Rheological and thermal stability of interpenetrating polymer network hydrogel based on polyacrylamide/hydroxypropyl guar reinforced with graphene oxide for application in oil recovery

2.1 Materials

HPAM, with 25 mol.% degree of hydrolysis, and 8 × 10⁶ Da molecular weight was supplied by SNF Co. GO (research-grade, Few-Layer: 0.43 gr/cc, Purity: 99%) was obtained from United Nanotech Innovation PVT Ltd, India. HPG with a molecular weight of 2 × 10⁶ Da was obtained from Shandong Shengyou Cementing Engineering & Technology Co., Ltd, China. Chromium triacetate was procured from Carlo Erba, Italy. The total dissolved solids (TDS) of saline water is 7616 ppm.

2.2 Preparation of nanocomposite hydrogels

The IPN hydrogel and nanocomposite hydrogels are prepared and schematically shown in Figure 1. First, 0.35 wt.% of HPAM as well as 0.35 wt.% of HPG powder were gradually added to the separate saline water and stirred regularly by a heater magnetic stirrer to obtain clear viscous solutions at room temperature and aged for 24 h. Then various amounts of GO (0.05, 0.1, 0.3, and 0.5 wt.%) were dispersed in saline water for 1 h by a magnetic stirrer to form a stable dispersion. Afterwards, the prepared HPG and HPAM were charged in GO solution respectively, and stirring was continued for another 1 h using an overhead mixer (Heidolph RZR 2020). Then, 1.4 wt.% of the chromium triacetate as a cross-linker was prepared in water and added to the aqueous mixture while stirring gently. The reaction mixture was heated at 185 °F (85 °C) for approximately 2 h to obtain a homogeneous solution. The final samples were dried in a vacuum oven for 24 h at 158 °F (70 °C). The polymer concentration of all samples was 0.7 wt.%.

Figure 1:

The schematic of the preparation procedure of HPAM/HPG/GO semi IPN nanocomposites hydrogel.

2.3 Characterization

3.1 FTIR characterization

FTIR was employed to evaluate the interaction between the IPN matrix and GO. As presented in Figure 2, the FTIR spectrum of GO displayed peaks at 3437, 1720, 1623, 1396, 1224, and 1046 cm⁻¹, corresponding to the O–H hydroxyl group, C=O carbonyl stretching, C=C, O–H deformation vibration, C–OH, and C–O–C (epoxy) stretching, respectively [38, 39]. In the spectrum of IPN hydrogel, the peak at 3449 cm⁻¹ is attributed to the overlap of the O–H bond of the hydroxyl group of HPG and the N–H band of the amide group of HPAM [21]. Besides, C–H stretching at 2825 cm⁻¹ is observed. The peak near 1682 cm⁻¹, is owing to the C=O stretching of the amide group of HPAM. CH₂ scissoring band appears at 1431 cm⁻¹. The peak at 1089 cm⁻¹ is due to CH₂–O–CH₂ stretching vibrations. Meanwhile, the peak around 617 cm⁻¹ is because of the N–H wagging vibration [21]. By comparing the FTIR spectrums of IPN hydrogel, with nanocomposite hydrogels at 0.1 and 0.5 wt.% of GO, it was observed that most of the characteristic peaks of IPN hydrogel shifted to a lower wavenumber for semi IPN nanocomposite hydrogels. The characteristic peak at 3449 cm⁻¹ in HPAM/HPG hydrogel shifted to 3446 and 3445 cm⁻¹ in HPAM/HPG/GO for 0.1 and 0.5 wt.% of GO. It indicates that the interaction between GO and matrix occurs by the N-H bond of HPAM in the IPN hydrogel matrix and O–H deformation vibration of GO. The other peaks appear at 2823 and 2824 cm⁻¹ for IPN nanocomposites at 0.1 and 0.5 wt.% of GO, indicating the stretching vibration of C–H. Moreover, the characteristic peak in IPN hydrogel moved from 1682 to 1664 and 1662 cm⁻¹ for IPN nanocomposites hydrogel at 0.1 and 0.5 wt.% of GO which was related to C=O of the amide group. However, the peak at 1431 cm⁻¹ in IPN hydrogel shifted to 1423 and 1422 cm⁻¹ for IPN nanocomposite hydrogels at 0.1 and 0.5 wt.% of GO, due to C–H bending vibration. Also, there is the C–O–C stretching around 1085 cm⁻¹ for IPN nanocomposites [12, 40]. These results indicate the link between the HPAM/HPG chains and GO through hydrogen bonding [34, 35, 39].

