O emulsions in presence of asphaltene at reservoir thermodynamic conditions

Journal Pre-proof Formation and stability of W/O emulsions in presence of asphaltene at reservoir thermodynamic conditions

Ismail Ismail, Yousef Kazemzadeh, Mohammad Sharifi, Masoud Riazi, Mohammad Reza Malayeri, Farid Cortés PII:

S0167-7322(19)34179-0

DOI:

https://doi.org/10.1016/j.molliq.2019.112125

Reference:

MOLLIQ 112125

To appear in:

Journal of Molecular Liquids

Received date:

25 July 2019

Revised date:

10 October 2019

Accepted date:

11 November 2019

Please cite this article as: I. Ismail, Y. Kazemzadeh, M. Sharifi, et al., Formation and stability of W/O emulsions in presence of asphaltene at reservoir thermodynamic conditions, Journal of Molecular Liquids(2019), https://doi.org/10.1016/ j.molliq.2019.112125

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© 2019 Published by Elsevier.

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Formation and Stability of W/O Emulsions in Presence of Asphaltene at Reservoir Thermodynamic Conditions. Ismail Ismaila, Yousef Kazemzadehb, Mohammad Sharifib, Masoud Riazic*, Mohammad Reza Malayeric,d, Farid Cortése a

School of Mineral Resources Engineering, Technical University of Crete, Crete, Greece Department of Petroleum Engineering, Amirkabir University of Technology, Tehran Polytechnic, Tehran, Iran c Enhanced Oil Recovery (EOR) Research Centre, IOR EOR Research Institute, Shiraz University, Shiraz, Iran d Institut für Verfahrenstechnik und Umwelttechnik (IVU), Technische Universität Dresden, D-01062, Dresden, Germany e Grupo de Investigación en Fenómenos de Superficie – Michael Polanyi, Facultad de Minas, Universidad Nacional de Colombia-Sede Medellín, Colombia

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Corresponding author: Masoud Riazi* E-mail: [email protected]

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Abstract In-situ formation of W/O (water-in-oil) emulsion in oil reservoirs is gaining increased attention particularly for EOR purposes. In this process, asphaltic components may serve as a stabilizer for W/O emulsions, which may result in higher viscosity of injected fluid front causing improved oil recovery. In this study, the effect of temperature and pressure on the formation and stability of W/O emulsions were evaluated by a high pressure-high temperature (HP-HT) test apparatus in order to simulate as close as possible to reservoir conditions. The experimental results showed that the stability of the emulsions reduced as temperature increased from 25 to 110°C. The relationship between pressure and the emulsion stability, on the other hand, is more complicated where an increase in emulsion stability was shown with pressure when it reaches a plateau at 27579.0 kPa (4000 psia) with a decrease afterwards. Based on the experimental results, different mechanisms dominate in each pressure interval. In the first pressure interval (3447.4 to 27579.0 kPa) the break-up of the dispersed phase droplets caused by the exerted shear energy, which makes the dispersed phase droplets smaller in size, where W/O surface coverage of the asphaltene was important. In the second interval pressure (27579.0 to 41368.5 kPa), the reduction of emulsifier (asphaltene) due to the increased pressure in the dead oil showed a much lower stable emulsion. Thus, at pressures below 27579.0 kPa with increased pressure, the mechanism of increased shear energy prevailed. For the pressure interval of 27579.0 to 41368.5 kPa though, as pressure increased, asphaltene precipitation decreased at the water/oil interface. Keywords: Emulsion Stability, Asphaltene, Interfacial Tension, Enhanced Oil Recovery

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1. Introduction Emulsions are two-phase systems which would be resulted from dispersion of at least one immiscible liquid in another [1-3]. Generally, they are thermodynamically unstable due to the interfacial tension between the phases [4-6]. Nevertheless, they can sometimes remain stable for weeks, months, and even years [7]. The stability of emulsions is usually improved in the presence of surfactants, polymers, inorganic salts or a combination of them [8]. Where, in turn, stability of the emulsion depends on the amount of surfactant available at the interface which is sufficient for the prevention of spontaneous separation of two phases [9, 10]. Through the formation of W/O emulsions in the front of injection fluid, the viscosity of injected water is increased and as a result, the mobility ratio would become favorable [11]. Under such circumstances, the coalescence of oil droplets gives rise to formation of larger oil droplets and improved oil mobility [12, 13]. Moreover, the formation of emulsion blocks the high permeable zones and causes the sweep efficiency to increase [14]. Separation of phases in emulsions is affected by thermodynamics of the system [15] as such that when oil and water form two separate phases, then the free energy of the system decreases [16]. Taking into consideration the unstable nature of emulsions, they tend to be separated into two different phases over time due to the high interfacial tension [1719]. In terms of thermodynamics, the Gibbs free energy is a criterion for spontaneity of a process, which can be expressed as: ∆𝐺 𝑓𝑜𝑟𝑚 = ∆𝐴𝛶12−𝑇∆𝑆 𝑐𝑜𝑛𝑓

