Silica/polyacrylamide nanocomposite for inhibition of asphaltene precipitation from unstable crude oils

Abstract Among various flow assurance problems, asphaltene precipitation is a major issue. This study aims to evaluate the potential application of silica-polyacrylamide nanocomposite to inhibit asphaltene precipitation for the first time. The nanocomposites were synthesized and characterized by Fourier transform infrared and field emission scanning electron microscopy techniques. The inhibitory effect of nanocomposites in water-based nanofluid on two unstable crude oils was evaluated using viscometry, asphaltene dispersion test, dynamic light scattering, and polarized microscopy techniques. The viscosity measurements indicate that nanocomposites postpone asphaltene onset of precipitation from 34 to 44 and 23 to 36 vol% nC7 for crude oil A and B, respectively. The maximum dispersion efficiency of nanocomposite was 69% and 79% at the 1% and 2.5% nanofluid volume dosages for crude oil A and B, respectively. Dynamic Light Scattering and microscopy results indicated the asphaltene size change to lower values in the presence of nanocomposites. Polyacrylamide attached to the silica surface promotes a surface with efficient functional groups that increase the asphaltene adsorption. Asphaltene adsorption leads to decreased aggregate size and controlling the asphaltene precipitation. The prepared nanocomposite was evaluated as an efficient dispersant and inhibitor that reveals the potential application of Silica-polyacrylamide for handling the asphaltene precipitation in reservoirs.


Introduction
Asphaltene is a significant problem in the oil industry, affecting the upstream, midstream, and downstream by precipitation and deposition, decreasing productivity, and increasing the repairing costs (Fakher et al. 2020), (Durand et al. 2010;Yang et al. 2016;Fakher et al. 2020;Durand et al. 2010;Yang et al. 2016).
Recent studies indicate that a combination of desirable properties is achieved by jointing the nanoparticles with proper materials (Hashmi and Firoozabadi 2010).In our recent study, polyaniline-coated Fe 3 O 4 nanoparticles were used for asphaltene adsorption from the model oil solution.The results indicated that the nanocomposite showed higher adsorption capacity than pristine Fe 3 O 4 (Hosseini-Dastgerdi, Meshkat, and Samadi 2021).Setoodeh et al. studied the adsorption capacity of polythiophene coated Fe 3 O 4 , and metal-organic framework Mil-101 (Cr) coated Fe 3 O 4 for asphaltene inhibition (Setoodeh, Darvishi, and Lashanizadegan 2018;Setoodeh, Darvishi, and Esmaeilzadeh 2018).According to the results, the high polarity of polythiophene increases the interactions between asphaltene and sulfur heterocycles of polythiophene, increasing the adsorption capacity of polythiophene-Fe 3 O 4 compared to pure Fe 3 O 4 .The application of NiO/poly (DVB) HIPE nanocomposites has shown a good adsorption capacity of this material for asphaltene from a model oil solution ( € Ozker and C ¸etinkaya 2019).Researchers have reported the practical application of Fe 3 O 4 -chitosan nanocomposite for asphaltene precipitation inhibition (Rezvani et al. 2018;Kazemzadeh, Sharifi, and Riazi 2018).The application of green nanocomposites for asphaltene adsorption and inhibition has been investigated by researchers.They showed that the novel nanocomposite could reduce asphaltene formation damage (Lopez et al. 2020, Mohammadi et al. 2019).For efficient stabilization of asphaltene in crude oil, some researchers have investigated the inhibitory effect of TiO2/SiO2 nanofluid by Near-IR (NIR) spectroscopy.The results indicate that asphaltene adsorption at 80% TiO2 nanocomposite has the most significant influence on the AOP postponement (Shnain et al. 2022, Mohammadi, Dadvar, and Dabir 2017a, Mohammadi, Dadvar, and Dabir 2017b).
Silica-PAM nanocomposite has been broadly used for polymer flooding applications due to the oil recovery increase of up to 60% (Thomas, Gaillard, and Favero 2012).Recent studies have shown that PAM is adsorbed on the surface of nanoparticles, forming a complex macromolecular structure that results in the viscosity improvement (Hu et al. 2017;Maghzi et al. 2014;Ye et al. 2013;Zhu et al. 2014;Maurya and Mandal 2016;Zhao et al. 2012;Rashidi et al. 2010).On the other hand, the active sites on silica -PAM nanocomposite surface may exhibit different interactions with asphaltene.In a study by Zou et al. (2019) the attraction force between asphaltene and hydrophobically modified polyacrylamide (HMPAM) was attributed to the long hydrocarbon chains of HMPAM and asphaltene (Zhu et al. 2014).Although the interaction between silica-PAM nanocomposite and asphaltene has not yet been investigated, the nanocomposites may be efficient in dealing with the asphaltene induced issues.
A detailed literature survey indicates no scientific publication on the potential application of nanoparticle-polyacrylamide nanocomposite for asphaltene precipitation inhibition.The nanoparticle-polyacrylamide nanocomposite increases the possibility and benefit of using nanoparticles in porous media, providing unique properties for reducing asphaltene formation damage and enhancing oil recovery.
Therefore, the main objective of this work is to investigate the potential use of laboratory synthesized silica-polyacrylamide nanocomposite (stabilized in water) on asphaltene precipitation of two unstable crude oils.In this regard, silica-PAM nanocomposite was synthesized via the vaper acid method.The potential application of different concentrations of nanofluid (1000 ppm nanocomposite in water) on asphaltene precipitation was investigated using viscometry tests and ADT.DLS and microscopic analysis investigated the asphaltene aggregation at optimum nanofluid concentration.The stability effect of nanofluid over time was also investigated.The interaction between asphaltene and nanoparticles was studied using FTIR and FESEM methods.

