Asphaltene Precipitation during Injection of CO2 Gas into a Synthetic

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Asphaltene Precipitation during Injection of CO2 Gas into a Synthetic Oil in the Presence of Fe3O4 and TiO2 Nanoparticles Sadegh Hassanpour,† Mohammad Reza Malayeri,*,†,‡ and Masoud Riazi† †

Enhanced Oil Recovery Research Centre, Department of Petroleum Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran ‡ Institute for Process Technology and Environment (IVU), Technische Universität Dresden, Dresden, 01062, Germany ABSTRACT: The production rate of oil reservoirs decreases gradually as reservoir pressure drops. This would occur due to various reasons, among them the precipitation and deposition of asphaltene. In this study, the impacts of two nanoparticles of titanium oxide and iron oxide with various concentrations were investigated for retarding the precipitation of asphaltene in the process of CO2 injection into synthetic oils. The experimental interfacial tension results clearly showed that both nanoparticles can effectively impede the precipitation of asphaltene. This has been sought in terms of change in the slope of interfacial tension curves. In this process, the adsorption of asphaltene by nanoparticles and interaction with precipitated asphaltenes at the interface between the two fluids were imperative in terms of adsorption capacity and affinity. The comparison of these two nanoparticles also revealed that Fe3O4 nanoparticles were more effective than TiO2 for reducing the precipitation of asphaltene at the interface between CO2 and oil and intensified the miscibility of CO2 into oil. Furthermore, the optimum weight fraction of both nanoparticles was found to be 1.0%. Finally, the reduction of the asphaltene precipitation intensity for the optimum concentration for oil solutions containing TiO2 and Fe3O4 nanoparticles was 17 and 18%, respectively. pressure cell filled with reservoir oil is the base of this method.10 Slim Tube. The slim tube technique is the most common approach for the determination of the miscibility of gas into oil. The method, in particular, evaluates the MMP, the minimum miscibility composition (MMC), the optimization of injection parameters, and composition of lean and enriched gas.11 Pressure/Composition (P/X) Diagrams. The P/X diagrams for miscibility gas−oil systems are plotted by determination of phase-behavior measurements in high pressure visual cells at reservoir conditions. The method is expensive, time-consuming, and cumbersome, requires a large volume of fluids, and is accompanied by noticeable experimental error.12 Vanishing Interfacial Tension (VIT). This method, which is used in this study, is a powerful and new technique which provides rapid determination of the MMP. It consists of measuring the IFT of the injected gas phase and the crude oil system for different pressures. The IFT is then determined by digitization of the images of the profiles of the sessile and pendant drops of crude oil enclosed in the vicinity of injected gas.13

1. INTRODUCTION The flow of heavy crude oil with high viscosity in tortuous passages of oil reservoirs could profoundly be decelerated. The problem can further be exacerbated if precipitation of asphaltene occurs which may result in heavier and denser crude oils.1 This, in turn, may cause permeability reduction, decreased water saturation, and wettability alteration within porous media. Asphaltenes are defined as an oil solubility class which are insoluble in light alkanes but are soluble in toluene or dichloromethane.2−5 They comprise aromatic polycyclic clusters variably substituted with alkyl groups and may contain a trace of metals (e.g., nickel, iron, and vanadium) and heteroatoms (N, S, O). The precise physicochemical properties of asphaltenes are difficult to define using existing analytical tools, and it remains the subject of ongoing research and contentious debates.5−7 Minimum miscibility pressure (MMP) plays a significant role for evaluation of the displacement efficiency of different gas injection processes in the oil reservoir.8 There are numerous experimental methods to assess gas/oil miscibility under reservoir conditions namely the rising bubble apparatus (RBA), the slim tube, pressure/composition (P/X) diagrams, and the recently developed vanishing interfacial tension (VIT) technique8 that are briefly described as follows: Rising Bubble (RBA). This method is ordinarily used to evaluate the gas/oil miscibility.9 The observation of changes in shape and behavior of a gas bubble that rises in a visual high © XXXX American Chemical Society

