HPAM nanofluids

Fuel 252 (2019) 622–634

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Full Length Article

Heavy oil recovery by surface modified silica nanoparticle/HPAM nanofluids ⁎

T

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Laura M. Corredor1, Ehsan Aliabadian1, Maen Husein , Zhangxin Chen, Brij Maini , ⁎ Uttandaraman Sundararaj Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr NW, Calgary T2N 1N4, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: Linear and nonlinear rheology Enhanced oil recovery (EOR) Flow in porous media Modified nanosilica particles Polymer flooding Hydrolyzed polyacrylamide

Recent studies showed that the presence of dispersed nanoparticles (NPs) can increase the efficiency of polymer flooding. The network structure of polymer/NP hybrid dispersion has a significant impact on oil recovery. In this work, the surface of SiO2 NPs was modified by chemical grafting of octyltriethoxysilane (SiO2-OTES), oleic acid (SiO2-OAA) and stearic acid (SiO2-SAA) in an attempt to stimulate higher degree of interaction with partially hydrolyzed polyacrylamide (HPAM). Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy were employed to characterize the modified silica NPs. In addition, ζ-potential, cryo-scanning electron microscopy (cryo-SEM), and linear and nonlinear rheology were used to evaluate the characteristics of the hybrid dispersion of HPAM/unmodified and modified SiO2 NPs. ζ-potential measurements showed that the addition of HPAM improved the colloidal stability of the modified and unmodified SiO2 NPs dispersed in deionized (DI) water. Moreover, the addition of HPAM reduced the size of the NP aggregates by effective steric repulsion. Small and large shear oscillatory deformation results showed that SiO2-OTES NPs improved the HPAM network significantly. Nevertheless, HPAM/NPs network formed with all the different SiO2 NPs severely decreased the intra-cycle shear-thickening behavior of the polymer. Lastly, the addition of 0.2 wt% SiO2-OTES NPs to the HPAM solution increased the ultimate oil recovery from 71.4% to 75.7% OOIP.

1. Introduction Two classes of polymers are primarily used in polymer flooding; namely synthetic polymers and biopolymers [1]. Partially hydrolyzed polyacrylamide (HPAM) is the most commonly used synthetic polymer to date [2]. HPAM solutions often suffer from viscosity loss due to harsh reservoir conditions; including high temperature, high pressure, and presence of chemical substances such as divalent cations and clay [3]. Xanthan gum (XG) is the most widely used biopolymer [4]. XG has been proposed as an alternative to HPAM due to its excellent tolerance to mechanical shearing and high salinity and temperature. Nevertheless, XG use is relatively limited compared with HPAM due to its higher cost, higher susceptibility to biodegradation and lower availability [5–7]. Recent studies revealed higher polymer flooding performance upon adding silica nanoparticles (NPs) to polyacrylamide solutions for medium and heavy oil recovery [8–12]. The best performance of these hybrid dispersions is obtained when the NPs are uniformly dispersed in the polymer solution [13,14]. Higher extent of dispersion can be

achieved by altering the silanol groups on the surface of the SiO2 NPs via attaching an organic functionality, either by physical association or chemical bonding. The principle of the physical modification method is the preferential adsorption of a polar group of a macromolecule on the silica surface through hydrogen-bonding or electrostatic interaction. SiO2 NPs have been modified by physical methods with fatty acids such as stearic acid [15], and oleic acid [16,17], and with cationic salts such as cetyltrimethylammonium bromide [18]. Modification of the surface by chemical reactions has received more attention, since it typically contributes to much stronger interaction between the modifiers and the nanosilica particles. Chemical methods involve modification either by grafting polymers or modifying agents on the nanosilica surface [19,20]. The resultant nanocomposites have been extensively studied for applications such as adhesives, biomaterials, protective coatings, microelectronics and proton exchange membranes [21]. However, their application for EOR processes has been limited. Ponnapati et al. [22] synthesized SiO2-ethylene oxide-based polymer/NPs hybrids using a “grafting from” method. They reported

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Corresponding authors. E-mail addresses: [email protected] (M. Husein), [email protected] (B. Maini), [email protected] (U. Sundararaj). 1 L.C. and E.A. made equal contribution to this work. https://doi.org/10.1016/j.fuel.2019.04.145 Received 13 February 2019; Received in revised form 22 April 2019; Accepted 26 April 2019 Available online 06 May 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature ASNP

DSC EOR FTIR HPAM KBr LAOS LVR MW OA OOIP OTES PV PF

S Intra-cycle strain-stiffening ratio SA Stearic acid SAOS Small amplitude oscillatory shear SEM Scanning electron microscopy SiO2-OAA Silica modified with oleic acid-Method A SiO2-OTES Silica modified with Octyltriethoxysilane SiO2-SAA Silica modified with stearic acid-Method A T Intra-cycle shear-thickening ratio WF Waterflooding Storage modulus G' G' Loss modulus ' GM Minimum-strain modulus GL' Large-strain modulus ω Angular frequency γ Strain ηM' Minimum-strain rate viscosity ηL' Large-strain rate viscosity

zwitterionic poly[2-methacryloyloxyethylphosphorylcholine (MPC) – co – divinylbenzene (DVB)] shell-coated silica nanoparticles Differential scanning calorimetry Enhanced oil recovery Fourier transform infrared Hydrolyzed polyacrylamide Potassium Bromide Large amplitude oscillatory shear Linear viscoelastic region Molecular weight Oleic acid Original oil in place Octyltriethoxysilane Pore Volume Polymer flooding

