudes and synthetic oils were investigated with and without the presence of emulsified water. They expressed that emulsified water didn’t have any influence neither on the onset point of precipitation nor on the precipitated amount of asphaltenes for the Middle East and Canada crude oils. However, the crude oil from Gulf of Mexico and the model oil from Athabasca bitumen were affected remarkably by the presence of emulsified water. They concluded that some asphaltene is more willing to interact with water at water/oil interface.14 All the abovementioned experiments have been done by deionized water in absence of any cations or anions. This makes the results of the investigation not sufficiently accurate since the water in reservoirs and also the water that has been used in smart water processes is brine water and contains a variety of cations and anions with different concentration. These ions may directly affect asphaltene precipitation and deposition mechanism and their absence bring into discussion the accuracy of the experiment’s results. Very limited studies can be found in the literature that has investigated the effect of salinity on asphaltene precipitation and deposition mechanism. Wang et al. in 2014 investigated the effect of deionized water and synthetic brine with 6.5 wt.% NaCl salt on the deposition of asphaltenes of two crude oil sample from the Gulf of Mexico. The samples were tested in the form of a water-in-oil emulsion in a capillary tube. They declared that adding littledeionized water can inhibit asphaltene deposition and reduce deposition rate up to 56%. The rate of asphaltene deposition, however, increased with synthetic brine water. In order to explore the effect of cations with high charge, Ferric ions (Fe3+) and aluminum ions (Al3+) were added to the system. The results showed that the deposition rate in presence of Ferric ions and aluminum ions increased more than the case that there was no water in the system.15 In 2016, Demir et al. studied the stability of asphaltene in the presence of CaCl2 and NaCl2 salts. Five different crude oils were probed with microscopy method. The results showed that as salt’s concentration increased, asphaltene aggregation was also increased. They expressed that CaCl2 salts had more effect on asphaltene aggregation compared to NaCl2 salts. Furthermore, the presence of inorganic compounds like sodium or calcium in asphaltene’s structure was observed to have an effective impact on asphaltene precipitation tendency.16 2 ACS Paragon Plus Environment

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Due to very limited research conducted in this area, in this study, the effect of different salts was discussed on the instability of asphaltene of one of the Iran’s oil fields named M1. For this purpose, synthetically brines with four different salts (MgCl2, CaCl2, MgSO4, and Na2SO4) in wide ranges of salinity were prepared to create water-in-oil emulsions in order to study the effect of water on asphaltene precipitation. A novel technique named indirect method was applied to detect asphaltene precipitation which was invented and applied by Tavakkoli et al. (2015) for the first time. 17 The term “indirect” refers to the detection of asphaltene precipitation with measuring the supernatant fluid’s absorbance after centrifugation of precipitant/oil mixtures. This method, unlike other methods such as microscopy, spectroscopy, interfacial method, reflective index measurement and conductivity method, can readily measure the amount of asphaltene precipitation.17 Furthermore, presence of emulsified water does not intrude the measurements which shows the advantage of indirect method over the other methods.14

2. Experimental procedure 2.1. Materials In this experiment, n-heptane (Dae-Jung, purity >99%) for precipitation of asphaltene also in addition with toluene (Dae-Jung, purity >99%) for dilution the supernatant fluid after centrifugation stage were used. Four different kinds of salt named MgCl2, CaCl2, MgSO4, and Na2SO4 (Merck, purity > 99%) were applied to make synthetic brines. Deionized water was supplied from the Water and Environment Laboratory of Shiraz University. 2.2. Water-in-oil emulsion preparation In this study, a wide range of salinity (from 5000 to 45000 ppm as the representative of low to medium salinity and 100000 to 200000 ppm as the representative of high salinity) were used in order to probe the effect of brine on the stability of asphaltene of one of the Iran’s oil field that specified in this study with M1. For this purpose, first, the crude oil was centrifuged to remove any sediments or water that may exist in the system. Second, by stoichiometric calculation, an appropriate amount of favorite salt was weighted and added to deionized water. A magnet stirrer was then put in the salt-saturated water for homogenization. After preparation of brine, while the oil sample was stirred on a magnetic stirrer, brine was added to it slowly. The sample was stirred at approximately 2000 rpm for 3 hours to be well-homogenized. After that, in order to ensure about emulsion formation, a microscope camera (HLOT) with the grandiosity of 500× was used. Once the formation of the emulsion was visually ensured, the oil sample was tested.