Received: October 13, 2017 Accepted: March 27, 2018

A

DOI: 10.1021/acs.jced.7b00903 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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which the effect of temperature, pressure, and composition was taken into consideration. They concluded that at low pressures as temperature increases, IFT decreases due to increased CO2 solubility. At high pressures, though, increased temperature led to higher IFT due to the decrease in CO2 solubility. They also showed that the type of paraffin would have the greatest impact on the slope of IFT versus pressure at a given temperature. In recent years, technological advancements of nanotechnology have given impetus for their utilization in the oil industry. Although their utilization in enhanced oil recovery (EOR) processes is in its infancy19, several investigations have shown promising results. Kazemzadeh et al.20 investigated the impact of adding Fe3O4 nanoparticles to the synthetic oil solutions on the precipitation of asphaltene using interfacial tension (IFT). They showed that the presence of Fe3O4 increased the slope of the IFT−pressure curve at elevated pressures to the same of those without nanoparticles indicating a reduced severity of asphaltene precipitation. This has mainly been attributed to the adsorption of asphaltene on the surface of the nanoparticles which is of interest for the oil industry especially for heavy crudes for two main reasons: (i) rapid depletion of asphaltene from heavy oil and (ii) use of nanoparticles as catalysts for the conversion of asphaltenes to usable light products.21 Esmaeilzadeh et al.22 found three nanoparticles of SiO2, TiO2, and CNT that would change the wettability of reservoir carbonate rocks from the strongly liquidwetting to the gas-wetting state. Numerous studies have also investigated the possibility of using TiO2 nanoparticles in EOR processes for the purpose of wettability alteration.23−26 Nassar et al.27 used Fe3O4 nanoparticles to investigate the effect of the adsorption and oxidation removal of four types of asphaltene. They found that the absorption rate, affinity, and capacity of the investigated nanoparticles to retard asphaltene precipitation depended on asphaltene molecular weight. The absorption rate and capacity increased when asphaltene molecular weight decreased and vice versa. Moreover, they also showed that in the absence of Fe3O4 nanoparticles at 298 K, the oxidation rate was different for the same four types of asphaltene while in the presence of nanoparticles, the oxidation rate was similar indicating the strong role of these nanoparticles as catalyst.27 Nanoparticles of the same metal oxide absorption capacity were

The VIT method unlike other techniques is capable of measuring MMP in oils of high asphaltene content. This approach is widely being used to understand the precipitation of asphaltene in gas injection processes.11,14 To do so, the utilization of a plane method is the easiest and most common approach to evaluate and to calculate the pendant drop for two immiscible fluids.15 The theory of the pendant drop was first presented in 1882 by Bashforth and Adams.16 Nonetheless as this theory was complicated, Andreas17 offered the following empirical equation for calculation of interfacial tension (IFT): γ=

ΔρgDe 2 H

(1)

where γ denotes the interfacial tension, Δρ is the density difference, and De is the equatorial diameter of the droplet, respectively. H is a correction factor which is proportional to the shape factor of the pendant drop, S, which is expressed as

S=

Ds De

(2)

Ds is the droplet diameter measured horizontally at a distance De away from the apex of the droplet. Bashforth and Adams16 proposed a correlation for the estimation of H value as a function of S values. Figure 1 illustrates the shape of the pendent drop in a typical liquid−gas system along with parameters defined in eq 2.

Figure 1. Relationship between dimensions of a pendant drop.

Hemmati−Sarapardeh et al.18 calculated the IFT and miscibility of a CO2−crude oil mixture experimentally in

Figure 2. Schematic diagram of the drop-shape analyzer (IFT700) for IFT measurement. B



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Table 1. Composition of the Crude Oil from Which Asphaltene Was Extracted component