wide range of flow conditions can be simulated. LAOS has been applied in polymer nanocomposites [56,57], polymer solutions [24,58], and colloidal gels [59,60] to link nonlinear viscoelastic behavior to microstructural evolution under nonlinear conditions. In the present work, three different fields (chemistry, rheology, and petroleum science) have been merged to show how to increase oil recovery, which distinguishes this study from other works in the literature. In phase one, bare SiO2 NPs were chemically modified with carboxylic acids (SiO2-SAA and SiO2-OAA) and silane (SiO2-OTES) to generate three differently modified SiO2 NPs. By modifying the SiO2 NPs surface, we expected to (i) increase the hydrophobicity of the NPs and, hence, enhance their dispersivity into the polymer solution; and (ii) tailor the interaction between the treated NPs and the polymer to tailor the mobility of the polymer solution and, consequently, increase the oil recovery. In phase two, the network structure of the hybrid dispersions of HPAM/SiO2 NPs was studied using rheology. The effects of adding modified SiO2 NPs on the linear and nonlinear viscoelastic properties were analyzed using frequency sweep test and intra-cycle shear-thickening behavior, respectively. In phase three, these different hybrid dispersions were injected into a linear sand-pack to evaluate their effectiveness in tertiary recovery. In our previous work [61], we studied the flow curves of the SiO2-SAA, SiO2-OAA and SiO2-OTES HPAM/XG hybrid dispersions and developed a model based on a multilayer perceptron (MLP) neural network to predict the viscosity of the dispersions. However, that study did not include the analysis of the linear and nonlinear viscoelastic properties of the dispersions, or their performance in displacing oil. To the best of our knowledge, there is no work in the literature that systematically synthesized the modified SiO2 NPs, evaluated the effect of hydrophobicity of the modified SiO2 NPs on the linear and nonlinear viscoelastic behavior and the network structure of an aqueous HPAM solution, and tested the performance of the HPAM/NP hybrid dispersions for heavy oil recovery as reported here.

that the injection of the 0.5 wt% nanohybrid could mobilize residual oil in the core, yielding 7.9% of the OOIP. Subsequent injection of brine yielded an additional 11% of the OOIP. The enhanced performance of the modified NPs resulted from polymer-grafted NPs clogging some water-occupied pore throats, which increased the local pressure, and hence mobilized the trapped oil in other pores. In another work, Choi et al. [23] studied nanofluid-enhanced oil recovery using hydrophobic zwitterionic poly[2-methacryloyloxyethylphosphorylcholine (MPC) – co – divinylbenzene (DVB)] shell-coated silica nanoparticles (ASNP). The results showed that both viscous and storage modulus of ASNP nanofluid were approximately 2 orders of magnitude higher than that of the bare silica nanofluid. The core flooding experiments showed that the oil recovery was 74.1%, which was higher than water injection (68.9%) and unmodified NP injection (72.7%). In our recent works [24,25], we showed that the slightly hydrophobic fumed SiO2 NPs (AEROSIL R816) are more efficient at forming a stronger network structure with HPAM molecules than the completely hydrophilic fumed SiO2 NPs (AEROSIL 300). Moreover, we showed that the surface chemistry of the NPs plays a key role in the formation of 3-D elastic structure in the polymeric nanofluids, independent of fumed SiO2 NP aggregate size. In porous media with a wide range of pore size distribution and geometry, hybrid dispersions may undergo various flow conditions [26,27]. To optimize the efficiency of the polymer flooding in EOR, understanding the flow behavior of such systems in porous media is of great importance. Rheometry, which deals with the deformation and flow behavior of materials, is a robust technique that provides a valuable opportunity to mimic both small and large deformation in porous media. Rheometry can capture viscoelastic properties that are affected by both inherent properties of a polymer molecule and external parameters. Molecular weight [28], degree of hydrolysis [29,30], and degree of branching [31] are some examples of inherent properties of a polymer molecule. Temperature [25,32,33], salt content [25,34–36], and shear/extensional deformation [3,37,38] are examples of external parameters that can significantly impact the viscoelastic behavior of such systems. Small amplitude oscillatory shear (SAOS) flow [3,39–42], flow curve [43–45], and extensional (shear-free) tests [46,47] are common methods used to investigate the network structure of polymer solutions. However, the applicability of these techniques is restricted due to some limitations like flow instabilities [48], shear inhomogeneity [49], and low viscosity of solution systems. Recently, large amplitude oscillatory shear (LAOS) test has been employed to assess the viscoelastic properties of different polymer solutions [50–55]. In LAOS, the input strain amplitude and frequency can be controlled independently and, hence, a

2. Materials and methods 2.1. Materials Fumed silica powder (7 nm) was obtained from Sigma-Aldrich (USA). The chemicals used for the surface modification of the SiO2 NPs were oleic acid (C18H34O2, 90%), stearic acid (C18H36O2, 95%), and octyl(triethoxy)silane (OTES, ≥97.5%) which were obtained from Sigma-Aldrich (USA). The solvents used in the reactions were: ethanol (EtOH, 99%) and hydrochloric acid (ACS reagent, 37%), both obtained from Sigma-Aldrich (USA); and cyclohexane (99.5%) and ammonium 623

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measurement, the samples were pretreated by ultrasonication for 5 min. A digital pH meter (model AB 15 plus, Fisher Scientific, USA) was used to measure the pH of the samples at 20 °C. The uncertainty of the pH meter was < 0.05 of the reported value.

hydroxide (28–30 wt% solutions of NH3 in water), which were obtained from Fisher Scientific (USA). The detailed synthesis of the surface modified SiO2 NPs used in this work has been reported previously by the authors [61]. n-Propanol (purity ≥ 99.5%, VWR, Canada) was used to determine the hydrophobicity of the NPs. The displacement tests were performed with Flopaam™ 3630 s (HPAM, hydrolysis degree 25–35%, MW ≈ 20 × 106 Dalton) obtained from SNF Floerger (USA). All chemicals were used as received.

5.2. Cryo-SEM images An FEI Quanta FEG 250 SEM equipped with a Gatan Alto 2500 cryostage and xTm version 4.1.12.2162 software was used for the cryo-SEM analysis. A small amount of the solution was loaded and frozen in a nitrogen slush for sample preparation. Then, the surface of the sample was coated with a layer of gold (around 10 nm). The images were obtained using either of secondary electron or backscattered electron detectors at an accelerating voltage of 5 to 10 kV. For cryo-SEM, 3–5 different locations were captured for each sample.

