Abstract
Polymer nanohybrids have displayed great potential in remobilizing oil droplets through porous media. This research aims at providing some insights into how the hydrolyzed polyacrylamide (HPAM) polymer and Al2O3 nanoparticles’ (NPs) hybrid can push crude oil toward the producers. An understanding of what the hybrid viscosity is when flowing through porous rocks was acquired by the rheological tests. Using the Du Noüy ring method, the interfacial tension (IFT) between the polymer nanohybrid and crude oil was studied. Contact angle experiments were employed to assess the ability of hybrid in reversing surface wettability. The results show that the hybrid can yield a 12% higher shear viscosity than the HPAM solution and the viscosity improvement dramatically depends on NPs’ concentration and temperature. With more than a 23% drop in the contact angle value, the results of contact angle experiments reveal the capability of the Al2O3 NPs in altering surface wettability. The measured IFT between hybrid and crude oil at different temperatures demonstrates that the adsorption of NPs on the oil–aqueous phase interface can significantly improve the capillary number. This article not only presents the underlying mechanisms of oil recovery during hybrid flooding but also provides a new reference for formulating a novel hybrid agent.
1 Introduction
Owing to the significant annual oil production growth rate over the last few decades, most of the oil reservoirs are experiencing a declining phase (1,2). As a result, considerable attention to the application of enhanced oil recovery (EOR) techniques has been captured for maintaining oil production from the reservoirs and supplying the global energy demand (3,4,5). The current rollercoaster of oil prices is also the main compelling reason for the shifted interest toward the cost-effective and more efficient EOR methods (6,7,8). To find out why the oil droplets are generally trapped in porous media, which results in large amounts of untouched pockets of oil in the reservoirs, there is a rich body of literature demonstrating that the unfavorable rock wettability, reservoir heterogeneity, and undesirable fluid mobility are some sources of disturbance and cause perturbations to the flow of oil droplets in pores and pore necks (9,10,11,12). In addition, to improve both macroscopic and microscopic sweep efficiencies, chemical enhanced oil recovery techniques have been proposed and are well known for remobilizing the trapped oil droplets in porous media (13,14,15). The distinctive physical and chemical features of nanomaterials have also attracted great interest from many researchers to study the flow behavior of nanofluids through porous rocks and scrutinize their possible effects on oil recovery enhancement and carbon geo-sequestration applications (16,17,18,19,20). A literature survey discloses that nanofluids not only can be injected into the hydrocarbon formations solely but also in combination with other EOR agents, such as polymers and surfactants (21,22,23). Among different presented nanohybrid formulations for ameliorating oil production from hydrocarbon reservoirs, nanopolymer sols have shown more favorable flow dynamics through porous media. Therefore, different underlying mechanisms of oil recovery enhancement have been proposed for the injection of polymer nanohybrids into the reservoir rocks (24,25,26).
Cheraghian et al. (27) studied the adsorption profiles of polymer and polymer nanohybrid samples on both carbonate and sandstone rocks. In terms of polymer adsorption reduction, the prepared polymer nanohybrids, which consisted of polyacrylamide (PAM) polymer and hydrophilic NPs, exhibited a desirable adsorption behavior onto the reservoir rocks with different surface charges. The authors claimed that the competitive adsorption behavior between NPs and polymer molecules can drastically reduce the amount of polymer adsorption onto the rock surface. Cheraghian and Khalilinezhad (28) conducted an experimental study to scrutinize the impact of clay NPs on the thermal properties of PAM polymers. They stated that the addition of clay NPs at a sufficient concentration to the polymer solution can prevent the network structure of the polymer from being destroyed at elevated temperatures. Khalilinezhad and Cheraghian (29) performed a numerical investigation to find out the possible effects of clay NPs on the performance of polymer flooding in improving heavy oil recovery. They reported that the exploited clay NPs can drastically enhance the sweep efficiency of polymer flooding by decreasing the amount of polymer adsorption and boosting the shear viscosity of the injectant. Haruna et al. (30) examined the role of novel graphene oxide nanosheets in the viscosifying ability of HPAM polymers under high-temperature conditions. The results unfolded that, compared with the conventional HPAM solutions, the stable graphene oxide nanosheets’ dispersions in aqueous HPAM have a higher shear viscosity and thermal stability due to the presence of strong hydrogen bonding between HPAM functional groups and the employed NPs.