2.3. Indirect method In this study, a novel experimental method called “indirect method” which was introduced by Tavakkoli et al. (2015) was utilized to specify the onset point of asphaltene precipitation together with asphaltene precipitation amount. The word “indirect” refers to the detection of onset point and the amount of precipitated asphaltene by measuring the absorbance of supernatant liquid after oil/n-heptane mixture being centrifuged. The indirect method was used in this study since it is more sensitive compared to other methods, it simultaneously specifies onset point and the amount of precipitated asphaltene and, it can be applied to oils with different ranges of asphaltene content.17 Furthermore, the Indirect method is reported to be the best method among the other experimental methods for determination of onset point in the presence of emulsified water.14 The experimental procedure of this method is explained as follow: After preparation water-in-oil emulsion, different mixtures of crude oil and n-heptane were prepared in the test tubes, ranging from pure crude oil to 90% n-heptane. The samples were shaken severely with hand and then permitted to remain motionless for 24 hours as the aging time at ambient temperature and pressure. Aging time refers to the time between sample preparation and deposit separation using a centrifuge. Next, the samples should be centrifuged at 10000 rpm for 25 minutes (13348 relative centrifuge force) in order to omit the instable asphaltene from the system. Appropriate centrifuge’s speed and time were calculated by referring to Tavakkoli et al..14, 17 Afterward, 1 ml of supernatant of each sample were taken and diluted with 40 ml of toluene. Thereupon, the absorbance of the diluted samples was measured at a wavelength of 700 nm with a spectrophotometer (GENESYS 10 UV), using air as the blank. After that, the effect of dilution was subtracted from the measured absorbance and figures of modified absorbance value versus volume percent of n-heptane were plotted. Sharp deviation from the linear behavior of absorbance data indicates the onset point of asphaltene precipitation. The detected onset point was considered as the reference point in this study. This procedure was applied both in the presence and absence of emulsion to detect the effect of salinity on asphaltene stability. For better understanding, a schematic of the procedure was provided as a supporting information (SI). 3 ACS Paragon Plus Environment