N2

CO2

C1

C2

C3

iC4

nC4

iC5

nC5

C6

C7

C8

C9

C10

C11

C12+

mole percentage

0.29

0.90

18.46

5.71

4.16

0.74

2.33

1.40

1.81

3.19

3.39

4.03

4.20

3.70

2.45

43.24

of 99.95%) were used. Asphaltene particles were extracted from one of the heavy oil reservoirs in southern Iran, and the composition of the crude oil is listed in Table 1. Other materials used in this study included iron oxide nanoparticles (Fe3O4) and titanium oxide (TiO2) with properties given in Table 2.35

also capable of reducing the aggregation and precipitation of asphaltene on the rock surface or tubing wall.28 Overall, the pros and cons of utilization of nanoparticles for EOR purposes are still at the know-how stage of development. More experimental endeavors are required before firm conclusions can be made. This is because nanoparticles are intended to be used in the oil industry as multifunctional chemical substances. Some of these nanoparticles can use thermal catalytic characteristics and suspend asphaltenes in crude oil and prevent their precipitation.29−31 It is particularly of great importance since the process of asphaltene adsorption on nanoparticles is reported to take place in a relatively short period of time which would serve as promising approach to delay the precipitation of asphaltene and possibly subsequent deposition.32 Magnetic properties of the metal nanoparticles are important for the adsorption of asphaltene.33,34 In particular, Fe3O4 and TiO2, within the weight fractions of 0.1 to 1%, have not yet been investigated to discern the interfacial tension (IFT) and precipitation of asphaltene in the asphaltenic synthetic oil by using the VIT method. In this study, IFT changes, as well as minimum miscibility pressure (MMP) and asphaltene precipitation were analyzed for synthetic oil solutions in both cases without and with nanoparticles (Fe3O4 and TiO2) with various concentrations while CO2 was also utilized as an injecting gas at a constant temperature and a wide range of pressure was studied. It is worth mentioning that the composition of synthesis oil used in this study was different to that of Kazemzadeh et al.20 to cover a wider range of volume fraction of chemicals. Finally, the two nanoparticles were evaluated in terms of adsorption capacity and affinity which helped to understand the reason for the different performance of nanoparticles to retard asphaltene precipitation.

Table 2. Properties of Purity, Size, Specific Surface Area (SSA), Color, and Density ρ of the Investigated Fe3O4 and TiO2 Nanoparticles nanoparticles

purity/%

size/nm

SSA/m2·g

color

ρ/g·cm‑3

Fe3O4 TiO2

99.5 99

15−20 20

82 145

dark brown white

0.85 0.46

2.3. Sample Preparation. In this study, the procedure of preparing the asphaltenic oil solution without/with nanoparticles was as follows. At first, a solution of toluene and nheptane with volume ratio of 60% to 40% (i.e., T60-H40) was prepared. The full description of asphaltene extraction has also been given elsewhere36 and avoided here for the sake of brevity. Then the weight fraction of 5% of asphaltene was added to this solution to make it consistent with the same asphaltene content in the investigated crude oil. Asphaltene dissolution in the oil solution was accomplished by using a magnet stirrer (at rotation per minute (rpm) of 600 for 1 h) and shaker (at a speed of 150 rpm for 24 h). If required, at the later stages, Fe3O4 and TiO2 nanoparticles with desired weight percentage were added to the synthetic oil solution. The ultrasonic homogenizer (600 W, 25 kHz) for 30 min was finally used to disperse nanoparticles in the solution, followed by 20 min agitation in a heater stirrer at 50 °C. The colloidal stability of nanoparticles in the solution can be characterized in terms of impacts of van der Waals attraction and electrostatic repulsion forces.37,38 2.4. Experimental Procedure. Before IFT measurements, the solution density was measured by a DMA HPM densitometer (Anton-Paar, Austria). These values were then used as input to the IFT700 unit .Table 3 presents the density of the investigated fluids (gas and oil) at various pressures and at a temperature of 323.15 K. As the mass of added nanoparticles to the solutions was marginal, it was assumed that the nanoparticles would have no considerable effect on the density of synthetic oil solutions. IFT varies with time until the mass transfer between the two phases of gas and oil seizes and reaches an equilibrium. To achieve the thermodynamic equilibrium faster, about 10 droplets of synthetic oil were added to the cell. After hanging the oil droplet at different pressures, it had enough time to reach thermodynamic equilibrium and the real amount of IFT was then calculated for the last 100 s.36 The measurement was repeated at least three times for each pressure, and repeatability of both dynamic behavior and IFT values and their accuracy was ensured.