Figure 2:

FTIR spectra of GO, IPN (0, 0.1, and 0.5) wt.% of GO.

3.2 Rheological behavior

Polymer flowability is a significant factor in EOR applications because high viscosity polymers solution cannot flow easily in the reservoir, while low viscosity polymers solution decreases polymer injection efficacy in the EOR [2]. To get an insight on this, the dependency of viscosity on the shear rate for IPN nanocomposite hydrogels at 200 and 240 °F was evaluated. As shown in Figure 3, the viscosity was boosted by adding GO, which was remarkable at 0.1 wt.% of GO, at the low shear rate, indicating the formation of physical interaction between GO and the matrix, which restrained the movement of polymer chains, resulting in a rapid rise in the viscosity [12, 41]. In other words, a large surface area of GO nanosheets can act as a multifunctional cross-linker and interact with polymer chains of the matrix, along with the chromium triacetate which created further cross-linking in the polymer network. This leads to an enhancement in the strength of the network structure of nanocomposite hydrogels [35, 42]. Moreover, nanoparticles tend to form agglomerates because of high interparticle interactions [10, 38]. As a result, a slight raise of viscosity was also observed with increasing GO to 0.5 wt.%. According to Figure 3, the viscosity of all samples diminished with a raise in the shear rate. However, the speed of this reduction is related to the GO contents. The nano samples with the addition of GO show better resistance to the increase of shear rate than neat IPN hydrogel, at both temperatures (200 and 240 °F). In fact, GO nanosheet gave greater strength to the IPNs, due to which, the network of IPN nano hydrogel could further withstand against stretch stress and deformation created by shearing fields [42].

Figure 3:

Viscosity versus shear rate of IPN nanocomposite hydrogels at (A) 200 and (B) 240 °F.

It was also found that all samples showed the non-Newtonian and shear thinning behavior, by soaring the shear rate, at both temperatures, which implied the dependency of the power-law to shear rate [11]. This was because of soaring the end-to-end distance as a consequence of unraveling the polymer chains, owing to shear fields. This causes a decrease in polymer viscosity, which implies the pseudoplastic fluid behavior [12]. The shear thinning behavior displays the dependency of rheological properties on the viscoelastic characteristics of the polymer matrix [5].

Figure 4 depicts the influence of GO contents on the viscosity of semi IPN nanocomposite hydrogels for four different shear rates at 200 and 240 °F. At low shear rates (0.1 and 1 1/s), the viscosity soared promptly by adding GO, which was noticeable at 0.1 wt.%, because of the good dispersion of GO into the polymer matrix to form the GO network structure. The viscosity continued to increase linearly with an increase in the GO to 0.5 wt.%. In contrast, at a high shear rate (100 1/s), viscosity was less sensitive to the augmentation of GO content. In other words, the reinforcement effect of GO nanosheet was more prevailing at a low shear rate compared to a high shear rate and also at both temperatures 200 and 240 °F [43, 44].

Figure 4:

Viscosity versus GO contents at (A) 200 and (B) 240 °F.

For further understanding of the effect of GO on rheology behavior, the activation energy (E) was studied by the Arrhenius–Frenkel–Eyring equation to appraise the dependency of viscosity to temperature [45] and was computed by Equation (1) [46].

where η 1 , η 2 , T 1 , T 2 , and R were the viscosities, temperatures, and gas constant (8.315 J gmol⁻¹⋅K⁻¹), respectively. As shown in Figure 5, the activation energy of the hydrogels reinforced with the addition of 0.1 wt.% of GO. This was mainly attributed to cross-linking between GO as a bridge reinforcer with matrix, causing a resistance to the mobility of the polymer chains, and creating an energetic barrier for polymer chains [9, 47]. In contrast, further increases in GO contents had no suitable effect on flow activation energy.

