Effect of Nanoparticles on Asphaltene Aggregation in a Microsized

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Thermodynamics, Transport, and Fluid Mechanics

Effect of Nanoparticles on Asphaltene Aggregation in a Micro-sized pore Xingxun Li, Yunmei Guo, Qiang Sun, Wenjie Lan, and Xuqiang Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00729 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Industrial & Engineering Chemistry Research

Effect of Nanoparticles on Asphaltene Aggregation in a Micro-sized pore

Xingxun Lia, Yunmei Guoa, Qiang Suna, Wenjie Lana, Xuqiang Guoa,b,* a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China b

China University of Petroleum-Beijing at Karamay, Karamay834000, China

* Corresponding author. E-mail address: [email protected]

Abstract During oil recovery process, the equilibrium state of asphaltene-oil solution system and colloidal system in porous media is altered, which could cause that asphaltene could precipitate from crude oil in rock pores. This severely results in the alteration of wettability, permeability and porosity of formation and damages oil reservoir formation. Nanoparticles are recently considered as effective asphaltene inhibitors. Several experimental works were carried out to study the impact of nanoparticles on asphaltene precipitation. The majority of these studies were performed using coreflooding experiments in bulk phase. However, few experimental investigations were performed to directly reveal the impact of nanoparticles on asphaltene aggregation based on the direct pore-scale visual evidence. In this work, a novel method was employed to make direct visualization and measurement on the kinetics of asphaltene aggregation in the presence of nanoparticles in a microcapillary. NiO, 1

ACS Paragon Plus Environment

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SiO2 and Fe3O4 nanoparticles were used. We directly measured the diameter and number and analyze size distribution of asphaltene aggregate particles in a micro-sized pore. The results indicate that the nanoparticles can significantly inhibit the aggregation of asphaltene particles in a micro-sized pore due to adsorption of asphaltene particles onto nanoparticle surfaces. The effect of nanoparticle concentration was investigated. Higher concentration of nanoparticles could contribute to the inhibition of aggregation of asphaltene particles. The asphaltene precipitation and aggregation can be prevented more effectively at higher concentration of asphaltene precipitant in the presence of nanoparticles.

1. Introduction

Asphaltene acts as a negative role in oilfields and refineries.1-5 In upstream, the aggregation and deposition of asphaltene particles could occur onto reservoir porous surfaces. They could block the pores of rock formation and significantly change the wettability of rock surfaces. This can severely alter the flow phase behavior and reduces the permeability and porosity of oil reservoirs, leading to damage of reservoirs and blockage of pipelines.6-8 In order to enhance oil recovery, the precipitated-asphaltene induced problems must be solved. Several methods have been generally used to prevent asphaltene precipitation. One is to inhibit asphaltene precipitation or remove the deposited asphaltene by solvent or mechanical treatment.9 The other one is to stabilize asphaltene in oil phase to postpone its precipitation.9 For 2


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example, some chemicals, such as amphiphiles, have been widely studied and employed to inhibit the asphaltene aggregation.10-13

Nanotechnology has been recently applied in oil upgrading and oil recovery processes.14 Several studies have addressed the potential and effectives of nanofluids on enhancing oil recovery and alteration of rock wettability. For instance, Giraldo et al. investigated change of wetting condition of sandstone core samples using alumina-based

nanoparticles

with

different

nanoparticle

concentrations.15

Hendraningrat and Torsæter conducted coreflooding experiments to study the effect of the wetting condition of rock surface on enhanced oil recovery processes by silica nanofluids, and concluded that wetting alteration significantly affected the mechanism of oil displacement by nano-EOR method.16 Esfandyari Bayat et al. investigated the impacts of nanoparticles on Enhanced Oil Recovery at elevated temperatures. Three types of nanoparticles were investigated, which are silicon dioxide (SiO2), titanium dioxide (TiO2), and Aluminum oxide (Al2O3).17 Monfared et al. revealed the impact of silica nanoparticle suspensions on wetting alteration of oil-wet calcite.18 They found that the strongly oil-wet calcite surface could be effectively changed to be more water wetting by silica nanoparticles suspension.18 Moghaddam et al. investigated the effect of a variety of nanoparticles, such as ZrO2, Al2O3, CaCO3, TiO2, MgO, SiO2, CeO2, and carbon nanotube (CNT), on the wetting condition of carbonate surface by imbibition and core floods tests.2