3. Nanoparticle characterization A Fourier transform infrared spectrometer, FTIR (model IRaffinity1s, Shimadzu, Japan) was used to analyze the SiO2 NPs. The background spectrum was obtained with dry KBr, and each spectrum was recorded over a range of wavelength of 4000–400 cm−1. Thermogravimetric analysis/differential scanning calorimetry (TGA/ DSC) measurements were performed by heating 6 mg sample of modified/unmodified NPs from 20 to 800 °C at a heating rate of 10 °C/min under air atmosphere (SDT Q600, TA Instruments, Inc., New Castle, DE). For FTIR and TGA/DSC, every sample was tested three times for reproducibility. The expected coupling mechanism between the silica surface and silanes is represented in Fig. 1. The hydrophobicity of the modified silica NPs was determined by using a dispersion stability test. For this procedure, 0.1 g of each NP type were dispersed in different mixtures of propanol/DI water (10 g) and ultrasonicated for 1 h. After that, the samples were left to settle for 10 min, and the amount of precipitate was determined for each propanol/DI water mixture.

5.3. Rheological characterization An Anton Paar MCR302 rheometer with a coaxial cylinder geometry (measuring bob and measuring cup had radii of 13.329 mm OD and 14.463 mm ID, respectively) was used to measure the rheological properties of the hybrid dispersions. Rheoplus/32 V3.62 software was connected to the rheometer to record the measurements. All the rheological measurements were done at a fixed temperature (T = 35 °C). To assure all systems were in the linear viscoelastic region for frequency sweep measurements, a strain amplitude equal to 1% was selected (based on strain sweep results). In the frequency sweep test, the angular frequency varied between 0.1 and 20 rad/s. Strain amplitude sweep test in which strain amplitude varied between 1% and 1000%, was designed to evaluate the nonlinear viscoelasticity of the hybrid dispersions. The angular frequency was set to 0.5 rad/s, to prevent any flow instability, torque overload and wall-slip effects.

4. Hybrid dispersion preparation Four different concentrations of unmodified and modified SiO2 NPs (1.0, 2.0, 3.0 and 4.0 wt%) were used to prepare hybrid dispersions. At first, the SiO2 NPs were dispersed in DI water and ultrasonicated for 1 h. Then, HPAM powder was added at 0.4 wt% to the aqueous dispersions under gentle stirring for 48 h.

5.4. Flooding experiments and set up Fig. 2 depicts a schematic of the flooding setup. It consisted of a pump, tubing, two transfer vessels, pressure transducer, sand-pack holder, graduated cylinders, and back pressure regulator. The sandpack characteristics and the injected fluid properties are listed in Table 1. For the flooding experiments, each sand-pack was fully saturated with water by imbibition method. The absolute permeability was calculated using Darcy’s law. For this procedure, water was injected at different flow rates and the corresponding pressure drop was recorded. The drainage process was carried out by injecting 2 PV of oil. After that, 0.5 PV of water were injected followed by 0.5 PV of HPAM/SiO2 NPs

5. Hybrid dispersion characterization 5.1. ζ-potential measurements ζ-potential values were measured at 20 °C with a Zetasizer Nano ZS unit (Malvern Instruments Ltd, UK) with uncertainty of ± 1 to 6% of the reported value. 35 specimens were tested in the ζ-potential measurements (20 samples + 15 reproducibility tests). Before every

Fig. 1. Reaction route of surface modification on silica particles with (A) SA and OA, and (B) OTES [62]. 624

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Fig. 2. Sand-pack flooding set up.

3400 cm−1 which corresponded to free silanol groups on the silica surface [64]. The bonding of the OA and SA molecules to SiO2 NPs was confirmed by the asymmetric and symmetric (C–H) stretching vibrations of –CH3 or –CH2 groups observed at ∼2851 cm−1 and the asymmetric and symmetric stretching vibrations of COO– situated at ∼1536 cm−1 and ∼1446 cm−1, respectively [65] (Fig. 4a and b).

Table 1 The sand-pack specifications and properties of the injected fluid. Properties

Values

Core holder length Core holder inner diameter Particle size Sand-pack porosity Sand-pack permeability Connate water saturation Pore volume Oil viscosity (T = 25 °C) Temperature

30.4 cm 2.54 cm 0.0105–0.0149 cm 35–40% 5–6.5 Darcy 3.0 ± 0.2% 50 ± 1.5 cm3 940 ± 5 cP T = 23–25 °C

6.1.2. TGA measurements The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of unmodified and modified silica NPs are shown in Fig. 5. The TGA profile of unmodified SiO2 NPs can be divided

Transmittance, %

dispersion. Finally, 2 PV of water were injected as a post-flush until the oil cut at the production outlet was less than 1%. All the fluids were injected at 0.1 ml/min. 6. Results and discussion 6.1. Nanoparticle characterization 6.1.1. FTIR measurements The FTIR spectra of unmodified and modified silica NPs after normalization of the peak area are shown in Figs. 3 and 4. The normalization for the FTIR spectra was performed by using peaks derived from bonds which are not involved in the modification reactions; namely peaks between 900 and 1300 cm−1. The bonding of the OTES molecules to SiO2 NPs was confirmed by the peaks at 2926 cm−1 and 2858 cm−1 which were ascribed to very intense asymmetric and symmetric stretching vibrations of C–H bonds of the CH2 groups in the octyl substituent [63]. Additionally, the peak at 960 cm−1 which was ascribed to Si–O–Si bond, and the disappearance of the peak at

4000

3200

2800

2200

1600

1000

-1

Wave number, cm SiO2

SiO2-OTES

OTES

Fig. 3. IR spectrum of SiO2, SiO2-OTES, and OTES. 625

400

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Transmittance, %

Transmittance, %

L.M. Corredor, et al.

(a) 4000

(b) 3200

2800

1000

1600

2200

4000

400

SiO2-OAA

2800

2200

1600

1000

400

Wave number, cm

Wave number, cm SiO2

3200

-1

-1

SiO2

OA

SiO2-SAA

SA

Fig. 4. IR spectrum of (a) SiO2-OAA, and (b) SiO2-SAA.