2. MATERIALS AND METHODS 2.1. Experimental Facility. The apparatus used in this study was an IFT700 unit which is schematically depicted in Figure 2. The apparatus contained an injection system (syringe pump with positive displacement feature), phase clear cube (high pressure IFT cell), laboratory camera with a high frame rate (CCD camera) and a proper computing system, and finally an image analyzer (PC computer, Lab View Image Processor). The IFT device made it possible to measure the IFT values between two fluids (gas/oil and water/oil) at ambient and reservoir conditions. It also provided the contact angle between a liquid and solid sample (rock) if required. The chamber fluid and the droplet were provided by the injection system that included two cylinder−piston systems. The injected fluid was then kept in a stainless steel chamber which provided safety requirements for tests. The visual part of this chamber was also made of quartz, enabling observation of the contents of the container without light refractions. The droplet images were then recorded by a CCD monochrome camera and at last the image analysis software calculated the IFT values between the investigated fluids at specified temperature and pressure by image processing. 2.2. Test Materials. In this study CO2 (with a purity of more than 99.99%), toluene, and n-heptane (Merck with purity

3. RESULTS AND DISCUSSION IFT experimental results confirmed that different trends of IFTpressure would be expected for the two nanoparticles C



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Table 3. Measured Densities ρ/g·cm−3 at 323.15 K and Different Pressures for CO2 and Synthetic Oil Solution P (MPa)

1

2

3

4

5

6

7

8

9

10

CO2 synthetic oil solution

0.01704 0.776

0.0355 0.7769

0.056 0.778

0.0788 0.779

0.1048 0.7801

0.1352 0.7811

0.172 0.7821

0.2191 0.7832

0.285 0.7843

0.3843 0.7852

mN/m, respectively. Moreover at lower pressures (first region), the IFT values of the synthetic oil with and without iron nanoparticles were comparably close together. At the end of the first and second regions, the IFT between the oil solution containing the nanoparticles and CO2 gas has a greater reduction compared to that without nanoparticles. This primarily lies in the behavior of nanoparticles at the CO2/ synthetic oil interfaces due to the adsorption of asphaltene onto the nanoparticles.32 Thus, the addition of nanoparticles to the asphaltenic synthetic oil causes the adsorption of asphaltenes onto nanoparticles in the vicinity of the interface between the gas and oil phases and moves them further toward the oil phase. The interaction between nanoparticles, gas, and oil would finally lead to the reduction of IFT. This is due to the properties of nanoparticles in terms of high adsorption affinity, high surface area to volume ratio, catalytic performance, and excellent dispersion rate.27,36,40 The results also showed that the higher is the concentration of Fe3O4 nanoparticles, the more reduction in the slope of IFT should be expected. 3.2. Effect of TiO2 Nanoparticles on IFT. Similar to Fe3O4 nanoparticles, the effect of titanium oxide nanoparticles (with weight fractions of 0.1, 1, and 3%) on IFT CO2 and asphaltenic synthetic oil for the range of operating conditions in this study is presented in Figure 4. The selected weight fraction of 3% of nanoparticles was merely to evaluate IFT changes at weight fractions above 1% as otherwise such concentrations would not be recommended for EOR purposes due to agglomeration of nanoparticles as will be discussed later on. The experimental results indicate that the addition of TiO2 nanoparticles with different concentrations to the oil solution reduced the IFT values. IFT curves in the absence of nanoparticles and solution with weight fraction of 0.1% of TiO2 nanoparticles have only a marginal difference in the first region, while the difference is greater in the second region with that of the solution without nanoparticles (see Table 4). By adding weight fractions of 1 and 3% of these nanoparticles to the oil solution, the reduction of IFT values was significant compared to those of the oil solution without nanoparticles and also to those of the one with 0.1% weight fraction of nanoparticles. Moreover as it can be seen in Table 4, the slope of all four IFT curves in the first region do not show any significant difference contrary to those solutions with Fe3O4 nanoparticles. In the second region, where the precipitation of asphaltene is expected to occur, with the addition of TiO2 nanoparticles to the synthetic oil solution, the reduction of IFT value for all pressures compared to the case without the nanoparticles is noticeable. In addition, in the second region, the difference between the slope of IFT curves for solutions without and with 0.1% weight fraction of TiO2 nanoparticles is marginal while for solutions with concentrations of 1 and 3% weight fractions of the nanoparticles, the reduction in the comparative slope is 2.3 and 2.8 times, respectively (Figure 4 and Table 4). Contrariwise, in the first region, the addition of weight fraction 3% of TiO2 nanoparticles to the synthetic oil solution caused slightly higher IFT values for all pressures compared to solution having 1.0% weight fraction of TiO2 nanoparticles. The experiment