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where ∆𝐺 𝑓𝑜𝑟𝑚 is the change in Gibbs free energy of emulsification, ΔA is the changed interfacial area, 𝛶12 is the interfacial tension between phases 1 and 2 (oil and water), T is temperature, and ∆𝑆 𝑐𝑜𝑛𝑓 refers to the configurational entropy. In most cases, T∆𝑆 𝑐𝑜𝑛𝑓 << ∆𝐴𝛾12, meaning that ∆𝐺 𝑓𝑜𝑟𝑚 would be positive [20]. This implies that the formation of emulsions is non-spontaneous and therefore they are thermodynamically unstable [21]. In the absence of stabilizers, emulsions become unstable by accumulation, coagulation, Ostwald process or a combination of these processes [22]. Stability or instability of the emulsions depends on several factors as given in the following order [23-25]: o o o o o o

Properties of water and oil Size of solid particles/heavy oil particles (asphaltene particles) Mixing rate or the applied shear stress Oil/water fraction in the emulsion Type and number of emulsifiers Free energy of the system 2

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Increase in temperature usually helps the separation of W/O emulsions. This originates from the decreased viscosity of the continuous phase, which tends to coagulate further. Temperature elevation also reduces the surface viscosity, which itself destabilizes the surface oil film in the water [26]. In on-going studies regarding food science, the effect of pressure on the formation and the stability of the emulsion has been studied. According to the results, the dispersed phase droplets become smaller with increased emulsification pressure, and the emulsion becomes more stable [27]. Furthermore, according to medication studies, the increase in the pressure on the emulsion build-up leads to the formation of a more stable emulsion with smaller droplets [28]. In the literature, there are a large number of studies on W/O and O/W emulsions. Tambe and Sharma (1994a,b) showed that despite the unstable nature of emulsions, the formation of kinetically stable emulsions is possible by emulsifiers and stabilizers. The formation of W/O emulsions during oil production is also feasible in heavy oil reservoirs in the presence of surfactants (i.e. asphaltene). This kind of emulsion has mostly been observed in the reservoirs with high-salinity formation water containing multivalent cations such as calcium and magnesium [29, 30]. However, emulsifiers (i.e. surfactants) form a film for protecting emulsion and preventing its rupture [31, 32]. Polar components that are sufficiently present in these solutions would cause asphaltene molecules to migrate to the water/oil interface [8, 33, 34] in which the high surface activity of asphaltene particles available in the crude oil, make it capable of moving to the interface and increasing the oil/water interfacial film and the stability of emulsions [8, 35]. In fact, the asphaltic compounds of the crude oil would act as natural surface-active agents where they accumulate at the interface and form w/o emulsions, which can be further stabilized by the salts present in the aqueous phase of the emulsions [34, 36, 37]. Meanwhile, Czarnecki and Kevin Moran (2005) presented a novel mechanism regarding the formation of w/o emulsions based on at least two types of chemicals. The first one is the asphaltene adsorption with a slow and irreversible process leads to the formation of a stable film while the second one is the rapid adsorption of low molecular weight materials such as surfactants. Furthermore, their results proved that there are two different mechanisms regarding emulsion stability at high concentrations and below the infinite dilution point, which implies the competition of asphaltene adsorption at the oil/water interface [30]. Cuiying Jian et al. (2016) showed that by increasing the asphaltene surface, the hydrogen bond formed between asphaltenes and water molecules reduces the interfacial tension between water and oil. Emulsions may not only consist of oil, water, and emulsifiers; but they may also contain solid particles or gas [38]. 3

Journal Pre-proof In previous studies, most of the proposed mechanisms were based on the formation and stability of emulsions at ambient temperature and atmospheric pressure. In this study, experiments at elevated temperature and pressure are conducted in order to simulate the emulsion formation and stability at reservoir conditions. This is because asphaltene precipitation is influenced by changing in temperature, pressure, and compositions [39, 40] and that the precipitation of asphaltene (as an emulsifier) can also change the formation and stability of the emulsion. Finally, the formation of W/O emulsions are then analyzed at thermodynamically different conditions.

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2. Experimental setup

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2.1 Experimental apparatus 2.2.1 HP-HT Emulsion Formation and Stability Analyzer The apparatus constructed for investigation of the emulsion stability at reservoir condition consisted of several parts including:  The main cell  Inlet valve for water  Inlet valve for oil  HP PT-100 thermometer  HP heater  HP fluid isolating valves for minimizing the dead volume in the system  Pressure gauge  Magnetic coupling  Easy to remove connections for cleaning  Electrical panel  The visual part which includes: o The interchangeable chamber for retaining HP optical glasses o The interchangeable chamber for taking photographic pictures of emulsion formed. o HD (High Definition) digital microscope o HP inlet and outlet valves

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Figure 1. The schematic view of the HP-HT emulsion formation and stability analyzer.

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The emulsion formation and stability analyses would begin by pumping oil and water to the main chamber. For this purpose, two different cylinders with separated lines were considered for oil and water injection, as indicated in Figure 1. After finishing the injection of fluids (oil and water) to the main chamber, the complete mixing of the fluids was performed through a strong magnetic coupling with a maximum rotational speed of 4000 RPM. It should be noted that, the magnetic force can affect the oil directly and can cause solid particles (i.e. asphaltene) in the oil to be deposited on bottom of the chamber. To prevent these phenomena, the magnetic coupling was embedded outside of the chamber. In other words, as the engine is started, the power is transferred to a rod that transform torque to rotate the propeller in the fluid chamber. An increase in pressure and temperature to the desired values is then applied to the fluids injected present in the main chamber. After that the mixing has been applied where it should be mentioned that the intermediate valve between the main chamber and visual chamber is kept closed until the point that complete mixing of fluid is achieved which is assumed to be after 5 min of mixing by the propeller. Once this is accomplished then the HP-HT fluid is ready to enter to the visual chamber, which is performed through the opening of the intermediate valve. It is noteworthy to remember that the fluid layer in the visual chamber is sufficiently thin for light being able to pass through dark fluids where the light source used was a SMD LED with a power of 36 W, which was adjusted with a dimmer. The visual chamber was also equipped with an HP drain valve for evacuation of the fluid from the chamber after 5