AlamiNia and Khalilinezhad (31) investigated the interactions of PAM polymer molecules and hydrophilic silica NPs with two different particle sizes by measuring the shear viscosity of the prepared polymer nanohybrids. The results proved that the silica NPs with larger particle sizes have a superior impact on the thickening characteristics of the PAM polymers, resulting in stabilizing the displacement front in porous media and enhancing ultimate oil recovery during the hybrid flooding. Hu et al. (32) explored the viscoelastic properties of partially HPAM polymers seeded by silica NPs. Their experimental observations revealed that the formation of a hydrogen bond between the carbonyl groups in HPAM and the silanol functionalities on the surface of silica NPs is the responsible factor for the improved salt tolerance and shear viscosity of the HPAM polymers at high temperatures. Khalilinezhad et al. (33) scrutinized the effect of the physical properties of HPAM polymers on the rheological characteristics of polymer nanohybrids consisting of silica NPs and HPAM polymer. They observed that the polymer content of the hybrid and the molecular weight (M w) of the employed polymer have undeniable impacts on the viscosity improvement of the hybrid. They asserted that silica NPs have a more significant effect on the viscosity improvement of polymers with lower M w. Furthermore, the wettability modification of reservoir rock and mobility improvement have been proposed as the main responsible mechanisms of EOR by polymer nanohybrid flooding. Xu et al. (34) performed an experimental study to clarify the hydrophobic effect between the molecules of hydrophobically associating PAM and modified silica NPs on the non-Newtonian behavior and oil recovery efficiency of the hybrid flooding. They declared that the combination of hydrophobically associating PAM and cetyl-modified NPs has a more substantial synergy for improving thickening characteristics of hybrid dispersions and enhancing oil recovery in high salinity conditions. They also suggested that the displacement mechanism of the hybrid can be attributed to the synergistic wettability and thickening modifications. Khalilinezhad et al. (35) numerically evaluated the role of silica and clay NPs in enhancing heavy oil recovery from heterogeneous reservoirs during polymer flooding. They expressed that the injection of the polymer nanohybrids into the stratified formations can dramatically improve the amount of oil production from both high and low permeable layers. Compared with conventional polymer flooding, the polymer nanohybrids can also sharply reduce the amount of water cut in producers.
Orodu et al. (36) examined the effect of Al2O3 NPs on the performance of bipolymers in enhancing oil recovery from sandstone rocks. They observed that the addition of Al2O3 NPs to the polymer solutions can dramatically improve the shear viscosity of the polymer in a wide range of shear rates. In addition, the nanocomposite showed better displacement efficiency in the reservoir, which results in higher microscopic and volumetric sweep efficiencies. Gbadamosi et al. (37) conducted a comparative study to investigate the EOR performance of different types of polymer nanohybrids. The results of multiphase flow experiments, contact angle tests, and shear-viscosity measurements displayed that, compared with silica NPs, the improvement of polymer ability in reducing IFT and modifying surface wettability is more favorable when the hybrid consists of either Al2O3 NPs or TiO2 NPs. Abdullahi et al. (38) assessed the effect of different types of electrolytes on salt tolerance and shear viscosity of the HPAM and Al2O3 NPs’ hybrid. They claimed that the hydrogen bonds between NPs and carbonyl groups in HPAM solutions can make a stronger three-dimensional network, which results in better elasticity and higher viscosifying ability of the nanosuspension even under high salinity conditions.
A closer look at the literature proves that, although there is a rich body of literature on the main responsible mechanisms of oil recovery by the injection of polymer and silica NPs’ hybrids, the flow dynamics of HPAM and Al2O3 NPs’ hybrids have not been completely explained and it has remained a topic of discussion. The purpose of this research is to explore the underlying mechanisms of oil remobilization in porous media due to the injection of HPAM and Al2O3 NPs’ hybrid. Thus, the rheological tests, contact angle experiments, and IFT measurements were carried out to inspect how Al2O3 NPs can improve the performance of HPAM polymers in remobilizing the trapped oil droplets in pores and dead-end pores of the porous media. The findings of this study not only introduce the main responsible mechanisms of oil recovery during the injection of HPAM and Al2O3 NPs but also present a new reference for formulating a cost-effective nanofluid for EOR applications.