Figure 3. IFT versus pressure for synthetic oil containing asphaltene with different weight percentages of Fe3O4 nanoparticles at 323.15 K: blue ⧫, without nanoparticles; purple ●, with weight fraction of 0.1%; red ■, with weight fraction of 1%.

Figure 4. IFT versus pressure for synthetic oil containing asphaltene with different weight percentages of TiO2 nanoparticles at 323.15 K: blue ⧫, without nanoparticles; purple ●, with weight fraction of 0.1%; red ■, with weight fraction of 1%; green ▲, with weight fraction of 0.3%.

investigated in this study. Accordingly, the impact of each nanoparticle on IFT was first discerned before their comparison. 3.1. Effect of Fe3O4 Nanoparticles on IFT. Figure 3 shows IFT equilibrium data as a function of pressure for the synthetic oil solution in the absence of nanoparticles and also with iron oxide nanoparticles with weight fractions of 0.1 and 1% for the range of operating conditions investigated in this study. It should be noted that solid lines correspond to the first and second regions of IFT- pressure curves for the attempted range of pressure, while the dashed lines represent the IFT extrapolation to zero in the first and the second regions to find out the minimum miscibility pressure (MMP). In other words, the extrapolation of the regression equation to zero IFT gives a VIT miscibility.9,39 As it can be observed in Figure 4, higher pressures caused the reduction of IFT between gas and oil in two cases of without and with nanoparticles and the slope of these diagrams monotonously decreased. The addition of iron oxide nanoparticles reduced IFT especially at high pressures (second region). For example, for a pressure of 6.3 MPa, when weight fractions of 0.1 and 1% of Fe3O4 nanoparticles were added to asphaltenic synthetic oil the amount of IFT decreased from 5.3 mN/m for the without nanoparticles case to 4.65 and 4.36 D



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Table 4. Curve-fitted correlations R2; minimum miscibility pressure (MMP) of the 1st and 2nd regions; % slope change of regions, and reduction of the severity of asphaltene precipitation in synthetic oil with different mass fractions of Fe3O4 and TiO2 nanoparticles at 323.15 K nanoparticle /weight fraction 0

region 1st 2nd

0.1% Fe3O4

1st

0.1% TiO2

2nd 1st 2nd

1.0% Fe3O4

1st

1.0% TiO2

2nd 1st 2nd

3.0% TiO2

1st 2nd

equation IFT/-, P /MPa IFT = −1.9344P + 17.438 IFT = −0.3157P + 6.709 IFT = −2.0853P + 17.842 IFT = −0.46P + 7.482 IFT = −1.9817P + 17.474 IFT = −0.3686P + 6.807 IFT = −2.256P + 18.318 IFT = −0.7714P + 9.26 IFT = −1.9807P + 16.617 IFT = −0.6536P + 8.039 IFT = −1.9581P + 16.927 IFT = −0.8857P + 10.25

R2

MMP/ MPa

% slope change (region 1/ region 2)

0.9986

9.01

84

0.9981

21.25

0.9991

8.56

78

6

0.9864 0.9975

16.27 8.82

81

3

0.992

18.47

0.9974

8.12

66

18

0.9827 0.9962

12 8.39

67

17

0.9988

12.3 55

29

0.999

8.64

0.9969

11.57

reduction of the severity of asphaltene precipitation/%

Figure 5. IFT versus pressure for synthetic oil containing asphaltene with different weight percentages of Fe3O4 and TiO2 nanoparticles at 323.15 K: (a) without nanoparticles (blue ⧫), with 0.1 wt % of TiO2 (green ▲), and Fe3O4 nanoparticles (red ●); (b) without nanoparticles (blue ⧫), with 1 wt % of TiO2 (red ●), and Fe3O4 nanoparticles (green ▲).