90

120

7. Hybrid dispersion characterization

80

7.1. ζ-potential measurements

Weight, %

80 40

70 60

0

50

Heat flow, mW

100

The ζ-potential values of the different NP dispersions are provided in Table 2. Typically, NP dispersions with ζ-potential values lower than −30 mV or larger than +30 mV are stable [69]. SiO2-OAA (−32 mV) and SiO2-OTES (−29.6 mV) NPs display better stability than SiO2-SAA (−26.1 mV) NPs in water, despite the borderline zeta potentials, due to the formation of a larger hydration layer. The hydration layer results from the hydrogen bonding between the silanol groups on the NPs with the water molecules. This conclusion is supported by the TGA results which showed that the weight loss associated to water for SiO2-SAA was only 6.34%, whereas for SiO2-OAA and SiO2-OTES, it was 10.99% and 31.83%, respectively. Examining Table 2, it is concluded that all the hybrid dispersions prepared in this study are stabilized chiefly by steric repulsion of the adsorbed polymeric chains. The adsorption of HPAM on the surface occurs via hydrophobic interactions between the polymer backbone (–CH2–CH2–) and the R-CH3 chain of the modifiers or by hydrogen bonding between the oxygen or nitrogen from HPAM and the hydrogen from the SiO2 surface (SiO–H⋯N–H or SiO–H⋯O–CNH2) or between the hydrogen from the HPAM and the oxygen of the SiO2 surface (SiO⋯HNH–CO–C) [70]. The effect of polymer adsorption onto dispersed particles in polymer solutions have been described in detail by Kawaguchi [71]. In this study, the effect of pH on the configuration of the adsorbed polymer molecules on the NP surface was not considered because the pH values of the hybrids did not change significantly (between 6.9 and 8.18).

-40

40 Exo up

30 0

-80 200

400

600

800

o

Temperature, C SiO2 TGA

SiO2-OAA TGA

SiO2-SAA TGA

SiO2-OTES TGA

SiO2 DSC

SiO2-OAA DSC

SiO2-SAA DSC

SiO2-OTES DSC

Fig. 5. TG/DSC profiles of SiO2, SiO2-OTES, SiO2-SAA, and SiO2-OAA.

into two regions, desorption of physisorbed water completed up to 400 °C and dehydroxylation process of adjacent –OH groups between 400 °C and 800 °C [66]. The percentage of weight loss related to each region was 25.7% and 6.0%, respectively. A three-step weight loss was identified in the TGA profiles of all modified NPs. The first step occurred below 200 °C which was attributed to the evaporation of physically adsorbed water and volatile solvents. The weight loss in this region was 6.34%, 10.99% and 31.83% for SiO2-SAA, SiO2-OAA and SiO2-OTES NPs, respectively. The second step was attributed to the thermal decomposition of each modifier grafted onto the surface of the NPs. For SiO2-OAA, the second step was observed from 220 °C to 330 °C (Fig. 5, purple line) [67] whereas for SiO2-SAA the second step was observed from 210 °C to 330 °C (Fig. 5, blue line) [68] and for SiO2-OTES from 200 °C to 370 °C [23] (Fig. 5, green line). The weight loss in this region was 11.7%, 13% and 15.5% for SiO2-OAA, SiO2-SAA and SiO2-OTES NPs, respectively. In the final step, the weight loss was attributed to the combustion of the degraded product. Fig. 6 shows the NPs dispersed in different mixtures of DI water/ propanol (W/A). It was observed that the precipitate of SiO2-SAA NPs and SiO2-OAA NPs increased as the concentration of DI water in the mixture increased, while the precipitate of SiO2-OTES NPs reduced. This test and the water weight losses in the TGA analysis suggests that the hydrophobicity of modified SiO2 can be ranked as follows: SiO2SAA > SiO2-OAA > SiO2-OTES.

7.2. Cryo-SEM results Fig. 7 depicts cryo-SEM images of aqueous unmodified and modified SiO2 dispersions before and after adding HPAM. The size of the unmodified SiO2 NP aggregates in DI water is larger than those of the modified SiO2 NPs (see Fig. 7A-D). This can be attributed to the hydrogen bonding between the –OH groups on the surface of the unmodified SiO2 NPs. The size of the aggregates of the unmodified NPs decreased after the surface was modified with OTES because the repulsive forces between OTES molecules prevent the aggregation of the NPs (see Fig. 7A (1) and B (1)). Smaller aggregates are also observed for SiO2-SAA and SiO2-OAA NPs. Their high hydrophobicity leads to the formation of larger aggregates which precipitated out, despite the ultrasonication, and, hence, could not be detected by the cryo-SEM images (see Fig. 7C (1-2) and D (1-2)). When HPAM was added to the aqueous dispersions, some polymer chains were adsorbed onto the SiO2 surface through hydrogen bonding or hydrophobic interaction. For the SiO2-SAA, SiO2-OAA and SiO2-OTES hybrid dispersions, the repulsive electrostatic forces between coated NP-coated NP and coated NP626

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Fig. 6. Dispersion stability test of 1.0 wt% (A) SiO2-SAA, (B) SiO2-OAA, and (C) SiO2-OTES in water/propanol (W/A) mixtures.

[72]. We believe the role of the depletion attraction forces is minimal for the modified NPs, especially since for the modified NP hybrid dispersions the interaction with the HPAM chains prevented the flocculation of the coated NPs, as evident from the different state of aggregration of silica NP before and after adding HPAM molecules seen in Fig. 7, hence minimizing the role of the depletion attractive forces. The adsorbed polymeric chains positively influenced the dispersivity of the NPs, reducing the size of the aggregates by effective steric repulsion (see Fig. 7A-D (3-4)). Some of the large aggregates formed by SiO2-SAA and SiO2-OAA NPs were captured in the cryo-SEM images after the addition of HPAM (see Fig. 7C (4) and D (4)).