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analysis. For safety reasons, a pressure safety valve (PSV) was used to avoid extremely high pressures with a rapid rise in temperature, but also working with crude oil in laboratory investigations the volume of the crude oil follow a volume criteria in which the main chamber has been designed to be as low as possible (50 cc( . However, the prepared emulsions were sampled and the number of water droplets distributed in the oil phase was analyzed on the surface of a lam )i.e. laboratory plate) using a highmagnification camera (Nikon microscope Eclipse E400, Nikon Corp., Japan).The structure of the emulsions is usually determined by the distribution of its statistical droplet size in which the distribution of droplet size gives good information on the efficiency of the emulsion-making process as well as many other information about the stability of the system with time. In other words, the size of the emulsion droplets is influenced by other properties of emulsion, including stability and rheology, valuable information is obtained from these particle size distribution studies where the advantage of our system is that the emulsion imaging device is designed to take pictures of droplets at high temperatures and pressures that mean information at reservoir condition. Meanwhile, the formation of emulsions through the dissolution of droplets is mainly based on the application of energy where this energy somehow increases the level of formation of emulsions. As a result, more fine droplets form and large droplets turn into fine ones and cause the stability of the emulsion. This energy can be provided by using various devices such as an ultrasonic bath, a mechanical stirrer, an ultrasonic transducer, a homogenizer, and a magnetic stirrer. The homogenizer in the device is capable of stirring a mixture of water and oil during different periods but since the purpose of this study is to investigate micro-emulsions in oil field, the homogenizers have to be set to a level that would allow for the formation of droplets in a range of micrometers. The rheological properties of emulsions such as stability, depend on various factors, such as droplet size distribution, surface film strength, average particle size, and other characteristics that change over time. These factors are investigated by using software to perform photographic analysis of the images. Therefore, all the analyzes performed (measuring surface tension and the RPM force of two-phase separation required) were analyzed two hours after emulsification has been achieved. After emulsifying and photographing the prepared emulsions at high temperatures and high pressures, a sample is taken to measure the surface tension and the centrifugation force required for twophase separation (30 cc). 2.2.2 Interfacial Tensiometer The schematic view of IFT measuring set-up is illustrated in Figure 2. As indicated, the apparatus is composed of an injection pump, a camera, and a quartz cell. To start measuring, the syringe filled with the sample is placed on the injection pump. A plastic 6

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pipe and a needle are attached to the two ends of the syringe. The quartz cell is also filled with the bulk fluid supposed to be a surrounding fluid. In this study, to measure the IFT between prepared emulsion and water, the syringe and the quartz cell are filled with emulsion and water, respectively. The W/O emulsion droplet is slowly injected upward through the needle in the quartz cell filled with water. The injection rate is kept noticeably low for the injected emulsion volume to be in the order of microliter. The picture taken from the emulsion droplet by the camera is finally analyzed by the software.

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Figure 2. Schematic view of pendant drop IFT apparatus

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Interfacial tension reported for each point, are equilibrium interfacial tension. Interfacial equilibrium tension is achieved at the threshold of falling drops. The average position time of the droplet on the needle is 30 minutes. Therefore, interfacial tension is measured after 30 minutes, when the droplet is present in the water bulk. Interfacial tension was measured three times in each test where the value of interfacial tension for each test is the average calculated from the three measurement’s values. The test error in each experiment or emulsion prepared is ± 0.3 mN/m. In other studies, the interfacial tension measurements have been measured between water and oil where in this one, the interfacial tension of emulsion (as the oil phase) and water were used to measure the stability of the emulsion formed at different pressures and temperatures. Several papers have suggested that a stable emulsion is associated with decreasing interfacial tension of water and oil [37, 41-43]. But none of the studies have indicated and how a W/O emulsion behaves in the adjacent water phase (i.e. water-filled in quartz cell). For this 7

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reason, the interfacial tension of emulsion and water was measured and reported in this study. However, Emulsion’s behavior formed at oil reservoir conditions with the contact of formation water, and injected water is the main goal of our investigation. Macroscopic measurement depends on the changes that occur in the microscopic level where changes in the microstructures of the emulsion are expressed in terms of the interaction between the droplets therefore in terms of stability. In the other hand the distribution of fluids and their behavior in the reservoir depends on parameters such as surface tension, wettability, capillarity, and surface mass transfer where interfacial tension of two phases is an important parameter for understanding fluid flow in the porous medium where the surface tension is a quantity for measuring the power of interfacial forces. In other words, the two-phase surface tension in terms of increasing the Gibbs energy per unit area in moles, are defined at constant temperatures and pressures. Surface tension measurements of water and of the stable emulsions formed can be important in order to demonstrate the stability of the emulsion from a macroscopic perspective where in an unstable emulsion, which has large droplets of water in oil, force is applied at the interface layer between the emulsion and the water, which indicates also the surface tension. Based on that when a drop of emulsion is placed in a bulky container, the water droplets tend to leave the emulsion phase and enter the aqueous phase if the emulsion is unstable. That mean more the emulsion is unstable, more the larger droplets tend to leave the emulsion phase and causing the intermolecular forces to collapse. Since water droplets with dimensions of about 1-10 μm in diameter from the emulsion phase are placed at the contact surface of the water and emulsion, the contact surface of the water and the emulsion increases, that lead to the increase of the surface tension.