Figure 5:

Flow activation energy versus GO contents.

The common types of polymer degradation are mechanical (shearing) and thermal degradation both of which affect the viscosity of the polymer. However, the effect of temperature is higher than the shear degradation [43]. Polymer solution will expose to diverse temperature gradients from the well surface to the reservoir. Because of this, the effect of temperature on the viscosity of semi IPN nanocomposite hydrogels was investigated. As illustrated in Figure 6, the viscosity diminished with an increase in the temperature from 200 to 240 °F, at all the shear rates. This behavior was due to an increase in chain mobility of polymers and the opening of entanglement of IPN hydrogel chains in the matrix. Moreover, the decrease in the number of cross-linking points into the IPN network at elevated temperatures, and, the undermined strength of the interaction between GO nanosheet and matrix, leads to slippage of polymer chains over GO sheet, causing a decline in the viscosity [5, 10]. However, IPN nanocomposite hydrogel showed great strength, compared to a neat IPN hydrogel because of the GO effect. In other words, when the polymers and GO are dissolved in saline water, the GO as a strong thermal enhancer interacted with polymer chains of IPN hydrogel, to form a durable solution. In addition, functional groups in GO provide cross-linking strength in the hydrogel network. This reinforces the thermal stability of the hydrogel and also reduces the rapid network breakage of the hydrogel with increasing temperatures and shear rates [12, 39]. Besides, the presence of chromium triacetate as a cross-linker can have a positive effect in improving the strength of the IPN network structure [1, 27]. However, despite the presence of a cross-linker in the 'IPN hydrogel without GO', its polymer network would weaken with an increase in temperature, due to the poor strength of cross-linking points, compared to IPN nanocomposites. As a result, cross-linking between polymer chains debilitated, resulting in rapid breakage in its network structure, causing a decrease in the viscosity of the solution [40]. Therefore, GO nanosheets as strong thermal reinforcement can hinder the degradation process and defer the further thermal degradation of the polymer matrix by increasing the temperature [31, 36], which causes the polymer solution to flow further in high permeability zones of a reservoir to ameliorate the oil recovery efficiency.

Figure 6:

The dependency of the viscosity on the temperature of IPN nanocomposite hydrogels, at 200 and 240 °F.

The viscosity stability of IPN nanocomposite hydrogels is another important rheological parameter in oil recovery and was determined at 200 and 240 °F, and at a constant shear rate of 100 1/s with the time, as shown in Figure 7a and b. Viscosity stability of IPN hydrogel and its nanocomposite hydrogels sharply decreased with time, especially in the first 15 min. The same effect was observed with an increase in temperatures until the arrival at the target temperature (200 and 240 °F). After this, the viscosity remained constant and no significant decline of the viscosity was observed with time under constant shear rate. This viscosity stability was because of the creation of a strong network in the matrix at 0.1 wt.% of GO [48]. However, with a raise in the GO content to 0.5 wt.%, this effect was not significant.

Figure 7:

The influence of time on the viscosity stability of IPN nanocomposite hydrogels at (A) 200 °F, (B) 240 °F, and (C) shear rate at 100 1/s.

It is also noteworthy from Figure 7c that, the decline in viscosity stability versus the time, at temperature 240 °F was greater than at temperature 200 °F. However, the viscosity stability of the cross-linked polymeric system containing HPAM and HPG hydrogel in the presence of GO at 0.1 wt/% was higher than the HPAM/HPG hydrogel without GO. This behavior was due to the role of GO nanosheet as a reinforcing bridge, leading to an enhancement in the strength of the hydrogel [34, 42]. As a result, the network structure of nanocomposite hydrogels displayed shear stability and also required more shear force and time to break its strength.