Table 2 ζ-potential and pH values of aqueous and hybrid dispersions of the modified and unmodified SiO2 NPs at 20 °C. NP type

HPAM concentration, ppm

NP concentration, wt%

ζ-Potential @ 20 °C

pH @ 20 °C

SiO2

4000 4000 4000 4000 0

1.0 2.0 3.0 4.0 2.0

−85.4 −84.5 −71.4 −66.3 −15.3

7.48 7.26 7.14 6.9 6

SiO2-OTES

4000 4000 4000 4000 0

1.0 2.0 3.0 4.0 2.0

−82 −76.7 −73.3 −63.2 −29.6

8.1 7.86 7.73 7.69 5.81

4000 4000 4000 4000 0

1.0 2.0 3.0 4.0 2.0

−75.3 −70.8 −67.3 −64.8 −32

8.18 8.11 8.10 8.0 5.29

4000 4000 4000 4000 0

1.0 2.0 3.0 4.0 2.0

−68 −63 −59.8 −56.1 −26.1

7.82 7.14 6.95 6.84 4.99

SiO2-OAA

SiO2-SAA

8. Rheological characterization 8.1. Linear viscoelastic properties Rheometry provides a distinctive, sophisticated tool to study the network structure of flocculated colloidal gels, yielding significant information on the nanofiller structural characteristics in a polymer solution [24,70]. The linear viscoelastic properties are affected by the network structure of polymer chains in the polymer solution systems, and this network structure can be captured in frequency sweep test [38,40,73]. The storage modulus (G') and the loss modulus (G'') are the two main parameters that represent the linear viscoelastic properties. G' provides information on the rigidity of the network structure, while G″ quantifies how readily an applied stress to the structure is relaxed or dissipated [74,75]. Fig. 8 shows G' and G″ of hybrid dispersions of different unmodified and modified silica NPs and HPAM under smallamplitude oscillatory shear (strain amplitude γ0 = 0.1%) for an angular frequency range from 0.1 to 10 rad/s at 35 °C. It should be noted that in log-log scale, the errors are within the data point markers. As can be seen in Fig. 8b and c, increasing the content of SiO2-SAA and SiO2-OAA NPs damages the network structure of HPAM molecules and decreases the storage modulus of hybrid dispersions, particularly in the low-frequency region. In other words, SAA-based and OAA-based

polymer (both negatively charged) contribute to disperse the NPs into the polymer solution and to keep the polymer in an extended configuration. However, the electrostatic repulsive forces do not play a major role in the adsorption of the polymer onto the coated NP surface. The van der Waals forces (hydrogen bonding) between NP-NP and NPpolymer are reduced following modifying the NP surface with OAA, SAA and OTES because the number of –OH groups available on the NP surface is reduced. In the meantime, the role of the –OH groups in polymer adsorption is not significant. Otherwise, these forces are significant in the absence of steric effects, i.e. for unmodified silica NPs 627

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Fig. 7. Cryo-SEM images of A) unmodified SiO2, B) SiO2-OTES, C) SiO2-OAA, and D) SiO2-SAA NPs dispersed in DI water or 4000 ppm of HPAM solution at low (1–3) and high magnification (2–4).

hydrophobic interaction between the backbone of HPAM and the hydrophobic functional group on SiO2-OTES NPs is the main driving mechanism to bring HPAM molecules to the vicinity of SiO2-OTES NPs. Due to large occupied space of silane on the surface SiO2-OTES NPs and the strength of hydrophobic affinity and electrostatic repulsion between HPAM molecules and SiO2-OTES NPs, the contact of HPAM molecules and SiO2-OTES NPs occurs with a minimal number of segments attached to the surface of SiO2-OTES NPs. This phenomenon increases the probability of creating configuration (A) in Fig. 9 for OTES-hybrid dispersions. In such configuration, polymer chains are moving and protruding freely into the aqueous phase with high probability for bridging between SiO2-OTES NPs and creating a 3D network formation. Frequency sweep results also confirm that by incorporating OTES NPs, the G' of hybrid dispersion increases and a stronger network is formed. Similarly, in SAA-based and OAA-based hybrid dispersions, hydrophobic interaction between the backbone of HPAM and the hydrophobic functional group of modified silica NPs control the adsorption of HPAM molecules. However, more hydrophobic affinity of SiO2-SAA and SiO2-OAA NPs compared to SiO2-OTES NPs may increase the number of HPAM segments attached to the SAA and OAA silica NPs. As a result,

hybrid dispersions have a weaker network structure compared to the HPAM solution. Conversely, incorporating unmodified and SiO2-OTES NPs improve the storage and loss moduli of HPAM solution, particularly at low-frequencies (see Fig. 8a and d). The reason for such differences in the network strength of hybrid dispersions originates from the surface chemistry of unmodified and modified silica NPs. Such different behavior of unmodified and modified silica NPs can be related to their dispersion state in HPAM solution and the configuration of adsorbed HPAM molecule on the surface of silica NPs. Firstly, the dispersion of unmodified and modified silica NPs with and without HPAM molecules was explained in the cryo-SEM image discussion. Secondly, different configurations of adsorbed polymer molecules onto the surface of carboxylic acid modified surfaces (SAA and OAA) and silane modified surface (OTES) are expected to have a significant effect in addition to the dispersion state [76]. Possible configurations of adsorbed HPAM molecule on the surface of modified NPs are depicted in Fig. 9. We showed that the modified silica NPs can be ranked based on their hydrophobicity as follows: SiO2-SAA > SiO2-OAA > SiO2-OTES. For SiO2-OTES NPs, due to the hydrophobicity of the modified chain,

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Fig. 8. Storage (G' , closed symbols) and loss (G'' , open symbols) moduli of the hybrid dispersions at 0.0, 1.0, 2.0, and 4.0 wt% of (a) unmodified and (b) SiO2-SAA NPs, (c) SiO2-OAA NPs, and (d) SiO2-OTES modified silica NPs. HPAM = 4000 ppm.

configuration (B) in Fig. 9 is predicted to be more likely in SAA-based and OAA-based hybrid dispersions. Any standing configuration for SAAbased and OAA-based hybrid dispersions is not consistent with the frequency sweep results. If the HPAM molecule can stand on the surface of SiO2-SAA or SiO2-OAA NPs, by increasing the content of SiO2-SAA or SiO2-OAA NPs, the probability of polymer bridging increases, and larger storage modulus should be expected, which is not the case for SAA-based and OAA-based hybrid dispersions (see storage modulus in Fig. 8b and 8c). Moreover, due to the unsaturated nature of the SiO2OAA NPs, the hydrophobic affinity of SiO2-OAA NPs is less than SiO2SAA NPs and as a result, the severity of disruption of HPAM network by the addition of OAA modified silica NPs is less than the addition of

Fig. 9. Possible configuration of HPAM molecules adsorbed at NP/water interface (A) random coil (high molecular weight polymers), (B) flat multiple site attachments (strong adsorption).