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2.2.3. Centrifuge The emulsions formed at high temperatures and pressures have different stabilities since they can be affected by various operating factors. The centrifuge’s energy has been used to measure the stability of the emulsions (resistance against the separation of two phases). Hematocrit centrifuge with a maximum force of 13,000 rpm in which 200 model of Hettich had been used in the experiments. 2.3 Methodology The investigation of formation and stability of the W/O emulsion is proceeded by injecting, 30 and 70 vol.% of seawater and dead oil, respectively into the main chamber. The effect of temperature was studied by selecting different temperatures of 25, 45, 75, 90, and 110 °C. Furthermore, the pressures studied are 3447.4, 6894.7, 13789.5, 20684.3, 27579.0, 34473.8, 41368.5 kPa. After the injection of fluids (oil and seawater) into the main chamber and adjusting the thermodynamic conditions (temperature and pressure), 8

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the magnetic propeller is turned on at a rate of 1000 RPM for five minutes and selected as basic criteria for all emulsion formulated and investigated. Afterwards, different pictures from the fluid sample were taken with time from the emulsion flowed from the main chamber to the visual chamber one. Considering the resultant findings and based on microscopic observation it has been found that the emulsion presents the highest stability after two hours as no noticeable changes in shape and size of droplets were observed. The prepared emulsions were mainly affected by turbulence caused by the energy of the agitator/propeller. After two hours though, an adequate balance time has passed for the intermolecular forces and the contact surface between the droplets to be balanced and become stable. Therefore, a base time of two hours was set to be criteria for taking pictures of emulsions. The results also show that the stability of the emulsion declines gradually over time. According to previous studies published on the surface film strength with time through viscosity measurements, it has been proved that the stability of the surface film increase in a time span of 2 to 4 hours [4, 44] This can be explained by the time required for the movement and accumulation of asphaltene in the surface between water and oil and the formation of a hard and viscoelastic layer [36]. The Image J software was used in order to analyze the images, and the distribution of dispersed water droplets in W/O emulsion. In addition, taking pictures were performed from different positions of the sample and the outlier data were removed for precise analysis by the software where in case of weak stability and diverse distribution of droplets at different points, the analysis was also performed based on the different pictures taken from various points of the sample. Figure 1 in the appendix shows a typical image of an emulsion sample as an input to the analyzing software. The picture is then converted into an 8-bit picture by the software, as indicated in Figure 1b, where the fluctuations within the image are eliminated. For more accuracy, the circularity of 0.9 to 1 for droplets was selected by the software for analyzing. Figure 1c presents the final analyzed picture from the software with the number and diameter of droplets organized as a separate file, which is accompanied by a histogram graph for comparison. As water volume/fraction increase in the emulsion, size of dispersed droplets is expected to increase, and the thickness of established film declines due to the presence of polar components of the crude oil (i.e. asphaltene). This phenomenon would decrease the repulsive forces among droplets where droplets start to join and flocculate due to the rupturing of the thin films around droplets. As a result, bigger droplets are formed, and the coalescence of the droplets occurs. Similarly, at low fractions of water, the dispersed water is low and smaller regarding the first case and does not change the behavior of the emulsion. Therefore, different emulsion was formulated with different water fractions

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after which the volume percentage of water was kept constant at 30 vol.% in the prepared emulsions. In order to determine the repeatability of experiments in the analysis of droplet distribution in each test sample, the results and images have been recorded three times at least. The calculated errors regarding calculation of average diameter of water droplets in oil, per recorded time, is less than 0.1 μm where it shows that, in the case where the test and the measurement is repeated, the average error is found to be less than 0.1 μm of droplet diameter. Microscopic measurements do not usually detect tiny droplets (less than 1 μm and to nanometer). Thus, in addition to microscopic measurements, macroscopic measurement is also required. In addition, the distribution of droplet size in microscopic dimensions affects other microscopic features of rheology also in order to ensure the results, microscopic measurements of surface tension and the necessary centrifugal separation energy measures were also performed. These two parameters help to cover the defects in the absence of the size of the droplet size where in case of any discrepancy in the discarding of small droplets (at the nanoscale), it can be covered in macroscopic measurements.

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2.3.1 Asphaltene and resin extraction The standard IP-143 method has been used to extract asphaltene. Later, the extraction of resin was also proceeded where resins were extracted from the deasphalted oil with the column chromatography method. The maltene (deasphalted oil + n-heptane) was adsorbed to a column of silica gel (Merck, 35−70 mesh ASTM); then the saturates and aromatics were washed by a solution of 70:30 n-heptane (Merck, mole fraction purity of >0.990) and toluene (Merck, mole fraction purity of >0.990); and finally, a mixture of acetone (Merck, mole fraction purity of >0.990), dichloromethane (Merck, mole fraction purity of >0.990), and toluene with the ratio of 40:30:30 was used to extract the resins from the column. 2.3.2 Crude Oil Analysis. The common laboratory procedure has been used to simulate distillation (SIMDIS) for description of oil which uses chromatography to estimate the carbon number distribution of an oil. It has been proceeded by gas chromatography (GC) with helium as a carrier gas. 2.3.3 Ion Concentration Analysis The well-known ion chromatography method has been used to describe the ion concentration of water used in this research.