2 Materials and methods
2.1 Materials
The utilized HPAM polymer called Flopaam™ 3330s with an average M w of 8 × 106 was kindly supplied by SNF Floerger and had a degree of hydrolysis of 30%. The specific area of employed α-Al2O3 NPs (50 nm; >99% in purity) was about 19 m2·g−1 and the density of NPs was 3.97 g·cm−3. They were completely hydrophilic and provided by Nanosany Corporation™ (Mashhad, Iran). The exploited crude oil for contact angle experiments and IFT measurements was relatively to the crude oil employed by Khalilinezhad et al. (33). It was a degassed stock-tank oil from one of the Iranian oil reservoirs characterized by a density of 0.872 g·cm−3 and a viscosity of about 64 cP at 25°C. It should be noted that all materials were exploited as received.
2.2 NPs’ analysis and characterization
The X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used for the characterization of the employed NPs. The XRD patterns of NPs were recorded via a diffractometer (Seifert PTS 3003). The SEM and TEM analyses of NPs were carried out using Tescan MIRA3 and Philips CM120, respectively.
2.3 Preparation of the polymer and hybrid samples
In this research, to scrutinize the impact of Al2O3 NPs on the performance of HPAM polymers in remobilizing the trapped oil droplets in porous media, the polymer and hybrid samples were formulated using the reported protocol by Khalilinezhad et al. (33). Therefore, the polymer samples were prepared by slowly mixing the desired amount of polymer powder with deionized (DI) water using a magnetic stirrer for about 2 days. To prepare hybrid samples, a specific concentration of NPs was completely dissolved in DI water by stirring the nanosuspension for about 1 h. Then, to break the agglomeration of NPs in DI water and completely disperse NPs into the solution, the probe sonicator (Sonic, VCX 750) was used and the ultra-sonication was done for about 30 min (40% A, 750 W). Next, the required amount of polymer powder was dissolved into the nanosuspension by stirring for about 2 days. It should be mentioned that to ensure that the samples were free of bubbles before conducting the experiments, all the prepared samples were stored for about 1 day without any agitation.
2.4 Rheological tests
The DV-III Ultra + Brookfield viscometer equipped with a temperature controller was used for characterizing the rheological and steady viscoelastic behaviors of both polymer and polymer nanohybrid samples. Before conducting any test, to guarantee that the setup was accurately calibrated, the performance of the viscometer was examined using standard oil and pure water samples with known viscosities. The shear stress and viscosity of the polymer and polymer nanohybrid samples were measured at different shear rates. The principles of the operation of the viscometer can also be found elsewhere (11).
2.5 IFT measurements
The IFT between crude oil and the prepared samples was measured by the Ring method (22) at ambient temperature using a Kruss tensiometer (Kruss, K100). In this technique, to measure the IFT between two liquids, the force acting on an optimally wettable ring as a result of the tension of the withdrawn liquid lamella when moving the ring from one phase to another is measured.
2.6 Contact angle experiments
Contact angle measurement of a surface is one of the efficient techniques to assess surface wettability. There are also different methods to measure the contact angle between displacing and displaced fluids (39). In this research, to evaluate the effect of Al2O3 NPs on the ability of HPAM polymers in reversing the surface wettability, the sessile drop method was exploited. As the second part of this research and oil recovery experiments will be conducted using the glass micromodels, the contact angle measurements were carried out using glass plates. Accordingly, several glass plates with a flat surface and the same size were provided and cleaned with ethanol. Then, they were dried in an oven for 1 h set to a temperature of 100°C. In the next step, to provide the oil-wet state, the plates were aged in crude oil at room condition for about 30 days. Finally, a drop of the aqueous phase was deposited into the plate and the image of the drop at equilibrium condition was captured and saved to analyze the contact angle. It should be noted that to get reliable results, the measurements were carried out for the selected five points on the plate and the average values were calculated. More details about the contact angle measurements using the sessile drop method can be found elsewhere (40).