3



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Nanoparticles are also considered as a promising additive agent to avoid large amount of asphaltene deposition due to their high adsorption and suspended capacities, and high surface area-to-volume ratio.3 Kazemazdeh et al. investigated the asphaltene adsorption onto iron oxide nanoparticles, such as SiO2, NiO2 and Fe3O4 nanoparticles surface.3 This can remarkably prevent asphaltene aggregation. They found that SiO2 was the most effective metal oxide-based nanoparticles.3 Hosseinpour et al. examined effects of various metal-oxide-based nanoparticles (CaCO3, Fe2O3, MgO, NiO, WO3 and ZrO2) on the thermodynamics of asphaltene adsorption.19 Nassar et al. selected Fe2O3, Co3O4, and NiO nanoparticles to investigate their impacts on adsorption of asphaltene particles in catalytic steam gasification and cracking.20 The greatest adsorption affinity on asphaltene particles were found on the NiO nanoparticle surface.20 Kazemzadeh et al. (2015) investigated the impact of Fe3O4 nanofluids on CO2-induced asphaltene deposition by interfacial tension and Bond number measurements.4 Shojaatiet al. (2017) used metal oxides nanoparticles to inhibit asphaltene precipitation.21 They show thatγ-Al2O3 nanoparticles could act as the most efficient asphaltene inhibitor.21 However, these recent studies were mainly carried out on planar surfaces or macroscopic core samples. They cannot provide direct information on the impact of nanoparticle on the asphaltene precipitation in micro pores. Our recent studies have proved that the experimental data in confined micro pores can represent the transport phenomena of reservoir fluids within porous medium more appropriately than the data measured on planar surfaces in an open space.22,

23

Recently, we have made direct pore-scale visualization on the growth 4


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kinetics of asphaltene particles in a micro-sized pore, and compared the results with the ones obtained on a flat surface in an open space.24,

25

Xu et al. (2017) used

microchannel devices to study the dynamic stability of emulsion, by investigating the influence of surfactants and nanoparticles.26 In this study, impact of metal oxide nanoparticles on asphaltene aggregation in a micro-sized pore was investigated. We used the technique recently developed in our lab24, 25 to perform direct pore-scale visualization on the kinetics of asphaltene aggregation and examine the effect of nanoparticles on inhibition of asphaltene aggregation in a micro-sized pore.

2. Experimental Section 2.1 Materials Crude oil used in this study was from Venezuela. It has approximately 14 wt% n-heptane asphaltene.24, 25 The dynamic viscosity was measured at 50 ℃, which is around 1000 mPa•s. 24, 25 The density was 0.975 g/cm3. 24, 25 We used toluene (Beijing Shiji, 99.5%) as crude oil solvent. n-heptane (NankaiRungong, 98.5%) was used to precipitate asphaltene from crude oil.24,

25

Glass square borosilicate micro-sized

capillaries (Beijing Zhong Cheng Quartz Glass) were used in experiments. They are with open ends, and highly-transparent with smooth optical paths to guarantee clear micro-visualization of the growth of asphaltene aggregates in a capillary. The dimension of each capillary is 20 (long) mm × 100 (wide) µm with a volume of 0.2 µL. Clean glass interior surfaces are of importance for the experiments since any contamination could cause alterations of wettability and adsorption behaviors.27 5



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Cleaning methods used in our previous work were employed to clean the micro-sized glass pore interiors before measurements.22-25, 28, 29 Nitric acid and sodium hydroxide solutions and acetone were used to rinse the micro-sized capillaries. Finally, the capillaries were

thoroughly

washed

by

DI

water.

Before

measurements,

microcapillaries were dried and kept in an ash-proof container to prevent capillary interior surface from any pollution or ash particles.24, 25 Three types of metal oxide nanoparticles were used in this study. They are SiO2 nanoparticles (JinleiNanophase Materials, 20 nm, ≥99.9%), NiO nanoparticles (Jinlei Nanophase Materials, 40 nm, ≥99.9%) and Fe3O4 nanoparticles (Jinlei Nanophase Materials, 20 nm, ≥99.5%).