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2.3.4 Phase Separation of Emulsions After taking images from the visual chamber, 40 cc of sample was taken from the mixing chamber through the drain valve for measuring the interfacial tension between the emulsion and distilled water. The centrifugal force required for the separation of oil and water was also evaluated by taking a sample of 30 cc and monitoring the attempted RPM for the complete phase separation of the W/O emulsion taking from the visual chamber through the drainage valve. The repeatability error of each test is 500 RPM. The micro-hematocrits centrifugation is determined using centrifugal hematocrit center (water to oil ratio) in different range. These types of centrifuges are rotated, with capillary tubes containing the specimen horizontally placed on it. After centrifuges were performed, using a graded plate, the amount of water in the continuous oil phase was obtained and by placing emulsion samples in centrifuges pipeline of 30 cc, they measure up the amount of water separation and the distance that separates about 30% of the seawater is obtained. This round is considered as a test for the separation of water and oil.

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2.3.5 Synthetic Oil

In this study, a synthetic oil with a solution containing 60%. vol. of toluene and 40% vol. of

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normal heptane was prepared. As stated, before the crude oil contained 7.71 wt.% of asphaltenes and 2.61% wt.% of resin. For this reason, the synthetic oil solution (heptol) has been added separately 7.71% by weight of asphaltene and 2.61% by weight of resin extracted from the crude

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oil, as per Table 4.

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Table 1. Compositions of prepared synthetic oil

Heptol Heptol+7.71 wt. % Asphaltene Heptol+2.61 wt. % Resin

n-Heptane Vol. % 40 37

Toluene Vol. % 60 56

Asphaltene Wt. % 0 7

Resin Wt. % 0 0

39

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0

2.50

3. Results and Discussion In water/chemical flooding, if the volume of either phase increases, then it transforms to a continuous phase. Thus, it can be said that the ratio of water to oil can be a decisive factor in the formation of emulsion type. In case of a heavy oil system, due to higher viscosity of crude oil, water droplets would have lower collision tendency than oil droplets [45]. 11

Journal Pre-proof The thermodynamic conditions of the reservoir would also play an important role in the formation and stability of emulsion hence parameters such as temperature, pressure and asphaltene must be discerned.

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3.1 Material 3.1.1 Crude Oil The investigated oil in this study was obtained from one of the southern Iranian oil reservoirs, with a gravity of 21°API. The compositional and SARA analyses performed on the original crude oil are presented in Tables 2 and 3. In addition, the Colloidal Instability Index (CII) of the crude oil used was calculated to be 0.94. This implies that the asphaltene is unstable and on the edge of precipitation [46].

C2

iC4

nC4

iC5

nC5

C6

Mole %

0.49 0.73 0.47 0.94 0.55 0.55 7.64 6.18 5.44 4.90 4.71 4.14 63.26

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C11

C12+

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Table 2. Compositional analysis performed on the crude oil used in this study

wt. %

Saturated Aromatic Resin

40.85 48.79 2.61

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7.71 Asphaltene Table 3. SARA analysis performed on the investigated crude oil 3.1.2 Seawater The compositional analysis performed on the seawater sample is presented in Table 4. Ions

Na+

HCO3-

Ca2+

Mg2+

Cl-

K+

SO42-

Total

Concentration (ppm)

18780

228

1250

1500

301

727

3360

26146

Table 4. Compositional analysis performed on the seawater used in this research 3.1.3 FT-IR analysis

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The FT-IR analysis of the crude oil is presented in Figure 3. It can be seen that the crude oil contains more acidic species than basic components since it possesses more COOH groups than NH2+. Based on that a polarity alteration of oil components like asphaltene is expected in case of water phase presentation where different behaviors are observed regarding the species present in water phase. Meanwhile, several studies [34-36, 47] showed that the main mechanism of asphaltene in the stability of the W/O emulsion is the formation of a sticky film (20-30 nm) with a mechanically strong 3D network at the W/O interface. Thus, emulsion would be stable due to the existence of molecules located at the interface, which delays the separation of two phases. One of the main existing molecule at the interface is asphaltene which is formed from both polar and non-polar functional groups and is known as surface active material [8, 34] where the dispersed phase exists as spherical droplets in the emulsion.

Wavenumber (cm-1)

R–CO–OH S=O R–CO–OH / NH2 / RSOOR / N–O / P–O RSOOR / R–CO–OH N–O

2993 1365 1226 1026 970

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Functional groups

Figure 3. FTIR analysis performed on the crude oil used in this research. 3.2 Effect of Temperature Figure 4 shows the different images of emulsions taken at different temperatures and pressure of 6894.76 kPa. As it is shown in Figure 5, as temperature increases the water droplets become larger and the emulsion stability noticeably deteriorates.

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Figure 4. Images of distributed water droplets in the continuous phase (oil) at pressure of 6894.74 kPa and different temperatures. Figure 5 shows the diameter frequency of formed droplets at different temperatures measured by Image J software. As temperature increases from 25 to 110°C, then the frequency of small droplets (<3 µm) decreases from 113 to 58, which confirms the decrease in stability of the formed emulsions. In other words, the rheological properties of emulsions such as stability, depend on various factors such as droplet size distribution where more fine droplets are present more the emulsion is stable and vise versa. According to the Le Chatelier's principle, whenever a change in concentration, pressure or temperature, the equilibrium will move in order to counteract the change [48]. In this

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case, a rise in temperature would cause the equilibrium of the system to shift in the endothermic direction in order to oppose the change in temperature.