Experimental materials
This work applied two heavy crude oil samples obtained from southwest Iranian oilfields.Table 1 represents the SARA analysis (ASTMD2007) of these two samples.The colloidal instability index (CII) of crude oils was calculated to determine the asphaltene precipitation tendency of crude oils (Yen, Yin, and Asomaning 2001).CII expression depends on the chemical composition of crude oil as follows: CII > 0.9 means an unstable crude oil with a high risk for asphaltene precipitation and deposition in the oil field.While CII< 0.7 represents a stable crude oil.As shown in Table 1, both crude oils fall into the unstable crude oil category.SiO 2 nanoparticles with a specific surface area of 170-230 m2/ g, Sulfuric acid, and polyacrylamide were obtained from Sigma Aldrich, Merk, and BDH laboratory supplies Poole, respectively.Normal heptane and toluene were purchased from Merk with more than 99% purity.

Experimental method
Silica-PAM nanocomposite was synthesized by the procedure described in detail in our previous article (Maleki, Hosseini-Dastgerdi, and Rashidi 2021).Initially, SiO 2 nanoparticles were functionalized using the vaper acid method with sulfuric acid over 130 0 C for 48 h.Next, the functionalized nanoparticle solution was mixed with polyacrylamide with a mass ratio of 1 to 1. Afterward, the mixture was sonicated for 15 min and then stirred for 3 h with the stirring speed of 200 rpm at laboratory temperature.The silica-PAM nanofluid was prepared by adding a specific amount of synthesized nanocomposites to distillate water to achieve the concentration of 1000 ppm by mass.The solution was first mixed using a stirrer magnet device for 15 min and then sonicated for 5 min at the laboratory temperature.In order to examine the stability of the synthesized nanocomposite in water, the mean particle size of the prepared nanofluid was monitored using the DLS technique (HORIBA, SZ-100) over a period of 24 h.The results indicate that the stability of nanofluid can be maintained for at least 24 h (supplementary material, Figure S2).
To prepare the treated crude oils, first, enough amount of crude oil was centrifugated for 30 min at 3000 rpm at room temperature to eliminate any extra solids.Next, the prepared nanofluid was added to crude oil with the volume ratio of 0.01, 0.025, 0.05, and 0.1.Finally, the mixture was stirred for 1 h at 200 rpm.
The viscosity experiments are utilized to investigate the effect of nanocomposite on asphaltene precipitation by measuring the AOP.Therefore, the viscosity of crude oil samples diluted with different concentrations of precipitant (n-heptane) is measured.The viscosity of the solution decreases by increasing normal heptane concentration.The fluctuation point observed in the viscosity versus n-C 7 concentration is the AOP.This fluctuation results from a sharp increase in the crude oil viscosity due to asphaltene precipitation (Mahmoudi et al. 2021a).The kinematic viscosity of treated and untreated crude oils was measured using an SVM3000-Anton Paar instrument.A particular amount of n-heptane was mixed with 10 ml of treated or blank crude oil and stirred for 30 min with 200 rpm for each test.The AOP of crude oils treated with different nanofluid dosages and blank oil were compared to investigate the effect of silica-PAM nanocomposite on asphaltene precipitation.
The asphaltene dispersion test is a method to evaluate the asphaltene precipitation tendency of different crude oils.The ability of treatment chemicals to stabilize asphaltene can also be investigated using this method.First, 9.25 ml of each treated crude oil at various concentrations of silica-PAM nanocomposites were prepared.Then each solution was mixed with n-heptane at the volume ratio of 8%.They stirred for 15 min at a speed of 200 rpm.The mixtures were placed in tubes and left to stand for 6 h.All tubes were monitored for the deposition level of asphaltene after 6 h and 24 h.The same procedure was also applied for blank oil to compare the effect of nanocomposite amounts on ADT results.The dispersion efficiency of nanocomposites was investigated as follows (Alemi et al. 2021a;Manek 1995;Melendez-Alvarez et al. 2016).
Where DE is dispersion efficiency and DLT and DLB are the deposition level of asphaltene (the height of deposit in tubes) for treated and blank oil, respectively.
In order to perform the microscopic tests, first, treated crude oils at optimum nanofluid concentrations (1% (vol/vol) for crude oil A, (2.5% (vol/ vol) for crude oil B) were prepared.The optimum nanofluid concentration was defined using viscosity experiments and ADT.Second, n-heptane at different volumetric ratios of 50%, 75%, and 100% was mixed with treated crude oils and blank oil.The solutions were stirred for 1 h at the speed of 200 rpm.Then, a small amount of solution was applied for optical microscopic analysis.In order to observe the structure, morphology, and size growth of asphaltene aggregates under the influence of precipitant, a polarizing microscope (Olympus) with a magnification of 40X was used.The microscopic studies were performed at 20 0 C.
The effect of nanocomposite on the asphaltene size distribution of crude oils was analyzed using HORIBA, SZ-100, and DLS equipment.The size distributions of titrated blank oil A and B were also provided for comparison.Blank crude oils and treated crude oils at optimum nanofluid dosage were mixed with n-heptane with a volumetric ratio of 8% (vol/vol) for 15 min with a speed of 200 rpm.The samples were allowed to stand for 6 h.The size distributions of asphaltenes present in supernatant solution were evaluated.