2.2 Methods Experimental methodology used in this study is similar with the one in our recent work.24 Synthetic diluted oils, which are oil-heptane-toluene mixtures, were used in this work. Crude oil was mixed with toluene (oil solvent) and n-heptane (asphaltene precipitant). The volume ratio of crude oil and toluene-heptane mixture is 1:40. The effect of concentration of n-heptane (asphaltene precipitant) was investigated. The concentrations of n-heptane used were 60 vol%, 70 vol% and 80 vol% in this study. Figure 1 shows the experimental setup used in this work. The oil sample was stirred in a beaker by a mechanical stirrer (Chengdong Xinrui Instrument, Jintan) at a stirring speed of 300 r/min. A water bath was placed on the heat pad to maintain a temperature of 20℃ for the oil sample (Figure 1). 6


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Figure 1 shows that the micro-sized capillaries were immersed in the oil sample for the growth of asphaltene aggregates in a small micro-sized pore. The aging times are t=0, 15, 30, 45 and 60 min. A microscope (Changrong S-T) equipped with a CCD camera (Aptina-LV500) was used to observe and capture the micro images of asphaltene aggregates in micro-sized pore. The optical microscope has a detection limit of is 0.5 µm. It is worth to note that the particle images of asphaltene aggregates can not be precisely obtained until the asphaltene particle grows to a visual observable micro-scale size.24, 25, 30, 31 The images of asphaltene aggregates at various locations of capillary were captured. 10~15 images were captured at different positions of capillary, which are near the inlet, middle or end of the capillary, so reliable results can be statistically obtained for data analysis. Image processing software (Image-Pro Plus 6.0, Image View software) was used to measure and analyze the number, size, area and perimeter of asphaltene particles. 24, 25

Figure 1 Experimental setup24 7



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3. Results and Discussion 3.1 Effect of Nanoparticles on Asphaltene Aggregates in a Micro-sized pore

In order to directly investigate the behavior of asphaltene aggregation in porous medium, the growth of asphaltene particles was investigated in a micro-sized glass capillary. n-heptane was used as asphaltene precipitant and toluene served as oil solvent. The impact of nanoparticles on aggregation of asphaltene particles was investigated at a pore scale. Three different kinds of nanoparticles were used, which are NiO, SiO2 and Fe3O4 nanoparticles. Figure 2 depicts the sizes of asphaltene aggregates at flocculation times of t=0, 15, 30, 45 and 60 min for 70 vol % heptane-toluene-oil mixture, without and with addition of nanoparticles.

As indicated in Figure 2, the mean particle size of asphaltene aggregates is small at t = 0 min, which is 4.6 µm. The mean particle diameters of asphaltene aggregates are 9.2 and 10.0 µm at 30 and 60 min, respectively, in the absence of nanoparticles.24 When NiO, SiO2 and Fe3O4 nanoparticles are used, the size of asphaltene aggregates is significantly reduced. The asphaltene aggregate sizes in the presence of nanoparticles are greatly smaller than those with no addition of nanoparticles, by 2.3~4.4 µm (Figure 2). There is no significant difference on the size of asphaltene aggregates when different nanoparticles (NiO, SiO2 and Fe3O4) are used (Figure 2). As shown in Figure 2, in the presence of nanoparticles, asphaltene particle size significantly increases from t=0 to 30 min because of fast aggregation of asphaltene particles. 8


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However, the significant asphaltene aggregation stops at 30 min, and the aggregate size starts to decrease with the flocculation time. The difference between the aggregation size in the presence and absence of nanoparticles becomes larger when the flocculation time increases. This might indicate that effect of nanoparticles on inhibition of aggregation of asphaltene particles could be more significant with time increase. Previous studies examined impact of contact period on the asphaltene adsorption on the nanoparticle surface.14, 19, 32 Longer contact time can contribute to greater asphaltene adsorption onto the surface nanoparticles.3 This inhibits asphaltene aggregation and causes suspended asphaltene aggregates with smaller size.

Figure 3 depicts the microscopic images of asphaltene particles without added nanoparticles and with addition of NiO, SiO2 and Fe3O4 nanoparticles. The images show that the nanoparticles can significantly prevent the flocculation of asphaltene particles. Asphaltene particles can aggregate together easily without nanoparticles.

9



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Figure 2. Mean asphaltene size at flocculation times of t=0, 15, 30, 45 and 60 min at concentration of 70 vol % with no nanoparticles24 and with addition of NiO, SiO2 and Fe3O4 nanoparticles at 2000 ppm.