Figure 6. Variation of threshold pressure (pressure at which the slope of IFT curves changes between the two regions) as a function of nanoparticle concentration at 323.15 K for synthetic oil solution containing asphaltene: , ∗, ▲ with TiO2 nanoparticles and ●, ■, ◆ with Fe3O4 nanoparticles.

Figure 7. MMP of the 2nd region for synthetic oil solution without and with different mass fractions of Fe3O4 and TiO2 nanoparticles at 323.15 K: ,∗, ▲ with TiO2 nanoparticles and ●, ■, ◆ with Fe3O4 nanoparticles.

was repeated but similar results were obtained. The unexpected result was probably occurred because the addition of nanoparticles to the oil solution to a certain level reduced the

IFT values and asphaltene precipitation but after a certain concentration, further increase in the amount of nanoparticles had a reverse impact which led to increased IFT values. This E



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of the experimental data has been calculated for the case of without nanoparticles, the maximum observed standard deviation of IFT data was 0.25 mN/m and the average standard deviation was 0.18 mN/m. Figure 6 compares the threshold pressure at which the slope of IFT changed between both regions for both nanoparticles and different concentrations. By adding nanoparticles to asphaltenic synthetic oils, the slopes change between the two regions of IFT curves at lower pressure. This means that the onset of asphaltene precipitation occurs at lower pressures. In other words, in this case, the solubility of injected gas in the oil solution decreased and asphaltene precipitation occured earlier than that of the nanoparticle-free oil solution case, which is unfavorable. It is necessary to emphasize that if the decrease in the threshold pressure is accompanied by MMP reduction, this phenomenon is considered to be negligible.36 The results show that with the addition of weight fractions of 0.1 and 1% of Fe3O4 nanoparticles to the synthetic oil solution, the threshold pressure occurred at lower pressures compared to that without nanoparticles. As for the addition of 0.1 and 1% weight fractions of TiO2 nanoparticles to the synthetic oil solution, the threshold pressures were different compared to those of Fe3O4 nanoparticles. They were 6.65, 6.3, and 6.3 MPa for weight fractions of 0.1, 1 and 3% of TiO2 nanoparticles, respectively. The threshold pressure, though, forthe cases of 0.0 and 0.1% weight fractions were more or less the same, and those for cases of weight fractions of 1 and 3% were also similar. The comparison of these two nanoparticles also shows that for the same concentration of nanoparticles, the addition of Fe3O4 nanoparticles to the oil solution compared to TiO2 resulted in a lower threshold pressure. In other words, the beginning of the second region starts at a lower pressure. It should be noted that the slope change at a lower pressure due to the lower solubility of CO2 gas in the oil solution and also the beginning of asphaltene precipitation at a lower pressure are not desirable phenomena. 3.3. Impact of Fe3O4 and TiO2 Nanoparticles on MMP of First and Second Regions. In addition to IFT data, the impact of nanoparticles with different concentrations on MMP is of prime importance. Table 4 presents IFT correlations as a function of pressure in both regions: the coefficient of determination of a linear regression (R2), MMP, % slope change of the first region to the second region, and the reduction of asphaltene precipitation intensity for various oil solutions. The data of this table shows that the addition of both nanoparticles to the synthetic oil solution reduces the MMP values in the first region, causing better gas miscibility in oil at lower pressures.The reduction of MMP in the oil solution containing Fe3O4 nanoparticles is greater to that containing

Table 5. Langmuir Parameters for Fe3O4 and TiO2 Nanoparticles:33 Langmuir Equilibrium Adsorption Constant, Qm and Adsorption Quantity, KL nanoparticles

Qm (g/m2)

KL (L/g)