Characterization of nanocomposites
FTIR analysis was provided using a Thermo-Avatar spectrophotometer to investigate the functional groups in the structure of asphaltene and the synthesized nanocomposite.At the end of the ADT tests, the precipitated asphaltene solids from the optimum ADT experiment were extracted using normal heptane at the solvent/deposit ratio of 40:1 (g/ml) according to the IP-143 standard (ASTM.D3279-90).The extracted solids were analyzed by FTIR to investigate the adsorption behavior and asphaltene-nanocomposite interactions (Alemi et al. 2021a).The same procedure was used to extract pure asphaltene from crude oils.In addition, FESEM (TESCAN-MIRA111) imaging method was applied to explore the structure of nanocomposites before the experiments and after the adsorption of asphaltene.

FTIR
The FTIR spectra of Silica-PAM in Figures 1 and 2 represent two sharp peaks at 3460 cm À1 and 1640 cm -1 that are assigned to antisymmetric stretching corresponding to the N-H groups and C ¼ O carboxyl group, respectively.The peak at 730 cm À1 is attributed to Si-O stretching vibration.According to the identified peaks, the possible interaction between the silica surface and PAM is the H-bonding between silanol functional groups of the silica surface and amide functional groups of PAM.The FTIR spectra of pure asphaltene indicate a sharp peak at 3415 cm À1 , which represents the vibrating hydroxyl groups (OH-) of asphaltene.Asymmetric aliphatic stretching vibration of C-H 3 and C-H 2 is observed at a wavelength of 2925 cm À1 and 2858 cm À1 .The peak at 1627 cm À1 represents the stretching vibration of C ¼ C aromatic bond.The peaks at 1459 cm À1 and 1375 cm À1 were attributed to methylene and methyl groups, respectively.The FTIR spectra of deposited asphaltene indicate a sharp peak at 3415 cm À1 which represents the overlapping of PAM stretching vibration of N-H groups and asphaltene hydroxyl groups.Also, the peaks at 2925 cm À1 , 2858 cm À1 , 1459 cm À1 and, 1375 cm À1 represent the aliphatic groups in the FTIR spectra of deposited asphaltene.The weak peaks around 730 cm À1 reflect Si-O stretching vibration.Sharp peaks of asphaltene and nanocomposite in the  deposited asphaltene spectrum is a sign of asphaltene presence on the nanocomposite surface.