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3. Result and Discussion 3.1. Comparison of Divalent Cations on Instability of Asphaltenes In order to investigate the effect of divalent cations on the instability of asphaltene, MgCl2 and CaCl2 were applied. Figure 1, shows the effect of Synthetic Brine of MgCl2 salt on the instability of asphaltene. As it is clear from Figure 1, in absence of emulsified synthetic brine, the onset point of asphaltene precipitation is about 53 vol.% of n-heptane. According to the trend, the absorbance before the onset point is constant and equals to 95. However, in the presence of emulsified synthetic brine, in all concentrations, the stability of asphaltene has been worsened. In 5000 ppm, 15000 ppm and 45000 ppm of MgCl2 the onset point of precipitationis equal to 43, 40 and 43 vol.% of n-heptane respectively. Moreover, the absorbance for 5000 ppm to 45000 ppm of MgCl2 is equal to 77, 70 and 79 respectively. This variation in the absorbance values can be justified by salting-in and salting-out effect which are explained as follow. Ions exhibit different behaviors in different concentrations. Within low salt concentration, ions accelerate the movement of surface active components (asphaltenes and resins) toward the water/oil interface and let these components to be more stable at the interface by forming bonding forces among them.18-21 This phenomenon is known as the salting-in effect. due to the salting-in effect, the solvability of polar organic components enhanced in the aqueous phase.22 Therefore, from 5000 ppm to 15000 ppm concentration, because of the salting-in effect, the amount of surface active components at the interface increased and consequently, more asphaltenes were brought out from the system which led to more absorbance reduction. This result is in consistent with works that have been done by other scholars. Tharanivasan et al. in 2012 declared that, in the presence of emulsified water, before the onset point of precipitation, the amount of yield (asphaltenes that were separated from the system by centrifugation method) increased. They attributed this to the interfacial asphaltenes which were adsorbed at the water/oil interface and were brought out of the system with water droplets. This consequently led to the higher amount of yield compared to the condition that there is no emulsified water in the system.3 Tavakkoli et al. in 2016 also drew the similar conclusion and attributed the absorbance reduction in the presence of emulsified water to the adsorbed asphaltenes at the interface of water/oil which were brought out of the system by centrifugation.14 Furthermore, when brine concentration increased from 15000 to 45000 ppm, the onset point of precipitation increased to 43 vol. % of n-heptane and the corresponding absorbance finds its value at 79. The increase in the absorbance can be ascribed to salting-out effect. The salting-out effect is a phenomenon in which the polar organic components such as asphaltenes tend to move toward the oil phase at high salt concentration.23 In fact, at high concentration of salt, inorganic ions such as (Ca2+, Mg2+, etc.) break the hydrogen bonds which were formed by water molecules around the hydrophobic organic molecules. Hence, the solvability of organic species decreased in the aqueous phase.24, 25 Therefore, as the brine concentration increased from 15000 to 45000 ppm, due to the salting-out affect, the presence of surface active components decreased at the water/oil interface. As a result, a number of surface active molecules, which are brought out of the system by centrifuge force, is decreased; and, the absorbance percentage is increased. Furthermore, the variations in the onset point of precipitation values can be justified by ion-bridging phenomenon which is explained as follow. Ions play another key role in the system which is known as the ion-bridging feature. Ionbridging that has a direct effect on asphaltene clustering and instability. In fact, asphaltenes molecules which are a presence in the aqueous phase at low salt concentrations, can play the role of polydentate ligands and form a chelate with available cations and promote asphaltene agglomeration that in turn leads to more asphaltene instability. According to the definition, Ligands are atoms or molecules with available electron pairs; they may be neutral or negatively charged. Ligands are usually known as electron donors imbibed to the metal ion (the electron acceptor) at the center of the complex.26-28 A ligand molecule which has more than one donor atom is denominated polydentate ligand.26-28 A polydentate ligand bonds to metal ions and forms ring with them in a process named chelation. The compound produced in chelation process is named chelate and the polydentate ligand is mentioned as chelating agent.26-30 In other words, in low salt concentration, the motion of asphaltene molecules is accelerated toward the interface by the ions that are present in the aqueous phase. For as much as asphaltene molecules are polar components, they can pass through the interface and present in water phase besides the ions. The cations in aqueous phase form bonds with asphaltene molecules. This helps asphaltene molecules be stabled by reducing their surface charge and prevent them from precipitation. But, on the other hand, these cations can form a chelate with asphaltene molecules at the same time and act as a bridge and attach several molecules of asphaltene to each other and promote asphaltenes clustering. The chelating effect is more intense than neutralization of surface charge by cations; and, as a result, this leads to asphaltene instability. Figure 2 shows how asphaltene molecules form a chelate with calcium ions. The same result was attained in other studies. Wang et al. in 2014 referred to chelating effect in their investigation and declared 4 ACS Paragon Plus Environment

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that presence of cations in the system promotes asphaltenes agglomeration by ions-bridging or chelate formation and accelerate asphaltene deposition in a flow loop.15 Demir et al. in 2015 expressed the similar conclusion and said that in the presence of Ca2+ and Na+, agglomeration tendency of asphaltene is increased. They attributed this phenomenon to the ion-bridging feature and declared that in the presence of cations with the higher charge, this tendency increases. Besides the charge of cations, the authors mentioned that presence of inorganic components such as Na + and Ca2+ in asphaltene’s structure has a significant effect on the chelate formation.16 Figure 3 shows the effect of synthetic brine in the range of 100000 to 200000 ppm of MgCl2 salt. As it is clear from the figure, in very high concentration of MgCl2, there is no instability observed in the system. In other words, the absorbance amount and the onset point of precipitation remain unchanged. It can be concluded that at very high salt concentration, because of salting-out effect, asphaltene molecules show more stability compared to low-to-medium salt concentration. A similar observation has been reported by Tavakkoli et al. in 2016.14 They investigated the effect of deionized water and synthetic brine with 200000 ppm of NaCl salt on the instability of asphaltene for several crude oils in the form of a water-in-oil emulsion. They declared that in the presence of deionized water, the onset point of asphaltene precipitation becomes worsen for one of the crude oil samples. But, when deionized water was replaced with synthetic brine, there was no change observed on the onset point and that was similar to the condition that there were no emulsions exist in the system. Figure 4 shows the effect of CaCl2 salt on the onset point of asphaltene precipitation in the range of low-to-medium concentration. The onset point of precipitation corresponding to brine with 5000 ppm concentration is found to be 48 vol. % and the corresponding absorbance value finds its value at 82. This reduction in absorbance continues up to 15000 ppm of salinity. At this point, the absorbance value and the onset point of precipitation are determined to be 75 and 46 vol. % respectively. This reduction in absorbance is due to salting-in effect. After the concentration of 15000 ppm, the absorbance starts to increase as far as at 45000 ppm of CaCl2 concentration, the absorbance value and the onset point of precipitation are found to be 78 and 48 vol.% of n-heptane respectively. This increase in concentration is due to salting-out effect. At a high-to-very high concentration of CaCl2 salt (100000-200000 ppm), a similar trend like MgCl2 salt for the concentration of 100000 ppm and above was observed. In this concentrations of CaCl2 salt, there is no instability observed in the system. In other words, the absorbance amount and the onset point of precipitation remain unchanged. It can be concluded that at very high salt concentration, because of salting-out effect, asphaltene molecules show more stability compared to low-to-medium salt concentration. Since the data is similar to figure 3, data was not shown at higher concentrations. By comparison between the Figures 1 and 4, it can be deduced that in all concentration of synthetic brine which was constructed by MgCl2 salt, the onset point of precipitation getting more instable. Since the charge of Mg2+ and Ca2+ are equal and both of them are bonded with chloride anions, it can be inferred that Mg2+ has a greater effect on the instability of asphaltene compared to Ca2+ cations. This phenomenon can be attributed to the ionic radius. Since the ionic radius of Mg2+ is lesser than ionic radius of Ca2+, it has a higher charge to surface ratio and as a result, Mg2+ cations exhibit a greater effect on the instability of asphaltenes through the chelate formation.