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solution. In contrast, for OTES-based hybrid dispersions, by increasing the concentration of SiO2-OTES NPs, the plateau storage modulus increases fivefold to 40 Pa for HPAM/4.0 wt% of SiO2-OTES NP solution (see Fig. 10d). Overall, the strain sweep results confirm that SiO2-OTES NPs improve the viscoelastic properties of HPAM solution significantly in the linear region.

SiO2-SAA NPs. 8.2. Nonlinear viscoelastic properties Due to converging and diverging pores in porous media, hybrid dispersions may experience large deformation, which can be extensional or shear. Thus, from a practical point of view, the material response will likely fall into a nonlinear regime of deformation. In this work, large shear deformation is studied to characterize the flow behavior of the hybrid dispersions. Fig. 10 depicts the G' of various hybrid dispersions at a constant frequency (=0.5 rad/s) and various strain amplitudes (1–1000%) (The loss modulus (G″) plot in the strain sweep test is included in the Supplemental information, Fig. S1). The errors in modulus for the strain sweep test are within the data point markers. The plateau storage modulus (modulus in linear viscoelastic (LVR) region) and the critical strain amplitude (the strain amplitude at which G' starts decreasing and reaches 90% of its original value) are the two main parameters which can be extracted from strain sweep test. In LVR, the applied strain amplitude is not large enough to disrupt the at-rest structures (i.e. storage modulus is independent of applied strain amplitude). However, by further increase in strain amplitude, the non-LVR is approached (storage modulus changes with applied strain amplitude) (see Fig. 10). As expected from the frequency sweep test, by increasing the content of SiO2-OAA NPs, the plateau G' did not change significantly compared with HPAM solution (Fig. 10c). In addition, the critical strain amplitude for the OAA-based hybrid dispersion did not shift significantly relative to HPAM solution. However, by increasing the content of SiO2-SAA NPs (see Fig. 10b), the plateau for G' decreases to 0.8 Pa for HPAM/4.0 wt% of SiO2-SAA NP solution from the plateau G' for HPAM solution, which is 8 Pa. Moreover, the critical strain amplitude reaches approximately 8% for HPAM/4.0 wt% of SiO2-SAA NP solution (the critical strain amplitude for HPAM solution is about 60%). SiO2-SAA NPs aggregates can adsorb HPAM chains and extract them from the aqueous media and thus prevent them from creating a rigid network, resulting in lower plateau modulus compared with HPAM 10 2

8.3. Intra-cycle shear-thickening behavior It is desirable to find a clear physical meaning for the measured storage and loss moduli in the nonlinear region. In the linear viscoelastic region, the output stress signals are simple sinusoidal functions. However, by increasing the strain amplitude and shifting into the nonlinear region, shear stress cannot be described simply by sinusoidal waves. The mathematical details of shear stress (τ) response in the linear and the nonlinear regions and the stress decomposition method can be found in our previous works [24,25,77]. When we are in one single cycle of oscillations at a constant strain amplitude, elastic and viscous local responses of a material at minimum and maximum instantaneous strains and strain-rates, which are denoted ' , GL' , ηM' , ηL' , respectively, can be very helpful to characterize the byGM nonlinear viscoelastic behavior and link the network structure to the rheological data. It should be noted here that all local nonlinear properties are compiled directly by the rheometer software (Anton-Paar Rheoplus/32 V3.62). To interpret local nonlinear properties quantitatively, shear-thickening and strain-stiffening ratios were defined as follows [78].

S=

T=

GL'

(1)

η' L − η' M . η' L

(2)

10 2

(a)

(b)

G'>Pa]

10 1

10 0

10 0

G' HPAM 3630 HPAM 3630/1.0 wt%-Unmod-SiO2 HPAM 3630/2.0 wt%-Unmod-SiO2 HPAM 3630/4.0 wt%-Unmod-SiO2

10 -1

10 -2 0 10

1

10

2

Ȗ[%]

10

10 2

10 -1

G'

10 -2 0 10

3

10

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-SAA HPAM 3630/2.0 wt%-SiO2-SAA HPAM 3630/4.0 wt%-SiO2-SAA 1

10

3

2

Ȗ[%]

10

10

10 2

(c)

10 1

(d)

10 1

G'>Pa]

G'>Pa]

,

Positive S and T represent intra-cycle strain-stiffening and intracycle shear-thickening behaviors, respectively, and negative S and T indicate intra-cycle strain-softening and intra-cycle shear-thinning

10 1

G'>Pa]

' GL' − GM

10 0

10 0 G'

G'

10 -1

10 -2 0 10

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-OAA HPAM 3630/2.0 wt%-SiO2-OAA HPAM 3630/4.0 wt%-SiO2-OAA 1

10

10 -1

2

Ȗ[%]

10

3

10

10 -2 0 10

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-OTES HPAM 3630/2.0 wt%-SiO2-OTES HPAM 3630/4.0 wt%-SiO2-OTES 1

10

2

Ȗ[%]

10

3

10

Fig. 10. Oscillatory strain amplitude sweep response of the hybrid dispersions of HPAM and the different silica NPs at a fixed angular frequency (0.5 rad/s). 4000 ppm of HPAM solution with (a) unmodified silica NPs, (b) SiO2-SAA NPs, (c) SiO2-OAA NPs, and (d) SiO2-OTES NPs. 630