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Figure 5. The frequency of water droplet diameters in oil at 6894.7 kPa and temperatures of 25, 45, 75, 90, and 110°C.

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Figure 6 shows the interfacial tensions between formed emulsions at different temperatures and distilled water. The emulsions in this study were very stable where visually a single phase can be observed. The densities of water and emulsions are measured and used as inlet data for the calculation of the interfacial. The following figure indicates that the interfacial tension rises from 6.4 to 27.8 mN/m with an increase in temperature from 25 to 110°C, where this increase of interfacial tension can be explained As the emulsion temperature increases, then the emulsion stability decreases due to lower solubility of asphaltene. Lower emulsion stability at standard conditions where interfacial tension is measured, also results in altering the equilibrium forces of the water and oil contact surfaces, leading to increased interfacial tension.

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interfacial Tension (mN/m)

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Figure 6. Interfacial tensions between formed emulsions at a pressure of 6894.7 KPa. and different temperatures and distilled water.

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Another sample from the formed emulsions at different temperatures was also investigated at different RPMs to determine the resistance of each emulsion against phase separation by the centrifugal force. Figure 7 shows the RPM needed for separating the two phases existed in the emulsion at different temperatures. As it can be seen also that the emulsion formulated at high temperatures show less resistance (low RPM required) of two-phase separation regarding emulsion formed at low temperature. In other words, as temperature increases from 25 to 110°C, RPM for phase separation reduces from 5000 to 2500 RPM.

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Figure 7. The required RPM for separating the continuous oil phase and the dispersed water phase in the formed emulsion at different temperatures by centrifugal force.

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3.3 Effect of Pressure The presence of asphaltene at the interface of water and oil can improve the stability of emulsion. The stability of asphaltene, in turn, varies with pressure change at a given temperature. By changing pressure, asphaltene molecules can be aggregated and then get precipitated. Therefore, pressure is a key factor in the stability of the W/O emulsions. Figure 8 presents the different images of W/O emulsions with a ratio of 70 to 30 vol.% (oil to water) at pressures of 3447.4 to 41368.5 kPa.

Pressure (kPa) Microscopic views

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Figure 8. The distribution of the water droplets in the continuous oil phase at 25℃ and at different pressures.

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As it can be seen from Figure 8, increase of pressure from 3447.4 to 27579.0 KPa, would cause the size of dispersed water droplets to decrease. On the other hand, from 27579.0 to 41368.5 kPa, the size and diameter of droplets may grow again. In order to analyze this behavior, firstly one has to consider that emulsion behavior is mostly affected by the water/oil interface as well as asphaltene functional groups. The latter are actually polar molecules, which can be spread in the water and react with it where this dispersion shows that water maintain the stability of asphaltene. Furthermore, the polarity of asphaltene at the water/oil interface increases due to the high atomic ratio of oxygen to carbon and high concentration of heavy metals [36] and this high polarity results from the increase in the reactions between polar molecules of water and asphaltene [49]. Additionally, formation/connate water in the oil reservoirs have different ions with different electrical charge, which affects the stability of asphaltene where two cases can exist i) in some crude oils, there is no asphaltene precipitation despite the high percentage of asphaltene and ii) there are cases in which oil is present with low asphaltene content that present a considerable asphaltene precipitation [50]. Numerous studies attributed this phenomenon to the thermodynamic conditions of the system (i.e. temperature, pressure, and oil composition). In accordance with the previous results performed by the present research group [51, 52] on the theory of "surface coverage", where 60% of the surface is not covered with asphaltene particles. It can thus be said that asphaltene particles can play a 18

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positive role in the stability of the emulsions by the time that 60% of the interface is not covered by asphaltene. Another main mechanism which can make the emulsion stable is the shear force exerted on the dispersed phase droplets causing them to become smaller and make the system more stable. As a result, the shear stress, applied to the system, can break-up the droplet of water, dispersed in the system, into fine droplets which would lead to smaller dispersed water droplets that need more time to flocculate and coagulate. As it is known, any change in temperature, pressure, and oil composition will cause the stability of the dispersed components in the oil to persist. For example, as the aromatic content of oil (a polarizable miscible solvent) increases, the asphaltene components form micelles, which will not increase as concentration increases. This phenomenon is mainly due to the contraction resulting from the molecular structure of asphaltenes. On the other hand, as the paraffin content of crude oil increases, the asphaltene components may become a solid mass phase. As for the CII index, crude oil is defined as a combination of pseudo-components that includes saturated, aromatic, resin and asphaltenes. The CII index defines asphaltene stability in terms of proportion of these components, which is defined as the mass ratio of asphaltenes and their constituents (saturates) to the total mass of asphaltene stabilizers and asphalt suspensions including resins and aromatics. If the asphaltene stability conditions are disturbed, these heavy molecules are present on the surface with water and oil in the aqueous phase, or deposit on the rock surface. Therefore, the relationship between asphaltene stability (as an emulsifier) and emulsion formation and stability is very important. If asphaltenes are present at the interface of water and oil then the conditions for forming emulsion can cause emulsion stabilization. Since the oil tested is dead oil, asphaltene precipitation would also increase at the water-oil interface. Therefore, the amount of sediment increases with increased pressure in dead oils. The highest amount of asphaltene present at the interface is related to the optimum emulsion stability point where the droplets have their lowest state due to the applied shear energy. Figure 9 shows the frequency of water droplet size for emulsions formed at different pressures. The figure shows that the frequency of small water droplets existed in the continuous phase of the oil is a function of the system pressure. As indicated, increase in pressure up to 27579.0 KPa intensifies the frequency of the small droplets and stability of emulsion, however, beyond that, it decreases.