FESEM
Figure 3(a, b) show the SEM image of neat nanoparticles and synthesized nanocomposites.As discussed in our previous article, the attachment of PAM to the nanoparticles' surface results in a uniform layer, and the distinct boundary on the surface of the nanoparticles vanishes (Maleki, Hosseini-Dastgerdi, and Rashidi 2021).The FESEM image of pure asphaltene (Figure 3e) shows a rough surface of interwoven aggregates.Figure 3(c, d) show the FESEM image of the deposited asphaltene after the optimum ADT experiments for crude Oil A and B, respectively.Compared to the FESEM of pure asphaltene, in the case of deposited asphaltene, the morphology of the surfaces is changed to large particles with distinct boundaries.The particles can be nanocomposites with asphaltene settlement on their surface.The results of FESEM confirm asphaltene adsorption on the surface of nanocomposites and their interactions for asphaltene stabilization.

Results of viscosity experiments
Figures 4 and 5 represent the effect of n-heptane titration on dynamic viscosity at different nanocomposite loading for crude oil A and B, respectively.By increasing the n-heptane concentration, the viscosity of the medium decreases until the AOP, which is the concentration where the first asphaltene aggregates in the range of 0.5 lm to 1 lm, appear in the solution (Hosseini-Dastgerdi and Meshkat 2019).Figure 4(a) indicates that nanocomposites increase the AOP from 34% in the case of blank crude oil to 44% for 1% nanofluid loading.Figure 4(b-d) show that by increasing the dosage of nanofluid to 2.5%, 5%, and 10%, the AOP decreases compared to 1%.The results indicate that the performance of nanocomposite for asphaltene aggregation inhibition depends on its concentration.In the presence of nanocomposite, asphaltene adsorption on the surface of nanocomposite occurs, leading to asphaltene stabilization and AOP increase.The static adsorption tests show a high affinity degree of asphaltenes between the silica-polyacrylamide nanocomposite compared to the silica nanoparticles.The results of static adsorption tests are reported in the supplementary material, Section 1.The replacement of silica functional groups by the attachment of PAM on the surface of silica nanoparticles leads to the exposition of amide groups and the alkyl chains for the interaction with the asphaltene.The presence of amide as the polar functional group and alkyl chain as the nonpolar functional group leads to a surface with high heterogeneity that interacts with asphaltene aggregates.Asphaltene adsorption can be attributed to hydrogen bonding between amide groups in the PAM structure and hydroxyl, carboxyl, pyridinic and pyrrolic groups in the asphaltene structure.Also, the alkyl chains of PAM interact with aromatic rings and alkyl chains of asphaltene via London dispersion forces.
Nanoparticle association at higher concentrations results in lower asphaltene adsorption and less postponement of asphaltene precipitation onset.Therefore, the optimum nanofluid dosage for crude oil A inhibition is 1%. Figure 5 indicates the viscosity results of crude oil B. Figure 5(a,b) show that by increasing the nanofluid dosage from 0% to 2.5%, AOP increases from 23% to 36%.On the other hand, Figure 5(c,d) indicate that more nanofluid dosage increment decreases the AOP compared to AOP of 1% and 2.5%.Consequently, the optimum nanofluid dosage of crude oil B is 2.5%.The results indicate that nanocomposite performance is directly related to crude oil characteristics and composition.
Figure 6(a,b) compare the AOP of silica nanoparticle and silica-PAM nanocomposite at the optimum dosage of nanofluid.Figure 6 indicates that the AOP of crude oil A increases from 35% in the presence of silica nanoparticles to 44% in the presence of nanocomposites.Figure 7 shows a higher performance of nanocomposite compared to silica nanoparticles for inhibition of asphaltene precipitation of crude oil B. From the molecular viewpoint, the interactions between different functional groups on the nanocomposite surface that do not interact with the silica surface and the polar functional groups of asphaltene heteroatoms O, N, S increase the asphaltene adsorption affinity on the surface of the nanocomposite.Therefore, higher available active sites on the surface of nanocomposite for asphaltene adsorption compared to neat silica nanoparticles approve the higher performance of Silica-PAM nanocomposite for asphaltene inhibition (Lopez et al. 2020).