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unmodified silica NPs leads to a similar effect on the behavior of the hybrid dispersions at higher strain amplitudes, well above the yield point of all systems (where the strain amplitude is much larger than the critical strain amplitude). As an example, for HPAM 3630/unmodified SiO2, the shear-thinning (T < 0) behavior starts at γ0 > 200% (see Fig. 11a, blue line). By increasing the weight fraction of the different types of modified and unmodified silica NPs, the value of the strain amplitude at which T changes sign to negative values, decreases. All hybrid dispersions exhibit an intra-cycle shear-thinning viscoelastic behavior (T < 0) at higher strain amplitudes. Such behavior could be explained in conjunction with the wide-spread bond breakup in the fumed silica NP flocculated structures, the orientation of the rigid polymer chains parallel to the flow direction and slippage at the NPpolymer interface [85–89]. Overall, introducing the different SiO2 NPs decreases the shear-thickening behavior of the hybrid dispersions, decreases the critical strain amplitude, and enhances the shear-thinning behavior above the yield point of all systems.

responses, respectively. In this study, only intra-cycle shear-thickening ratio of the hybrid dispersions are presented in Fig. 11 (the strain-stiffening ratios are included in the Supplemental information, Fig. S2). As can be seen in Fig. 11, for HPAM solution without silica NPs, T value is approximately zero up to γ ≈ 60%, then T reaches a positive maximum value at γ ≈ 250% followed by a decreasing trend back to approximately zero (see black line in Fig. 11a). The positive T reveals the occurrence of intra-cycle shear-thickening response. Generally speaking, in the low-frequency region, the nonlinear viscoelastic behavior is mainly described using a mechanism controlled by strain-rate-induced effects [79]. T > 0 could be related to the rate of flow-enhanced formation of network bonds and the rate of bond breakage for HPAM solution when deformation is applied in a single cycle. When the former is greater than the latter in one cycle, intra-cycle shear-thickening behavior occurs. Dupuis et al. [80] also studied the rheological properties of solutions of high molecular weight partially hydrolyzed polyacrylamide. They suggested that the shear-thickening behavior of such polymers originates from the formation and destruction of aggregates, which are related to intra- and inter-molecular interactions. More details on the shear-thickening behavior can also be found in Hu and Jaimeson [81], and Bharadwaj et al. [79]. However, by increasing the concentration of SiO2-SAA NPs, the maximum positive value decreases and shear-thinning (T < 0) behavior arises (see Fig. 11b). For HPAM/4.0 wt% of SiO2-SAA NP, the transition to nonlinear region starts at lower strain amplitudes (γ = 10%), and the maximum of T disappears. The same intra-cycle shear thickening behavior is observed for the addition of other silica NPs (see Fig. 11a, c, and d). The shear-thickening ratio (T ) decreased as unmodified and modified silica NP concentration increased. This phenomenon could be attributed to immobilization of polymer chains at the interface which makes a portion of the chains unavailable for deformation-induced structuring in the polymer phase [82–84]. Such behavior is observed for the whole range of concentration of all hybrid dispersions. Surprisingly, introducing the different types of modified and

80

8.4. Core flooding results In the last part, the performance of all hybrid dispersions of HPAM/ NPs for displacement of heavy oil was evaluated. Figs. 12 and 13 show the pressure difference of sand-pack and the cumulative oil recovery versus pore volume of the injected fluid at 25 °C, respectively. The pressure difference drops during secondary water flooding (see WF#2 in Fig. 12) and remains low and constant around 2.5 psi. During the injection of the HPAM solution and the HPAM/NP dispersions (see PF in Fig. 12), the pressure difference increased. The pressure differences observed for 0.5 PV of HPAM/1.0 wt% SiO2-SAA and HPAM/1.0 wt% SiO2-OAA dispersions were lower than those of the HPAM solution (0.5 PV) due to their lower viscoelastic properties and higher mobility ratio. The details of viscoelastic property reduction of SiO2-SAA and SiO2OAA NPs was explained earlier. In contrast, the higher-pressure differences of the HPAM/1.0 wt% SiO2-OTES dispersions can be related to higher viscoelastic properties, lower mobility ratio, and pore blockage

(a)

60

(b)

T [%]

40 20 0 -20

HPAM 3630 HPAM 3630/1.0 wt%-Unmod-SiO2

-40 -60 -80

10

0

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-SAA

HPAM 3630/2.0 wt%-Unmod-SiO2

HPAM 3630/2.0 wt%-SiO2-SAA

HPAM 3630/4.0 wt%-Unmod-SiO2

HPAM 3630/4.0 wt%-SiO2-SAA

10

1

Ȗ[%]

10

2

80

3

10 10

0

10

1

Ȗ[%]

10

2

(c)

60

10

3

(d)

40

T [%]

20 0 -20

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-OAA

-40 -60 -80

10

0

HPAM 3630 HPAM 3630/1.0 wt%-SiO2-OTES

HPAM 3630/2.0 wt%-SiO2-OAA

HPAM 3630/2.0 wt%-SiO2-OTES

HPAM 3630/4.0 wt%-SiO2-OAA

HPAM 3630/4.0 wt%-SiO2-OTES

10

1

Ȗ[%]

10

2

3

10 10

0

10

1

Ȗ[%]

10

2

10

3

Fig. 11. Intra-cycle shear-thickening ratio of the hybrid dispersion of HPAM and all silica NPs. 4000 ppm of HPAM solution with (a) unmodified silica NPs, (b) SAA NPs, (c) OAA NPs, and (d) OTES NPs. 631