Fe3O4 TiO2

0.012 0.007

0.001 0.002

Figure 8. Variation of the shape factor S versus pressure for synthetic oil containing asphaltene with different weight percentages of Fe3O4 and TiO2 nanoparticles at 323.15 K: blue ⧫, without nanoparticles; purple ●, with 0.1% weight fraction of TiO2; red ■, with 1% weight fraction of TiO2; green ▲, with 0.3% weight fraction of TiO2; orange , with 0.1% weight fraction of Fe3O4; black ∗, with 1% weight fraction of Fe3O4.

might occur because the nanoparticles could stick together and form flocculation that would not have a prevailing impact on reducing asphaltene precipitation. Thus, the IFT of the synthetic oil solution containing 3% weight fraction of TiO2 in the range of attempted pressures has increased compared to that of the oil solution containing 1% weight fraction of TiO2 nanoparticles. Figure 5 compares the two nanoparticles (TiO2 and Fe3O4) as a function of IFT versus pressure for different concentrations. As can be seen the decreasing trend of IFT in the first region of oil solution containing Fe3O4 nanoparticles are almost similar, in contrast to that of the oil solution containing the TiO2 nanoparticles; this trend changes and shows further reduction by adding the nanoparticles. In the second region in all solutions containing both nanoparticles, a sharp drop of IFT was experienced, except for the solution containing 0.1% weight fraction of TiO2. It can be concluded that in the second region, the decreasing trend of the IFT−pressure diagram for the addition of Fe3O4 nanoparticles is slightly greater that that of the solution with TiO2 nanoparticles (refer to Figure 5 and Table 4). This is due to the better adsorption and possibility of layer coverage of asphaltene by Fe3O4 nanoparticles on the gas/ oil interface. It is worth mentioning that the standard deviation

Figure 9. Variation of droplet shapes for synthetic oil containing asphaltene with weight fraction of 1% Fe3O4 nanoparticles at a temperature 323.15 K with increase of pressure. F



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absorption. Thus, the surface area of Fe3O4 is higher than that of TiO2 and absorbs more asphaltene. 3.4. Effect of TiO2 and Fe3O4 Nanoparticles on Shape Factor (S). Small droplets usually tend to be spherical because surface forces depend on area. In principle, one can determine the interfacial tension from measurements of the shape of the droplet. In the case of the pendant drop, the most convenient and measurable shape dependent quantity is defined in eq 2. Figure 8 shows the variation of this factor versus pressure for different concentrations of Fe3O4 and TiO2 nanoparticles. As can be seen from Figure 8, the S factor increases with pressure. That is, Ds increases with pressure that corresponds to larger S values and consequently greater H values. According to eq 1, an increase in H factor causes IFT to decrease. In other words, the pressure increase causes the droplet shape changes from a balloon to a cylindrical shape (s ≈ 1). Also, as can be seen in Figure 9, the pressure increase causes a smaller volume and droplets contact surface area with CO2. Furthermore, the addition of nanoparticles to the synthetic oil solution led to a larger S factor. This would, in turn, mean higher miscibility of the gas in oil; thus smaller IFT and finally lesser precipitation of asphaltene at the interface of gas/oil would be expected.