3.3 Morphological characterization

To better understand the dispersion of GO for IPN hydrogels, the morphology of the nanocomposite hydrogels was evaluated by SEM. As observed in Figure 8a–c, GO is well dispersed in a mixture solution of HPAM and HPG and formed a uniform solution with the addition of 0.1 wt.% and somewhat uniform at 0.3 wt.% while some agglomerate of the GO particles was observed with the further increase in GO to 0.5 wt.%, as shown in Figure 8d. This was due to the strong interparticle attraction of GO to form agglomerates in the matrix of IPN hydrogel [49, 50]. In addition, GO particles trend the sedimentation in solution [18], which may cause plugging of the pores in the reservoir, due to nonuniformed dispersions and sedimentation of particles [51]. Therefore, the GO concentration is the key factor for the performance of the interpenetrated polymer network.

Figure 8:

SEM images of IPN nanocomposites hydrogel: (A) 0.1, (B) 0.3, (C) 0.5 wt.% of GO, and (D) agglomeration of GO particles at 0.5 wt.%.

3.4 Electrical characterization

For further evaluation of the dispersion of GO in the IPN hydrogel systems, the electrical conductivity of IPN nanocomposite hydrogels was also investigated. As can be observed in Figure 9, with the addition of GO up to 0.1 wt.%, the conductivity of nanocomposite increased, this change in electrical conductivity was due to the good dispersion of GO nanosheets within the IPN matrix, to form a strong network structure. As a result, a large number of the electrons are transported within the matrix, owing to the creation of the interconnecting conductive channels [31, 50]. Moreover, this raise in electrical conductivity corresponded to the formation of stable conductivity paths [31]. However, electrical resistivity decreased when GO contents increased up to 0.5 wt.%. This may be due to poor GO dispersion and decline in the distance between the GO sheets with increasing GO contents. This leads to a decrement in the passing of electrons through the matrix and hinders the formation of the electrical network, indicating an agglomerate of GO particles in the IPN matrix [52]. Therefore, the electrical conductivity of IPN nanocomposite hydrogels improved by reinforcing the effect of GO but up to 0.1 wt.% of GO. This behavior was also supported by morphology and rheological studies.

Figure 9:

Electrical conductivity versus GO contents.

3.5 Aging effect

Viscosity loss of polymer solutions is a significant element in polymer injection, especially in long-term aging under harsh conditions of the reservoir in EOR applications. Because of this, the influence of thermal aging on the viscosity behavior of IPN nanocomposite hydrogels, at temperatures of 200 and 240 °F were investigated. As highlighted in Figure 10, the shear viscosity of all samples decreases remarkably by increasing shear rate and temperature. This reduction of viscosity was dependent on GO concentration. Indeed, the physical interaction between GO and matrix undermined significantly, resulting in a decrease in the strength of the IPNs network structure [11]. Interestingly, with the addition of GO to 0.1 wt.%, the sample displayed higher thermal stability than all samples. This behavior is because of the good dispersion of GO in the IPN hydrogel matrix and makes a potent physical interaction with HPAM and HPG chains [10, 18]. Besides, the presence of chromium triacetate and HPG in the matrix further strengthens the IPN network and improves the shear stability [1, 21, 22]. In addition, the cross-linked network of polymer solutions caused decreasing accessibility of divalent cations such as Ca²⁺ and Mg²⁺ to the HPAM chains [2], and the polymer solution presenting greater thermal stability. According to this result, the GO nanosheet as a reinforcing bridge enhanced the strength of the IPN network and boosted the thermal stability and viscosity stability of the polymer solution at high temperatures for the long-term. In contrast, with an increase in the GO contents up to 0.5 wt.%, viscosity decreased. This effect could be due to the formation of the agglomerate of GO, which notably reduced the interaction between GO and matrix, because of a decline in the cross-linking in the network. This behavior was supported by morphological studies. Therefore, based on these results, IPN nanocomposite hydrogel by adding 0.1 wt.% of GO, showed less viscosity loss than the other samples at both temperatures, for long-term aging.

Figure 10:

Viscosity vs. shear rate of IPN nanocomposite hydrogels after aging at (A) 200 °F and (B) 240 °F.