Figure 3. Representative microscopic images of asphaltene particles at 70 vol % n-heptane with no added nanoparticles24 and with addition of NiO, SiO2 and Fe3O4 nanoparticles at 2000 ppm at t= 30 min.

The size distributions of asphaltene aggregates are studied in the presence and 10


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absence of nanoparticles (Table 1). In the absence of nanoparticles, the particle sizes of asphaltene aggregates are in very wide ranges. For instance, they range from 1.5 to 71.5 µm at t=30 min and 1.5 to 80.5 µm at 60 min.24 The particle sizes are mainly from 2.5 to 14.5 µm.24 Table 1 indicates that the size ranges of asphaltene aggregates become narrow when nanoparticles are added to the diluted oil. For example, in the presence of 2000 ppm SiO2, the particle size ranges are narrowed down to 1.0~27.0 µm at t=30 min and 1.0~23.0 µm at t=60 min. The aggregates are mostly with sizes of 1.5~9.5 µm.

11



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Table 1. Size distribution of particle of asphaltene aggregates at 70 vol % n-heptane without nanoparticles24 and with nanoparticles at 2000 ppm at different flocculation times Different nanoparticles

Flocculation time 30 min

60 min

Without 24 nanoparticles

2000ppm NiO

2000ppm SiO2

2000ppm Fe3O4

The particle number of asphaltene aggregates is investigated in the presence and absence of nanoparticles. For instance, the number of asphaltene aggregates with no 12


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nanoparticles is around 21 at t= 30 min at 70 vol % n-heptane,24 while the numbers of asphaltene aggregates with added NiO, SiO2 and Fe3O4 nanoparticles are approximately 82, 141 and 81, respectively, at nanoparticle concentration of 2000 ppm. The number of asphaltene aggregates with added nanoparticles is significantly more than the one without nanoparticles, by 60~120. The results could be explained by adsorption effect of asphaltene particles onto nanoparticles.3 This can prevent the asphaltene particles from aggregating to become large particles. Instead, the asphaltene molecules adsorbed on nanoparticles are suspended in the oil bulk phase. Important properties of nanoparticles, such as high surface area-to-volume ratio, good adsorption capacity and high degree of suspension,3 could contribute to suspend asphaltene and inhibit aggregation process.

3.2 Effect of Nanoparticle Concentration on Asphaltene Aggregation in a Glass Micro-sized Pore

The effect of concentration of nanoparticles on asphaltene aggregation in a microcapillary was studied. Figure 4 indicates the mean diameter of asphaltene aggregates in the presence of nanoparticles at 2000 and 10000 ppm. The results indicate that the asphaltene aggregate size decreases when higher concentration of nanoparticles is used. For instance, the particle diameter of asphaltene aggregates in the presence of 10000 ppm NiO at t= 30 min is 5.8 µm, which is smaller than the one 13



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for 2000 ppm NiO, by around 1µm. At 10000 ppm, the particle diameter of aggregate stops increasing with flocculation time at t=30 min. After t=30 min, the aggregate size slightly decreases with time. The data trends are similar to the ones at 2000 ppm. However, the impact of added nanoparticles on asphaltene aggregation inhibition is less remarkable with the increase of contact time than the one at 2000 ppm. Figure 5 depicts representative microscopic images of particles of asphaltene aggregates without nanoparticles and in the presence of NiO nanoparticle at 2000 and 10000 ppm at t=30 min. The results show that higher concentration of nanoparticles could enhance stability of asphaltene in the fluid. This might agree with recent finding from literature3. Kazemzadeh et al. stated that increasing the concentration of metal oxide nanoparticles (NiO, SiO2 and Fe3O4) could enhance the ultimate oil recovery.3 They proposed that the most effective asphaltene inhibitor on efficiency of enhanced oil recovery process was the SiO2 nanoparticles, and NiO and Fe3O4 nanoparticles followed.3 However, there is no significant difference on size of asphaltene aggregate with addition of various kinds of nanoparticle (NiO, SiO2 and Fe3O4) used in this study.

14


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Figure 4. Mean size of asphaltene aggregates at concentration of 70 vol % n-heptane with addition of nanoparticles at 2000 and 10000 ppm.