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PF

20 10 0

0.5

1

1.5

2

2.5

3

Pore Volume

80

WF #2

PF

1

1.5

2

50 40

15

30

10

20

2.5

10

WF #3

0 0.5

1

1.5

2

2.5

3

Pore Volume Fig. 14. Cumulative oil recovery and pressure drop as a function of pore volume of the injected fluids at 25 °C (green line – HPAM/1.0 wt% SiO2-OTES, light purple line – HPAM/0.5 wt% SiO2-OTES, grey line – HPAM/0.2 wt% SiO2OTES). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

pores. By adding NPs, if they can enter the smaller pores, they can block these pores and force water to enter the larger pores and displace the remaining oil and increase the ultimate oil recovery [91]. Regarding the pressure difference and the cumulative oil recovery of HPAM/1.0 wt% SiO2-OTES, it can be concluded that the aggregate size of 1.0 wt% SiO2OTES NPs is not sufficiently small to go to the smaller water-wet pores, block them, and improve oil recovery compared to HPAM solution. To test this hypothesis, two lower concentrations of OTES NPs (0.2 and 0.5 wt%), which should have smaller aggregates, were injected into the sand-pack (Fig. 14). The cumulative oil recovery of the HPAM solution increased to 75.7% and 73.8% of OOIP upon adding 0.2 and 0.5 wt% of SiO2-OTES NPs, respectively. Higher recoveries at lower OTES concentrations suggest better propagation of the NPs inside the sand-pack and blockage of water-wet pores. In general, it can be concluded that the performance of the HPAM/ NPs for EOR processes is mainly related to the size of the NP aggregates and the interaction of the polymer and the NP in the aqueous solution, i.e. the network structure of the HPAM/NP dispersion. The NPs (SiO2OAA and SiO2-SAA NPs) that negatively affected the viscoelastic properties and the network structure of the polymer solution yielded lower oil recovery than that of the HPAM solution and the NPs with positive effect (SiO2-OTES NPs) on the network structure of HPAM solution. The SiO2-OTES NPs system exhibited better dispersion, and was stable in HPAM solution, thus yielding higher oil recovery. It should be mentioned here that the effects of salinity and temperature on the viscoelastic properties and heavy oil recovery of the hybrid dispersions were not assessed in this study.

20

0.5

60 20

0

43.1%

40

0

70

0

71.6% 71.4% 70.9% 62.6% 59.3%

WF #3

60

0

PF

5

Fig. 12. Pressure drop as a function of pore volume of the injected fluids at 25 °C (black line – HPAM, blue line – HPAM/ 1.0 wt% SiO2-SAA, purple line – HPAM/ 1.0 wt% SiO2-OAA, red line – HPAM/ 1.0 wt% SiO2, green line – HPAM/ 1.0 wt% SiO2-OTES). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Oil Recovery (%)

WF #2

25

30

0

80

30

WF #3

Oil Recovery (%)

WF #2

Pressure (psi)

Pressure (psi)

40

3

Pore Volume Fig. 13. Cumulative oil recovery as a function of pore volume of the injected fluids at 25 °C (light blue line – water, black line – HPAM, blue line – HPAM/ 1.0 wt% SiO2-SAA, purple line – HPAM/ 1.0 wt% SiO2-OAA, red line – HPAM/ 1.0 wt% SiO2, green line – HPAM/ 1.0 wt% SiO2-OTES). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of the SiO2-OTES NPs in the sand-pack (to have a better understanding of viscosity change at different shear rates, the flow curves of all hybrid dispersions are included in the Supplemental information, Fig. S3). The pore blockage mechanism of NPs in porous media and its effect on ultimate oil recovery will be discussed later. At the end of the extended water flooding (see WF #3 in Fig. 12), the pressure differences reached their initial value of ∼2.5 psi for the HPAM solution and the HPAM/1.0 wt% unmodified SiO2, HPAM/1.0 wt % SiO2-OAA, and HPAM/1.0 wt% SiO2-OAA dispersions. The higher pressure drop observed for HPAM/1.0 wt% SiO2-OTES dispersions, at the end of the extended water flooding, suggest a reduction in the permeability of the porous medium by virtue of the higher adsorption/ retention of the NPs in the sand-pack [90]. The cumulative oil recovery after water flooding and (0.5 PV) HPAM flooding was 43.1% and 71.4% OOIP, respectively (Fig. 13). It is observed that the addition of 1.0 wt% of unmodified SiO2 and SiO2OTES NPs to the HPAM solution did not change the performance of the polymer flooding, while the addition of 1.0 wt% of SiO2-OAA and SiO2SAA NPs reduced the cumulative oil recovery from 71.4% OOIP (for 0.5 PV HPAM solution) to 59.3% and 62.6% OOIP, respectively. The main reason for lower cumulative oil recovery for water flooding is viscous fingering. Adding polymer molecules increases the viscosity of the injected fluid and decreases the mobility ratio and prevents viscous fingering to a certain extent. It is well documented in the literature that in water-wet systems, water (wetting phase) occupies the smaller pores and oil (non-wetting phase) mainly resides in the larger pores [3]. As a result, during water flooding, water tends to enter smaller pores and bypasses large amount of oil residing in the larger

9. Conclusions By altering the silanols of the surface of SiO2 NPs, three differently modified SiO2 NPs were synthesized and characterized using TGA, DSC, and FTIR. The colloidal stability, dispersion state of the NPs, and the flow behavior of the aqueous solution of HPAM/different SiO2 NPs were investigated using ζ-potential, cryo-SEM, and linear and nonlinear rheology, respectively. Finally, the performance of HPAM/all SiO2 NPs while displacing heavy oil were evaluated in a linear sand-pack set up. In the linear viscoelastic region, it was shown that bare SiO2 NPs chemically modified with silane (SiO2-OTES NPs) improved the viscoelastic properties of HPAM solution compared to carboxylic acids modified NPs (SiO2-SAA and SiO2-OAA). Such improvement originated from better dispersion state of SiO2-OTES NPs and standing configuration of HPAM on the surface of SiO2-OTES NPs, which increased the probability of polymer bridging. However, in the nonlinear region, all silica NPs decreased the intra-cycle shear-thickening behavior and led to severe intra-cycle shear-thinning after yielding of HPAM/ SiO2 NPs 632

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network. The displacement tests showed that adding either SiO2-SAA and SiO2-OAA NPs to HPAM solution, both of which had a negative effect on the HPAM network structure, reduced the oil recovery of the HPAM solution from 71.4% OOIP to 59.3% and 62.6% OOIP, respectively. In contrast, incorporation of 0.2 wt% SiO2-OTES NPs to HPAM solution increased the oil recovery to 75.7% OOIP.

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