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Figure 9. The frequency of water droplet diameters in the oil phase at different pressures of 3447.4, 6894.7, 13789.5, 20684.3, 25579.0, 34473.8, and 41368.5 kPa and temperature of 25°C.

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Figure 10 shows the frequency of small droplets (<3 µm) at different pressures. The figure shows that the highest frequency of small droplets (<3 µm) is related to the pressure of 27579 kPa, in which confirms stronger stability of the emulsions. High frequency/presence of small droplets means more stable emulsions.

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Figure 10. The frequency of the small water droplets with the size of <3 µm observed in the oil phase at 25°C and different pressures.

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Figure 11. The interfacial tension between emulsions and distilled water at different pressures and a temperature of 25°C. As it is evident in Figure 11, the highest and the smallest interfacial tensions are related to the emulsion formed at the pressure of 3447.4 to 27579.0 kPa, respectively. Primarily, the stability of the emulsion is a function of asphaltene stability in the crude oil where 21

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asphaltene role as a natural surfactant in the crude oil near to the precipitation point is imperative compared to other pressures in which asphaltene is stable. In this regard, asphaltene has a positive effect on the stability of the W/O emulsions by forming a viscoelastic layer around the droplets. The thickness of this asphaltic layer at the oil/water interface increases with the concentration of asphaltene in which the interfacial tension between water and oil decreases in the presence of asphaltene by the accumulation of asphaltene at the water/oil interface, which prevents the coalescence of droplets. The functional groups containing asphaltene molecules are electrically charged and their activity increases due to higher hydrophilicity. As the surface becomes more active, interfacial tension decreases and the elastic film increases [53]. Furthermore, higher concentration of asphaltene molecules at the water/oil interface causes a decrease in the interfacial tension between water and oil. The presence of asphaltene at the interface unbalances the forces and decreases the interfacial tension between the two phases. The decrease in interfacial tension may cause the stability of the emulsion. Therefore, lower interfacial tension somehow indicates better stability of the formed emulsion. Further increase in the concentration of asphaltene, the hydrogen bond formed between asphaltene and water molecules causes a decrease in the water/oil interfacial tension. For investigating the validity of resultant findings, the samples prepared from different emulsions have been evaluated under the separation test by centrifugal force using different RPMs. Figure 12 shows the required RPMs for the separating of the prepared emulsions at different pressures. As illustrated, the RPM required for separating the two phases of the emulsion in the pressure of 27579.0 kPa is 10000 RPM, which is 4 times higher than that of 3447.4 kPa.

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3.4 Impact of Asphaltene on the Emulsion Stability at Elevated Pressures Emulsion stability is due to the presence of asphaltene particles located at the interface between two liquid phases which postpone the tendency of separation of two fluids [33]. These particles are molecules with polar and non-polar functional groups in their structures, which are known as surface active agents.

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The stability of asphaltene depends on the ratio of resin to asphaltene of the crude oil in the reservoir [54]. There are several studies that have been performed on the effect of resin on the precipitation of asphaltene[55]. The striking point is the impact of resin on the onset point of the asphaltene precipitation. Resins are usually adsorbed by asphaltene and surrounds them to prevent the asphaltene precipitation [56]. For this reason, the oil reservoirs containing high resins are more stable where it shows that as the ratio of resin to asphaltene increases, the instability of the formed W/O emulsion also increases. Figure 15 shows the frequency of water dispersed droplets with area less than 3 µm2 in four phases of crude oil, toluene, and heptane solution (described in detail in material section). In addition to crude oil and the heptol solution prepared with a volume ratio of 60 to 40 toluene and heptane respectively, a heptol solution with the same volume ratio as the previous one containing 2.61 wt.% resin and another heptol solution with 7.71 wt.% was investigated at different emulsification pressures.

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Figure 13. Frequency of droplets smaller than 3 µm water in oil in the presence of a continuous oil phase, heptol, heptol with 2.61% by weight of resin, and heptol with 7.71% by weight of asphaltene at three pressures of 13789.5, 27579.5 and 41368.5 kPa. 23

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Polar bond: if water is not present in the system, the absorption of polar components probably occurs between polar surfaces of the molecules having polar atoms (N,S,O) [57]. The surface precipitation: asphaltene deposition occurs when oil is a weak solvent for heavy components [58]. The acid-base interaction: this reaction occurs between the parts having opposite electrical charges [59].