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Industrial & Engineering Chemistry Research 100 crude oil

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N-HEPTANE VOL% Figure 1. The result of the indirect method for crude oil M1 after correction of the dilution effect, in the presence and absence of emulsified synthetic brine, formed/prepared by MgCl2 salt in the range of low to medium concentrations.

Figure 2. Chelate formation of asphaltene’s molecules with magnesium cations that leads to instability of asphaltenes.


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N-HEPTANE VOL% Figure 3. The result of the indirect method for crude oil M1 after correction of the dilution effect, in the presence and absence of emulsified synthetic brine, formed/prepared by MgCl2 salt in the range of high to very high concentrations.

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Figure 4. The result of the indirect method for crude oil M1 after correction of the dilution effect, in the presence and absence of emulsified synthetic brine, formed/prepared by CaCl2 salt in the range of low to medium concentrations.



3.2. Comparison of Divalent and Monovalent Inions on Instability of Asphaltenes In order to investigate the effect of divalent and monovalent anions on the instability of asphaltenes of crude oil M1, MgCl2 and MgSO4 salts were selected. Figure 5 shows the absorbance diagram versus the volume percent of nheptane for synthetic brine which was prepared by MgSO4 salt. As it is observed from the figure 5, the onset points of precipitation in both 5000 and 45000 ppm brines are the same and equal to 48 vol. % of precipitant. In 15000 ppm of salinity, due to salting-in effect and chelate formation, the onset point moves toward lower vol. % of n-heptane and it finds its value at 46 vol. % of n-heptane.. By comparing the MgSO4 and MgCl2 salt concentrations in the range of 5000 to 45000 ppm, it can be concluded that presence of monovalent anions has a greater effect on the instability of asphaltene compared to the divalent anions. This observation can be attributed to two factors; size and charge of divalent anions. Since the size of sulfate anions is greater than chloride anions, it causes a limited number of asphaltene molecules to be present in the aqueous phase. Furthermore, the presence of heteroatoms like S2-, N2- and O2- with the same charge of sulfate anions is another effective factor for reduction of asphaltene molecules in the aqueous phase. Reduction in the number of asphaltene molecules in the aqueous phase makes the interaction between cations and asphaltene molecules become lighter. As a result, the degree of asphaltene clustering due to ion-bridging gets lower. Similar results have been reported by Lashkarbolooki et al. in 2014 when they investigated the effect of divalent and monovalent anions on the complex formation of Mg2+ with asphaltene molecules. They declared that the intensity of complex formation of Mg2+ with asphaltene molecules in the presence of sulfate anions decreased compared to the condition that chloride anions present at the system. They attributed this phenomenon to the size and charge of sulfate anions.25 At a high-to-very high concentration of MgSO4 salt (100000-200000 ppm), a similar trend like MgCl2 salt for the concentration of 100000 ppm and above was observed. In fact, there were no changed observed on the onset point of precipitation of asphaltenes and that is because of the salting-out effect. In addition, it has been reported that as the negative charge of anions increase, the stability of emulsified water will be decreased in the system and as a result, the available surface for the presence of asphaltene molecules at the interface of water/oil reduced that leads to less chelate formation 31.