3 Results and discussion
3.1 NPs’ analysis and characterization of polymer nanohybrid
Based on the depicted SEM and TEM images of the employed Al2O3 NPs (Figure 1a and b) and regarding the hexagonal and octahedral site inherent in the crystal structure of the alumina, it is clearly observed that the morphology of the employed NPs is relatively non-spherical in shape. In addition, the average size of NPs can be estimated in the order of a few nanometers and they have a narrow particle size distribution. Figure 1c also displays the XRD pattern of the Al2O3 NPs. The presented XRD peaks demonstrate that the employed NPs are mainly composed of alumina oxide (>99% in purity).
Characterization of the employed Al2O3 NPs: (a) SEM image, (b) TEM image, and (c) XRD pattern.
Besides, the chemical analysis of the prepared samples was conducted using Fourier transform infrared spectroscopy (FTIR) to evaluate the structure of the HPAM and Al2O3 NPs’ hybrid and the formation of strong bonding between HPAM functional groups and the employed NPs. Figure 2a exhibits FTIR spectroscopic analysis of the Al2O3 NPs. Based on the presented results, the main peaks were detected at 598 and 695 cm−1, which demonstrate Al–O stretching in the octahedral structure. Furthermore, at 3,410 cm−1, an adsorption peak was observed that displays the stretching of the –OH bond. Figure 2b depicts the FTIR spectroscopic analysis of the HPAM and Al2O3 NPs’ hybrid. The captured peaks (650 and 1,310 cm−1) show Al–O stretching in the tetrahedral structure and symmetric bonding vibration of Al–O–H by the –OH group on the alumina surface. Besides, the appeared adsorption peak at 1,660 cm−1 confirms the stretching vibration of the carboxyl group in the acrylamide polymer. Accordingly, a strong hydrogen bond is formed between the surface of Al2O3 NPs and amide groups of HPAM. In addition, a hydrogen group in HPAM and an oxygen group on the Al2O3 surface can create a hydrogen bond, which results in forming strong hydrolyzable crosslinks of Al2O3–HPAM. Similar to the reported procedure by Khalilinezhad et al. (33), the stability of the prepared samples was investigated using the time-dependent transparency of the samples. Thus, the visual observations of the hybrid samples kept for about 21 days revealed that the samples have proper stability as no sedimentation, coagulation, and flocculation were observed for a reasonable period of time. Regarding the employed NPs’ concentration and sonication time during the preparation process of the hybrid samples, it is reasonable to have a stable nanofluid. Moreover, these results are inconsistent with the reported results by Choudhary et al. (41).
FTIR spectra for: (a) pure Al2O3 NPs and (b) HPAM (0.26 wt%) and Al2O3 NPs (0.1 wt%) hybrid.
3.2 Rheological tests
3.2.1 Rheological characterization and temperature dependence of polymer
Different crucial factors, such as hydrophilic nature, polymer content, solution salinity, and temperature, affect the rheological characteristics of polymers (33). There is also a rich body of literature on the salt tolerance improvement of the EOR polymers by different types of NPs (24). Therefore, in this study, the assessment of the effect of salinity on the performance of the hybrid in oil remobilization has been excluded. On the other hand, experimental observations have proven that NPs have a significant impact on the improvements of the thermal stability of EOR polymers (42,43). In this study, first, the rheological characteristics of the polymer solutions with different polymer contents were evaluated under low- and relatively high-temperature conditions. Figure 3a displays the shear viscosity of polymer solutions with different polymer contents at 25°C. Based on the presented trends when the polymer concentration is about 0.15 wt% or higher, the viscosity is heavily dependent on the shear rate and it decreases by increasing the exerted shear rate on the fluid. However, for the polymer solution with a polymer concentration lower than 0.15 wt%, the polymer sample behaves like Newtonian fluids and is independent of shear rate. In fact, the concentrated polymer solution (with polymer content of more than 0.15 wt%) has a higher degree of polymer chain entanglement and a larger hydraulic radius of the chain, which results in the expansion of the shear-thinning region and improvement of solution viscosity. Figure 3b also depicts the shear viscosity of the polymer solution at 80°C. It reveals that the shear viscosity of the polymer is strongly dependent on temperature. Generally, regarding the physical and chemical properties of the HPAM polymers, the nature of HPAM polymers is not tolerant of high temperatures. At elevated temperatures, the thermal motion of the HPAM molecules increases, which results in increasing the mobility of HPAM chains and impairing the intermolecular interaction of its hydrophobic group. Consequently, hydrodynamic, polymer chain entanglements, and viscosity of the HPAM polymer decrease (37). Furthermore, experimental observations have demonstrated that there is a critical temperature to observe the aforementioned phenomenon. When the temperature is lower than 35°C, activation energy is low and the temperature does not significantly affect the viscosity. In contrast, when the temperature is higher than 35°C, the activation energy is high and viscosity is dramatically reduced by increasing the temperature (44). Accordingly, based on the presented data in Figure 3b, the shear viscosity of the polymer samples sharply decreases at 80°C. In addition, similar to the described trends at 25°C, the rheological behavior of polymer solution at 80°C is a function of polymer content. When the polymer concentration is more than 0.15 wt%, the viscosity gradually reduces with an increase in the shear rate, indicating shear-thinning behavior. However, for the cases with a polymer content of lower than 0.15 wt%, the Newtonian behavior is observed.