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As shown in Figure 13, in the absence of asphaltene and resin in the continuous phase system (heptol), the emulsion stability gradually increases. An increase in the number of droplets smaller than 3 µm2, with an increase in the emulsification pressure proves that the stability of emulsion is due to the increases of shear stress introduced into the dispersed phase. In the presence of 2.61% by weight of the resin in heptol, the stability of the emulsion increases with pressure. Taking into consideration both cases with percentage of asphaltene present in the system is zero (heptol and heptol with 2.61% by weight of resin), the amount of small water droplets (i.e. less than 3 µm2) in oil increases with increasing pressure. While the stability of the emulsion is lower one compared to that when there is a presence of asphaltene in the system. It is observed that the presence of asphaltene in synthetic oil increases the frequency of small water droplets in oil (i.e. less than 3 µm). The behavior of heptol system with 7.71% by weight of asphaltene is similar to that of crude oil containing the same amount of asphaltene. As observed, increase in pressure to a value of 27579.0 kPa in both crude oil and heptol containing 7.71% by weight of asphaltene, the stability of the emulsion increases and then decreases at higher pressures above this pressure. Based on this experiment it can be said that the two major mechanisms result in maintaining the stability of emulsion are shear stress introduced to the system and asphaltene precipitation at W/O interface. The asphaltene available in the hydrocarbon systems precipitate through one of the following interactions:

As pressure approaches a certain value due to more intensively interaction between molecules, a shear force would be exerted on the dispersed phase droplets causing them to become smaller and make the system more stable. The reason for the size reduction of dispersed water droplets in the oil phase may be a result of mainly the shear energy present at high pressure leading to break up of large droplets of the dispersed phase (oil) and the increased power of water connection to the hydrophilic acid part of asphaltene. For dead oil, there is a linear relation between the pressure and the amount of precipitated asphaltene [60]. As pressure increases, the crude oil density increases. This would, in 24

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turn, result in an increase of the solubility of asphaltene in crude oil thus the solubility of asphaltene increases with pressure in the bulk and as a consequence, the amount of asphaltene decreases at the water and oil interface. At higher pressures (above the pressure of 27579.0 kPa), the amount of asphaltene present at the interface would be reduced to stabilize the emulsion. In addition, high pressures would have a negative impact on the hydrophobic and electrostatic power of the asphaltene. Above the optimum pressure or the inflection pressure value (27579.0 kPa), the stability of the emulsion due to the unavailability/absence of asphaltene decreases for the stability of water in oil emulsion. Figure 15 presents a schematic of the asphaltene role at different pressures for the stability of water in oil emulsion investigated in this study.

Figure 14. Schematic of asphaltene role in the formation and stability of emulsions at different pressures. As shown in Figure 14, as pressure increases, the dominant mechanism for improved emulsion stability is the increase of shear energy to the water in oil droplets in the range of zero to 27579.0 Kpa which is confirmed by testing the same procedure for synthetic oil (heptol). The dominant mechanism for weaker emulsion stability is the paucity of asphaltene at the interface in the range of 27579.0 to 41368.5 kPa. Asphaltenes are polar molecules that can react with water. This high polarity is due to increased interactions between polar water molecules and asphaltenes.

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4. Conclusions The following conclusions can be drawn from the experimental results obtained in this study:  As temperature increased, the water droplets in the continuous oil phase became larger leading to weaker stability of the W/O emulsion. Therefore, the emulsion phases would be separated at a lower RPMs when attempted in a mixer. Furthermore, as temperature increased from 25 to 110°C, the interfacial tension between the formed emulsion and water went up from 6.4 to 27.8 mN/m.  As for the impact of pressure, the stability of the emulsion increased up to a pressure of 27579.0 kPa. Consequently, the interfacial tension is expected to decrease thus the two phases present in the emulsion were separated at higher RPMs. Beyond this pressure though, any further increase in pressure resulted in weaker stability of emulsion as water droplets became larger in the continuous oil phase. The interfacial tension was also increased and the required RPM for the separation of two phases was reduced. The results also implied that the emulsion stability before and after the optimum pressure depended upon the asphaltene precipitation. In both pressure regions/intervals, below and above the pressure of 27579.0 kPa, the influence of asphaltene in the stability of emulsion is different. In the first region (i.e. low pressures up to 27579.0 kPa), the stability of the emulsion increased due to the smaller droplets formed in the dispersed phase by the turbulent flow originating from the exerted shear stress. In the second region (i.e. above 27579.0 kPa), with increased pressure, the amount of precipitated asphaltene at the oil/water interface decreased, causing an increase in water droplets and a reduction in the stability of the emulsion. In the second region, the reduction of emulsifier (precipitated asphaltene) at the oil/water interface resulted in larger droplets. Generally, the mechanism of intensified shear energy dominated the pressure range of zero to 27579.0 kPa. For the range of 27579.0 to 41368.5 kPa, though, the diminishing amount of asphaltene dominated.  The results of synthetic oil analysis show that asphaltene acts differently on the interface of water and oil under the influence of pressure changes. The presence of asphaltene at the interface of water and oil increases the stability of the emulsion. At higher pressures of up to 27579.0 kPa due to the presence of asphaltene at the interface, the stability of the emulsion would monotonously enhance. For pressures above 27579.0 kPa though the solubility of asphaltene increases causing lower stability of the emulsion. Nomenclature CII Colloidal Instability Index 26

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There is no Conflict of interest

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Journal Pre-proof Highlights New apparatus was invented by the authors which describe the effect of temperature and pressure on the formation of W/O emulsions

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Stability of W/O emulsions were evaluated in order to simulate as close as possible to reservoir conditions thermodynamic conditions.

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The experimental results showed that the stability of the emulsions reduced as temperature increased from 25 to 110°C.

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The relationship between pressure and the emulsion stability, on the other hand, is more complicated

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An increase in emulsion stability was shown with pressure when it reaches a plateau at 4000 psia with a decrease afterwards.

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