Figure 5. Representative microscopic images of asphaltene aggregates at 70 vol % n-heptane with no added nanoparticles24 and in the presence of NiO nanoparticle at 2000 and 10000 ppm at t=30 min

Table 2 indicates the particle size distribution of asphaltene aggregates with no added nanoparticles and in the presence of NiO nanoparticles at 2000 and 10000 ppm. As 15



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shown in Table 2, the particle size distribution becomes narrower when concentration of nanoparticles increases. For instance, at t = 60 min, the particle size ranges of asphaltene aggregates are mainly from 1.5 to 80.5 µm in the absence of nanoparticles.24 While, the size ranges are from 2.0 to 36.5 µm, and from 2.0 to 24.0 µm in the presence of 2000 and 10000 ppm nanoparticles, respectively.

Table 2. Size distribution of particles of asphaltene aggregates at 70 vol % n-heptane with no added nanoparticles24 and in the presence of NiO nanoparticles at 2000 and 10000 ppm at different flocculation times Concentration

Flocculation time

of NiO nanoparticles

30 min

60 min

Without 24

nanoparticles

2000ppm NiO

10000ppm NiO

16


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The impact of nanoparticles on the number of asphaltene aggregates is also considered. The numbers of asphaltene aggregates in the presence of NiO nanoparticles at 2000 and 10000 ppm are 82 and 186, respectively, at flocculation times of 30 min. The number of asphaltene aggregates significantly increases with the nanoparticle concentration. In the absence of nanoparticles, the number of asphaltene aggregates is only around 20.24 When nanoparticles are added to the diluted oil sample, the presence of nanoparticles prevents asphaltene flocculation. Smaller and more asphaltene aggregates can be observed when higher concentration of nanoparticles is used.

3.3 Asphaltene aggregation at different concentrations of asphaltene precipitant in the presence of nanoparticles

Asphaltene aggregation behavior in a microcapillary at various concentrations of asphaltene precipitant (n-heptane) in the presence and absence of nanoparticles was investigated. Three n-heptane concentrations are used here, which are 60, 70 and 80 vol%. Figure 6 (a) (b) and (c) presents the sizes of asphaltene aggregates with no addition of nanoparticles and in the presence of 2000 ppm at 60 vol%, 70 vol% and 80 vol% heptane, respectively. Our recent studies have proved that the concentration of asphaltene precipitant could affect the kinetics of aggregation and the aggregation mechanism in a micro-sized pore. 24 As shown in Figure 6, the growth of asphaltene aggregates is faster at higher concentration of n-heptane without addition of 17



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nanoparticles. The inhibition of asphaltene aggregation caused by nanoparticles is more significant at higher concentrations of asphaltene precipitant. Figure 6 (b) presents that the average diameters of asphaltene aggregates in the presence of 2000 ppm NiO nanoparticles are smaller those with no addition of nanoparticles at 70 vol% n-heptane, by 2.5-3.6 µm. For 80 vol% n-heptane, the particle sizes of asphaltene aggregates in the absence of nanoparticles are significantly larger than those in the presence of NiO nanoparticles, by 6.3-12.6 µm (Figure 6(c)). Comparatively, the effect of nanoparticles on inhibition of asphaltene aggregates is less remarkable at lower concentration of asphaltene precipitant. As shown in Figure 6 (a), at 60 vol% heptane, the aggregate sizes in the absence of nanoparticles are only slightly larger than the ones in the presence of nanoparticles at the flocculation times of 15 and 30 min. As the flocculation time increases, the difference becomes significant. Our data agrees with recent findings in literature well.3 The most severe asphaltene flocculation can be caused by the presence of heptane and the absence of nanoparticles. The higher concentration of n-heptane in the diluted oil (heptane-toluene-oil mixture) could cause more adsorption of asphaltene aggregates on the surfaces of nanoparticles. This could mitigate the severe precipitation and aggregation of asphaltene particles.

Small water-wet micro-sized glass pores were used to represent reservoir porous medium in this study. The micro-capillary experiments could capture the kinetics of particle growth and phase change behavior at a pore-scale. However, other conditions of pore fluids in real reservoir porous media, such as, high pressure, temperature and 18


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dynamic condition of capillary flow, were not involved in this study. These will be investigated in details in our following studies. The effect of nanoparticles on asphaltene aggregation in oil-wet pores will be also studied in our future work.