Dependence of the viscosity of polymer on the shear rate: (a) T = 25°C and (b) T = 80°C.
A literature survey shows that the favorable influence of hydrophilic NPs on the ability of EOR polymers in improving oil recovery is affected by the physical properties of polymers, such as polymer content and polymer M w (33). In addition, it is suggested that the amount of polymer concentration should be higher than the critical association concentration (CAC) of the polymer. The CAC values can also be obtained using several techniques, such as light scattering determination and variation of the fluorescence when micellization occurs. In this research, similar to the reported procedure by Hu et al. (32), the rheological behavior of the polymer solution was studied to obtain an estimation of the CAC. Moreover, regarding the typical exerted shear rate (<35 s−1) on the fluid through porous media (45), the viscosity of the samples at shear rates from 13 to 31 s−1 was studied (Figure 4a) to determine the CAC. The results exhibit that as the curve has two distinct slopes for each shear rate at 25°C, the curve can be divided into two parts at a polymer content of about 0.25 wt%. Below that, the viscosity increases slowly with an increase in the polymer content. But, at higher concentrations (more than 0.25 wt%), a rapid increase can be observed. These results suggest that the CAC for the employed polymer is about 0.25 wt%. Similar trends for 80°C (with a relatively higher CAC) can be seen in Figure 4b. Regarding the results of CAC estimation using the rheological behavior of polymer, 0.26 wt% was chosen as a fixed polymer concentration for preparing the polymer nanohybrids and examining the effect of Al2O3 NPs on the oil recovery performance of polymer flooding. Figure 5 displays the viscosity and shear stress of the polymer sample with a polymer concentration of 0.26 wt% as a function of the shear rate at two different temperatures.
Viscosity of the polymer at different polymer contents: (a) T = 25°C and (b) T = 80°C.
Rheological behavior of polymer sample (fixed polymer content of 0.26 wt%) at two different temperatures: (a) shear viscosity and (b) shear stress vs shear stress.
3.2.2 Rheological characterization of polymer nanohybrid
Based on the literature, the impact of hydrophilic NPs, such as Al2O3 NPs, on the viscosifying ability of HPAM polymers displays a non-monotonic behavior and there is a critical concentration of NPs at which the NPs have a superior effect on the HPAM viscosity (37). Below the critical concentration, the hybrid viscosity increases by increasing the concentration of NPs. However, for the concentrations above the critical concentration, the hybrid viscosity decreases by increasing the NPs’ concentration. Gbadamosi et al. (37) claimed that, for the concentrations above 0.1 wt% Al2O3 NPs, the NPs cannot be fully dispersed in the polymer solution resulting in forming aggregates, diminishing the functionality of the NPs in the polymer solution, and decreasing the hybrid viscosity.