TiO2 nanoparticles. In addition, the presented data in Table 4 confirmed that for a weight fraction of 1% for both nanoparticles, the difference between the slopes of these curves in the first region is marginal compared to that without nanoparticles. On the contrary, the addition of nanoparticles to the synthetic oil containing asphaltene, caused an increase in the slope of the second region. As shown in Figure 7 and Table 4, the MMP values in the second region decreased by adding the Fe3O4 and TiO2 nanoparticles. The results show that the reduction of this parameter is greater in the synthetic oil solution containing Fe3O4 nanoparticles. In other words, gas miscibility in the oil solution is obtained easier and at lower pressures using Fe3O4 than TiO2. The data of Table 4 also shows that the slope change of the second region to the first region becomes moderate by adding both nanoparticle types to the synthetic oil. The point in which the slope changes is the key indication of the onset of asphaltene precipitation. If the difference between the first and second slopes reduces then a lower intensity of asphaltene precipitation is expected.36,40 Hence, according to the experimental results of this study, using 0.1 and 1% weight fractions of Fe3O4 nanoparticles reduces 6 and 18% of the intensity of asphaltene precipitation, whereas, weight fractions of 0.1, 1, and 3% of TiO2 nanoparticles reduced the severity of asphaltene precipitation by 3, 17, and 29%, respectively. Since the synthetic oil and gas compositions for all attempted tests in this study (in the range of operating conditions investigated) were the same, then the difference in severity of asphaltene precipitation would be attributed to the presence of nanoparticles. Hassanpour et al.36 also showed that adding weight fractions of 0.01, 0.1, and 1.0% of Co3O4 nanoparticles to the same asphaltenic synthetic oil solution had reduced the severity of asphaltene precipitation at the interface of the gas/oil by 7%, 14%, and 13%, respectively. In 2011, Nassar41 reported that asphaltene absorption on the surface of metal oxide nanoparticles depended on the asphaltene type and nanoparticle substrate, as well as the strength of interaction between them. Therefore, it is necessary to understand the Langmuir parameters. These include KL as the Langmuir equilibrium adsorption constant, which corresponds to the affinity of binding sites (L/g), a parameter that explains the ability of a specific adsorbent (nanoparticles) to be in equilibrium with adsorbate (asphaltene) which is termed the adsorption quality. The other parameter, Qm known as adsorption quantity, is the maximum amount of adsorbed asphaltenes per unit surface area of nanoparticles for complete monolayer coverage (g/m2). The latter would be used to determine the affinity of different metal oxides surfaces to contact asphaltene. Thus, Qm and KL are called the adsorption capacity and the adsorption affinity, respectively. The Qm parameter is the mass to surface ratio (for nanoparticles, the reverse of specific surface area (SSA) in Table 2), and the KL parameter is the volume to mass ratio (for nanoparticles, the reverse of density in Table 2). Table 5 presents Qm and KL, according to the definition of these two parameters, for both investigated nanoparticles. As can be seen in Table 5, based on the quantities of adsorption quantity, that is, Qm, and adsorption quality, that is, KL, Fe3O4 has a higher Qm compared to that of TiO2 and contrariwise TiO2 nanoparticles have larger KL in comparison with Fe3O4. It means that with respect to quantity, Fe3O4, and with respect to quality, TiO2, would be prone to better

4. CONCLUDING REMARKS Nanoparticles of various types can potentially be utilized for EOR purposes though their application is at a know-how stage of development. The following conclusions can be drawn from the experimental results presented in this study: • The change in the slope of IFT curves as well as the pressure where this occurs were the key parameters which could indicate the affinity of asphaltene precipitation onto nanoparticles. • TiO2 and Fe3O4 nanoparticles were able to reduce the IFT between CO2 gas and synthetic oil solution and increased the miscibility of gas in the oil leading to reduced intensity of asphaltene precipitation. The intensity of asphaltene adsorption is characterized in terms of adsorption capacity and affinity and the data showed that the surface area of Fe3O4 is higher than that of TiO2 so it absorbs more asphaltene. • The optimum weight fraction of both nanoparticles based on evaluation of IFT and MMP values was 1.0% for the range of operating conditions in this study. • The reduced intensity of asphaltene precipitation in the optimum concentration for oil solutions containing Fe3O4 and TiO2 nanoparticles was, respectively, 18 and 17%. • Adsorption quality between adsorbate, asphaltene, and adsorbents (nanoparticles) for TiO2 is greater than that for Fe3O4. Adsorption quantity between adsorbate and adsorbents for Fe3O4 is larger than for TiO2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mohammad Reza Malayeri: 0000-0001-8376-8055 Masoud Riazi: 0000-0003-0572-1766 Notes

The authors declare no competing financial interest. G



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ACKNOWLEDGMENTS The authors would like to express their gratitude to S. Aftab and S. Shariat for their help to set up the experimental test facilities. The crude oil used in this research project was supplied by the National Iranian South Oil Company (NISOC) which is gratefully acknowledged.

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