Results of asphaltene dispersion tests
Table 2 shows the dispersion efficiency of nanocomposite at different dosages for crude oil A and B. The effect of aging time on dispersion efficiency has also been evaluated.However, the dispersion efficiency did not change very much over time.According to the results, adding 1% of nanofluid to blank crude oil A increases the dispersion efficiency of nanocomposites.More increase in nanofluid dosage reduces nanocomposites' performance for stabilizing and dispersing the asphaltene aggregates.The dispersion efficiency of crude oil B shows the enhancement of nanocomposite performance up to 2.5% nanofluid dosage.More enhancement of nanocomposite loading decreases the dispersion efficiency for crude oil B. ADT results confirm the optimum 1% and 2.5% nanofluid concentrations for crude oil A and B, respectively.aggregates (4000-7000 nm) was omitted, and a single peak in the range of 1000-1700 nm appeared (Figure 8b).The size distribution of treated crude oil B is 1000-2000 nm, which is much lower than the size distribution of blank crude oil in the range of 1000-7000 nm (Figure 8c, d).The reduction in the size of asphaltene aggregates after introducing nanocomposites in the bulk of crude oils proves the controlling effect of nanocomposites for asphaltene aggregate growth.Several molecular interactions such as p À p, hydrogen bonds and van der Waals results in the formation of wide size asphaltene aggregates.Asphaltene adsorption on the nanocomposite surface prevents the collision of low size aggregates and the formation of large aggregates.In this sense, the increase in the nanocomposite dosage results in smaller aggregate size.The heterogeneities of the nanocomposite surface are expected to create several adsorption sites for asphaltene, increasing adsorption affinity and decreasing asphaltene self-association.
Figures 9 and 10 represent the micrograph of blank crude oils and treated crude oils A and B (at the optimum dosage of nanofluid) at different n-heptane concentrations, respectively.According to the results, increasing    effectively control the asphaltene aggregation process.The results of the AOP postponement using silica-PAM compared with the AOP postponement using some other nanoparticles reported in the literature are shown in Table 3.The results indicate the efficient stabilizing effect of silica-PAM compared with other nanoparticles.

Conclusion
This study examined the performance of Silica-PAM nanocomposite in a water-based nanofluid for the inhibition of asphaltene in two unstable crude oil through viscosity measurements, ADT, DLS, and microscopy techniques.Silica-PAM nanocomposites were synthesized by the vapor acid method and characterized by FTIR and FESEM techniques.FTIR and FESEM analysis results indicated the presence of asphaltene on the surface of nanocomposites, revealing asphaltene adsorption by nanocomposite.The viscosity experiments indicate that nanocomposites can effectively increase the AOP of asphaltene from 34 to 44 and 23 to 36 vol% nC 7 for crude oil A and B, respectively.The result proves the inhibitory effect of Silica-PAM against the asphaltene precipitation.The maximum dispersion efficiency of nanocomposite was 69% and 79% at the 1% and 2.5% nanofluid volume dosages for crude oil A and B, respectively.DLS analysis results indicate that in the presence of nanocomposites, the size distribution range of asphaltenes moves to lower values.Polarized microscopy results show that untreated crude oil consists of large aggregates that change to more dispersed, small size aggregates after nanocomposite treatment.In this sense, strong hydrogen bonding and London dispersion forces between the nanocomposites and asphaltenes lead to successful adsorption of asphaltene aggregates resulting in inhibition of asphaltene aggregation and precipitation.In conclusion, the prepared nanocomposite should be considered as

Figure 1 .
Figure 1.FTIR spectra of SiO 2 -PAM nanocomposite, pure asphaltene and extracted asphaltene from crude oil A after the ADT test.

Figure 2 .
Figure 2. FTIR spectra of SiO 2 -PAM nanocomposite, pure asphaltene and extracted asphaltene from crude oil B after the ADT test.

Figure 6 .
Figure 6.Effect of nanoparticle type at the optimum dosage on the AOP point of crude oil A. Arrows indicate the AOPs.

3. 5 .
Results of dynamic light scattering tests

Figure 8
Figure8(a-d) represent the asphaltene size distribution of blank and treated crude oils A and B, respectively.According to Figure8(a), the blank crude oil A shows a 600-7000 nm bimodal size distribution.After treating the crude oil at optimum nanofluid dosage, the peak related to large

Figure 7 .
Figure 7. Effect of nanoparticle type at the optimum dosage on the AOP point of crude oil B. Arrows indicate the AOPs.

Figure 8 .
Figure 8.The results of DLS analysis for (a) Blank crude oil A, (b) Treated crude oil A at optimum nanofluid dosage, (c) Blank crude oil B, (d) Treated crude oil B at optimum nanofluid dosage.

Table 1 .
The properties of investigated crude oils.

Table 2 .
Calculated dispersion efficiencies of treated crude oils at different settling times.

Table 3 .
Comparing the inhibitory effect of different nano particles for asphaltene precipitation from crude oil.