Accordingly, the effect of Al2O3 NPs with lower concentrations than the critical concentration on the viscosity of HPAM was assessed at a typical reservoir shear rate (< 40 s−1). The hybrid samples were prepared with a fixed polymer concentration of 0.26 wt% and different NP concentrations ranging from 0.025 to 0.1 wt%. Moreover, the viscosity measurements were carried out at two different temperatures. Figure 6 depicts the viscosity of the hybrid samples as a function of NPs’ concentrations. Based on the results (Figure 6a), the viscosity of the polymeric sample is significantly enhanced by increasing the NPs’ concentration, particularly at lower shear rates. In other words, the hybrid has a 12% higher viscosity than the polymer solution. Regarding the FTIR spectroscopic analysis and the physicochemical properties of NPs, due to the high surface area of NPs, the functionality of NPs and their bonding with the HPAM polymer increased. Furthermore, the interactions between functional groups of NPs and the amide groups of the HPAM polymer can improve the formation of strong electrostatic hydrogen bond and, due to the adsorption of macromolecules on the surface of NPs, the employed NPs can play a physical crosslinker role between different polymeric chains, which results in developing the network structure of the polymer and increasing the viscosity of the solution. It should be mentioned that, compared with the SiO2 NPs, the favorable mechanical resistance of the Al2O3 NPs can also reduce the shear degradation of polymers (46). Thus, the hybrid samples consisting of Al2O3 NPs display higher viscosity at both low- and high-shear rates.
Viscosity of hybrid sample as a function of NPs’ concentration: (a) T = 25°C and (b) T = 80°C.
Figure 6b shows the viscosity of hybrid samples with different NP concentrations at 80°C. Similarly, the sample viscosity is still improved by increasing the NP concentrations. Compared with the results depicted in Figure 6a, it exhibits that, similar to the polymer solutions, the viscosity of the hybrid sample is dependent on temperature. In fact, the network structure of the hybrid can be partially destroyed at elevated temperatures, leading to a decrease in the intermolecular forces between the polar groups of polymer structures and a reduction in the affinity of polymer molecules to the surface of NPs. As a result, due to the weak bonds between NPs and polymer molecules, the network structure of the hybrid is weakened, which results in the viscosity reduction of the hybrid (42). In addition, regarding the thermal properties of Al2O3 NPs, electrostatic interactions between oppositely charged surface of Al2O3 NPs can slightly improve the adsorption of polymer molecules on the surface of the Al2O3 NPs. Finally, at elevated temperatures, the viscosity of the hybrid sample decreases due to the thermal degradation but with a lower degradation rate compared with the polymer solutions (37).
Figure 7 compares the shear viscosity of the polymer sample at 25°C and the hybrid sample at 80°C. As the results show, the employed NPs can minimize the negative impact of temperature on the viscosifying ability of EOR polymers. Thus, by employing even a small amount of Al2O3 NPs (0.1 wt%), the HPAM polymer can be utilized for improving oil recovery from high-temperature formations.
Comparison of shear viscosity of polymer sample (at 25°C) and hybrid sample (at 25°C).
3.3 IFT measurements
Experimental studies have proven that EOR polymers are mainly responsible for the improvement of mobility ratio in porous rocks. The polymers have also been well known for their little impact on IFT and rock wettability (47). There is also a growing interest in improving the ability of polymers to reduce IFT or reverse rock wettability by introducing different types of chemical agents, such as NPs, to the polymer solution (24). In this section, the impact of the introduction of Al2O3 NPs to the polymer solution in reducing IFT is evaluated. Figure 8 exhibits the IFT values of the crude oil-hybrid sample at two different temperatures. As the results show, the addition of Al2O3 NPs to the polymer solution can drastically reduce the amount of IFT by a factor of 50% at 25°C. This behavior can be attributed to the adsorption of NPs on the oil-aqueous phase interface that can reduce the IFT. As a result, NPs can act as a surfactant at the oil-aqueous phase interface; however, the extent of the IFT reduction is generally less than the amount that can be obtained through EOR surfactants (48).
IFT of oil/aqueous phases at different temperatures as a function of the concentration of NPs.
Figure 8 also displays a decreasing trend for IFT of crude oil-aqueous phase at 80°C, which is similar to the observed trend at 25°C. The presented significant reduction in IFT at 80°C, similar to the surfactant systems, can be related to the adsorption behavior of NPs at elevated temperatures. In fact, the adsorption of NPs onto the interfaces increases by increasing temperature, which results in further reduction of IFT. Therefore, the IFT decreases by increasing temperature (49). As the IFT reduction can decrease the capillary force and deform the trapped oil droplets in porous media to pass the pore necks (42), these results demonstrate that the formulated hybrid can dramatically improve the flow behavior of crude oil through porous media, which results in the significant oil recovery enhancement.