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Figure 5. The result of the indirect method for crude oil M1 after correction of the dilution effect, in the presence and absence of emulsified synthetic brine, formed/prepared by MgSO4 salt in the range of low to medium concentrations. 8 ACS Paragon Plus Environment

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3.3. Comparison of Divalent and Monovalent Cations on Instability of Asphaltenes To investigate the effect of divalent and monovalent cations, MgSO4 and Na2SO4 salts were applied. Figure 6 illustrates the effect of Na2SO4 salt on the instability of asphaltenes. As it is obvious from the figure, the Na2SO4 salt has a little effect on the instability of asphaltenes in all range of salt concentration from 5000 to 45000 ppm. The onset points of precipitation are the same and it is equal to 50 vol. % of n-heptane. Furthermore, the variations in the absorbance are negligible as in that the absorbance finds its value at 90, 85, and 87 for 5000, 15000 and 45000 ppm salt concentration. By comparison the Figure 5 and 7, it can be inferred that divalent cations have more effect on the instability of asphaltenes compared to monovalent cations in all range of salt concentration. It can be deduced that as the charge of cations increases, the instability of asphaltene increases too. Furthermore, at a high-to-very high concentration of Na2SO4 salt (100000-200000 ppm), a similar trend like MgCl2 salt for the concentration of 100000 ppm and above was observed. There was no change neither on the onset point of precipitation nor on the absorbance amount because of salting-out effect.

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N-HEPTANE VOL% Figure 6. The result of the indirect method for crude oil M1 after correction of the dilution effect, in the presence and absence of emulsified synthetic brine, formed/prepared by Na2SO4 salt in the range of low to medium concentrations.

4. Conclusions In this study, the effects of emulsions which had been prepared by different salts (MgCl2, CaCl2, MgSO4, and Na2SO4) in a wide range of concentrations on the instability of asphaltene were investigated through measuring the onset point of precipitation with a novel technique called “indirect methods”. The following conclusion can be drawn from this study:    

Presence of all the salts from the low to mid concentration in the system results in asphaltene instability. The main mechanism in instability of asphaltenes could be attributed to the chelate formation which has been reported to can due to salting-in effect in the system. In the low salt concentrations (from 5000 up to 15000 ppm) the salting-in effect helps asphaltene molecules move toward the interface and ultimately leads to more chelate formation and asphaltenes clustering. The results showed that divalent cations have a more effective role in forming chelate compared to monovalent cations. 9 ACS Paragon Plus Environment


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  



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When the charge of the cations is the same, the cations which have a higher charge to radius ratio cause chelates to form more easily. Divalent anions hinder asphaltene aggregation more than monovalent anions. In the medium salt concentrations (i.e. from 45000 ppm) the salting-out effects forced asphaltene’s molecules to stay in the crude oil bulk rather than putting them at the interface and as a result, reduced the chelate formation. Whereas, at the high-to-very high salt concentration (i.e. from 100000 to 200000 ppm) asphaltene instability was not observed due to already complete salting-out effect. The instability of asphaltenes decreased according to the following order: crude oil with no brine > MgCl2 > CaCl2 > MgSO4 > Na2SO4.

Supporting Information Schematic of the Indirect Method.

Acknowledgements The authors acknowledge Dr. Mohammad Tavakkoli from Department of Chemical and Biomolecular Engineering of Rice University for fruitful discussions and assistance. The authors also would like to thank Ms. Zeinab Derikvand from Department of chemical and petroleum engineering at Shiraz University for her valuable assistantship during our work. The crude oil and its SARA analysis used in this research project have been supplied by National Iranian South Oil Company (NISOC) which is gratefully acknowledged.


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TOC/ GRAPHICAL ABSTRACT

100 crude oil

90

5000 ppm MgCl2 80

Absorbance @ 700 nm

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15000 ppm MgCl2

70

45000 ppm MgCl2

60

5000 ppm MgSO4 15000 ppm MgSO4

variation of onset point of asphaltene precipitation in presence of different ions

50 40

45000 ppm MgSO4

30 20 10 0 0

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N-HEPTANE VOL%


Investigating the Effect of Salinity on the Behavior of Asphaltene

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