(a)

(b)

(c)

Figure 6. Effect of nanoparticles on asphaltene aggregation at different n-heptane concentrations of (a) 60 vol%, (b) 70 vol% and (c) 80 vol%.

3.4 Error Analysis During the measurement and analysis of particle size of asphaltene aggregates, errors of particle sizing should be considered in this study. In addition to the contributions of 19



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instrument (optical limit of microscope), imaging (resolution) and human factors to the source of sizing error of particles,33-35 the sizing accuracy is closely related to the irregularity of particle shape (nonhomogeneity or nonsphericity) and particle size distribution.33, 34, 36-39 Evans and Napier-Munn (2013) and Mariano and Evans (2015) proposed that it would be of importance to estimate the errors associated with size distribution or number of particles,40,

41

In Table 3, the standard deviations of

asphaltene aggregate diameters are analyzed and presented based on various size ranges, for 70 vol % n-heptane−toluene−crude oil mixture with no nanoparticles and 2000 ppm NiO nanoparticles. As shown in Table 3, the size errors of asphaltene aggregates in the presence of 2000 ppm NiO nanoparticles are smaller than those in the absence of nanoparticles. Large errors are resulted from the large particle size and low size distribution (small number of particle measured).

Table 3. Size error of asphaltene aggregate of 70 vol % n-heptane−toluene−crude oil mixture with no nanoparticles24 and 2000 ppm NiO nanoparticles No nanoparticles24

2000 ppm NiO nanoparticles

Size

Size

Size error (µm)

Size

Size error (µm)

range

distribution

(Standard deviation)

distribution


(µm)

(%)

0-2.5

11.2

0.35

8.2

0.22

2.5-5.0

23.2

0.78

36.5

0.70

5.0-10.0

41.6

1.24

43.9

1.28

10-20

13.6

3.11

8.2

2.61

20-40

7.2

6.14

3.3

4.52

40-80

3.2

14.84

0

—

(%)

20


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The asphaltene aggregates with irregular and non-spherical shapes were analyzed based on their projected area. The particle size measured in this study is referred to the equivalent spherical diameter. These certainly cause significant deviation from their true values36. The size error in image recognition could strongly depend on shape factor.35, 42 Shape factor (circularity) of asphaltene aggregate particle can be obtained based on the microscopic images captured in this study. Circularity is defined as

C =

∙ 43, 44 ,

where C is circularity of asphaltene particle, (≤ 1, 1 is for a sphere), A

is projected area (µm2) and P is perimeter (µm).43,

44

The projected area and

perimeter of asphaltene particle were measured by Image-Pro Plus 6.0.

Figure 7 shows the shape factor (circularity) of asphaltene aggregate as a function of particle size. It can be seen that the circularity decreases significantly with aggregate size. Larger sizes of asphaltene aggregate are more irregular-shaped. Combined with the size errors in Table 3, the nearly spherical particles (circularity is close to 1), such as the small aggregates with a size range from 0 to 2.5 µm, have small size errors below 0.4 µm. On the other hand, the less regular-shape aggregates with smaller shape factors have larger size errors. For example, the size errors of asphaltene aggregates are over 4 µm for the particles within the size range from 20 to 40 µm. These meet the finding in literature.35

21



24

(a) no nanoparticles

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(b) 2000ppm NiO

Figure 7. Shape factor (circularity) of asphaltene aggregate of 70 vol % n-heptane−toluene−crude oil mixture with (a) no nanoparticles24 and (b) 2000 ppm NiO nanoparticles

4. Conclusions

In this study, a novel experimental methodology recently developed in our lab was employed to directly observe and measure asphaltene aggregation behavior at a pore scale. We investigated effect of nanoparticles (NiO, SiO2 and Fe3O4) on the inhibition of asphaltene particle aggregation in a water-wet micro-sized pore. The particle number, diameter and size distribution of asphaltene aggregates were directly measured and analyzed in a small pore. This work could improve the understanding of the impact of nanoparticles on asphaltene inhibition at a pore scale. Furthermore, a direct pore-scale visualization and measurement on asphaltene aggregation behaviors in a small pore is of significance to solve flow problem of pore fluids for enhanced oil recovery purpose.