3.4 Contact angle experiments
A wettability change toward more water wet is of key importance in carbon geo-sequestration applications and oil recovery from complex systems, such as fractured limestone reservoirs. As stated earlier, contact angle experiments were conducted on glass surfaces to assess the effect of nanofluids on wettability modification. First, to ensure that the glass surface is in an oil-wet state, a water droplet was deposited on the glass surface and the image of the drop at equilibrium condition was captured. Figure 9a displays the measured contact angle for the selected five points on the glass surface. As shown in this figure, the average value for the contact angle is about 101°, which proves that the aging process made the oil-wet surface.
Comparison of contact angle of (a) oil-wet surface and (b) surface treated with the polymer nanohybrid.
Similar to the literature (42), to examine the effect of nanofluids on wettability reversal, the glass plates were laid vertically in the prepared nanofluids to avoid deposition of NPs on the glass surface for about 2 days. Based on this procedure, the wettability modification can only be attributed to the adsorption of NPs on the glass surface. It should be mentioned that 0.05 wt% of NPs was chosen as a fixed concentration for preparing polymer nanohybrid during contact angle experiments. Regarding the results of rheological tests and IFT measurements, the aforementioned concentration of NPs is considered an optimal concentration from both economical and technical perspectives. Based on the depicted results in Figure 9b, treating the surface of oil-wet glass plates with the prepared nanofluid alters the wettability to more water-wet or intermediate states for all the selected points. In addition, the average value of contact angle can be decreased from 101° to about 78°. Inconsistent with the reported results by Gbadamosi et al. (37), the observed trends for the wettability modification of glass plates can be ascribed to the adsorption behavior of NPs on the glass surface. In other words, NPs have an affinity to adsorb on the surface on which they flow and as a result, the adsorbed NPs on the surface can change the wetting state of the surface (50). Figure 10 compares the advancing contact angle (C A) and receding contact angle (C R) before and after treating with the nanofluid. As can be seen, both C A and C R can be significantly decreased by the adsorption of NPs on the surface of glass plates.
Comparison of advancing and receding contact angles of (a) oil-wet surface (first point), (b) surface treated with the polymer nanohybrid (third point), (c) oil-wet surface (first point), (d) surface treated with the polymer nanohybrid (third point).
4 Conclusion
In this article, the impact of Al2O3 NPs on the ability of EOR polymers in remobilizing oil droplets through porous media was scrutinized. The experimental results show that, due to the presence of a strong hydrogen bond between HPAM polymer and the NPs, the presented formulation for the polymer nanohybrid has a higher shear viscosity than conventional HPAM solution at both low and high temperatures. Furthermore, the results of IFT measurements at two different temperatures have proven that the employed NPs can move toward the interface of the fluids and reduce the IFT between crude oil and the aqueous phase at both 25°C and 80°C by a factor of 50%. Furthermore, the contact angle experiments have shown that the suggested polymer nanohybrid has a significant ability in reversing the wettability of glass surfaces toward a more water-wet state by decreasing the contact angle from 101° to 78°. The results of this study have revealed that the addition of even a small amount of Al2O3 NPs to the polymer solution can dramatically increase the capillary number in porous media even at elevated temperatures by improving several crucial factors.
Acknowledgment
The authors greatly appreciate the Research Institute of Flood Science and Technology (in Mashhad) and Research Institute of Petroleum Industry (in Tehran) for providing technical facilities.
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Funding information: The authors state no funding involved.
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Author contributions: Sina Mobaraki: investigation, writing – original draft; Hamid Tabatabaee: supervision; Reza Shiri Torkmani: investigation, writing – original draft; Seyed Shahram Khalilinezhad: investigation, formal analysis, writing – review and editing, supervision; Saeed Ghorashi: writing – review and editing.
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Conflict of interest: The authors state no conflict of interest.
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Data availability statement: All data generated or analyzed during this study are included in this published article.
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