22


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The results show that nanoparticles can act as asphaltene inhibitors, which can effectively prevent asphaltene aggregation and enhance the asphaltene stability in a microcapillary. The asphaltene particles can aggregate together easily in the absence of nanoparticles. On the other hand, the existence of nanoparticles can prevent the flocculation of asphaltene particles. This could mainly result from high surface area-to-volume ratio, good adsorption capacity and high degree of suspension of nanoparticles. In the presence of nanoparticles, suspended asphaltene aggregates with smaller size were observed in a microcapillary. The number of asphaltene aggregates is more than the one in the absence of nanoparticles, and size distribution becomes narrower. The effect of nanoparticle concentration was investigated. Higher concentration of nanoparticles contributes to the inhibition of asphaltene aggregation. In addition, the effect of concentration of asphaltene precipitant in the presence of nanoparticles was also investigated. Higher concentration of heptane could cause more asphaltene particles adsorbed onto the surfaces of nanoparticles. The effect of nanoparticles on inhibition of asphaltene aggregation is more remarkable when higher concentration of asphaltene precipitant is used.

Acknowledgements This work was supported by Science Foundation of China University of Petroleum, Beijing (2462016YJRC005, 2462017BJB05) ，

Science Foundation of China

University of Petroleum, Beijing (No. 2462018BJC004) and Science Foundation of CUPBK (RCYJ2017A-02-001, RCYJ2017A-03-001). 23



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29



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For Table of Contents Only

30


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Figure 1 Experimental setup24



Figure 2. Mean asphaltene size at flocculation times of t=0, 15, 30, 45 and 60 min at concentration of 70 vol % with no nanoparticles24 and with addition of NiO, SiO2 and Fe3O4 nanoparticles at 2000 ppm.


Page 32 of 40

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Figure 3. Representative microscopic images of asphaltene particles at 70 vol % n-heptane with no added nanoparticles24 and with addition of NiO, SiO2 and Fe3O4 nanoparticles at 2000 ppm at t= 30 min.



Figure 4. Mean size of asphaltene aggregates at concentration of 70 vol % n-heptane with addition of nanoparticles at 2000 and 10000 ppm.


Page 34 of 40

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Figure 5. Representative microscopic images of asphaltene aggregates at 70 vol % n-heptane with no added nanoparticles24 and in the presence of NiO nanoparticle at 2000 and 10000 ppm at t=30 min



(a)

Page 36 of 40

(b)

(c)

Figure 6. Effect of nanoparticles on asphaltene aggregation at different n-heptane concentrations of (a) 60 vol%, (b) 70 vol% and (c) 80 vol%.


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(a) no nanoparticles

24

(b) 2000ppm NiO

Figure 7. Shape factor (circularity) of asphaltene aggregate of 70 vol % n-heptane−toluene−crude oil mixture with (a) no nanoparticles24 and (b) 2000 ppm NiO nanoparticles



Page 38 of 40

Table 1. Size distribution of particle of asphaltene aggregates at 70 vol % n-heptane without nanoparticles24 and with nanoparticles at 2000 ppm at different flocculation times Different nanoparticles

Flocculation time 30 min

Without 24

nanoparticles

2000ppm NiO

2000ppm SiO2

2000ppm Fe3O4


60 min

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Table 2. Size distribution of particles of asphaltene aggregates at 70 vol % n-heptane with no added nanoparticles24 and in the presence of NiO nanoparticles at 2000 and 10000 ppm at different flocculation times Concentration

Flocculation time

of NiO nanoparticles

30 min

Without nanoparticles

24

2000ppm NiO

10000ppm NiO


60 min


Page 40 of 40

Table 3. Size error of asphaltene aggregate of 70 vol % n-heptane−toluene−crude oil mixture with no nanoparticles24 and 2000 ppm NiO nanoparticles No nanoparticles24

2000 ppm NiO nanoparticles

Size

Size

Size error (μm)

Size

Size error (μm)

range

distribution


distribution


(μm)

(%)

0-2.5

11.2

0.35

8.2

0.22

2.5-5.0

23.2

0.78

36.5

0.70

5.0-10.0

41.6

1.24

43.9

1.28

10-20

13.6

3.11

8.2

2.61

20-40

7.2

6.14

3.3

4.52

40-80

3.2

14.84

0

—

(%)


Effect of Nanoparticles on Asphaltene Aggregation in a Microsized

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