Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water shut-off treatments

Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water shut-off treatments

Journal Pre-proof Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water...

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Journal Pre-proof Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water shut-off treatments

Reena, Abhinav Kumar, Vikas Mahto, Abhay Kumar Choubey PII:

S0167-7322(19)34525-8

DOI:

https://doi.org/10.1016/j.molliq.2020.112472

Reference:

MOLLIQ 112472

To appear in:

Journal of Molecular Liquids

Received date:

11 August 2019

Revised date:

17 December 2019

Accepted date:

5 January 2020

Please cite this article as: Reena, A. Kumar, V. Mahto, et al., Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water shut-off treatments, Journal of Molecular Liquids(2018), https://doi.org/ 10.1016/j.molliq.2020.112472

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© 2018 Published by Elsevier.

Journal Pre-proof

Synthesis and characterization of cross-linked hydrogels using polyvinyl alcohol and polyvinyl pyrrolidone and their blend for water shut-off treatments Reena*, Abhinav Kumar$, Vikas Mahto$, Abhay Kumar Choubey* *Division of Sciences & Humanities, Rajiv Gandhi Institute of Petroleum Technology, Jais229304, India $Department of Petroleum Engineering, Indian Institute of Technology (Indian School of

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Mines) Dhanbad-826004, India

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ABSTRACT

Polymer gels have been proved useful for profile control and reducing excess water

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production from hydrocarbon-bearing reservoirs. In the present work, gels consisting of

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polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) a d a blend of these two polymers

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(PVA and PVP) crosslinked with organic crosslinker was developed and studied. The optimum pH and polymer crosslinker ratio for gel systems was between 8-9 and the ratio was

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1/0.25 respectively. The viscoelastic properties and thermal properties of blends were

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characterized using rheometer and differential scanning calorimetry (DSC) respectively. Rheological data indicated that elastic modulus is higher than loss modulus (G’>G”) which shows that developed gel systems are the viscoelastic in nature. From the DSC analysis, free and bound water contents were distinguished. DSC result indicated that the gel systems could be used for excess water management in the temperature ranges of 87-69 ℃. The developed gel systems were further characterized using Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM) to investigate hydrogen bonding and microstructure of the systems respectively. The effectiveness of the prepared gel systems for controlling excess water production was tested under in-situ condition using sandpack flooding study. The study basically involved the detailed in-situ gelation mechanism of the developed gel systems. The developed gels and blend showed better stability in the porous 1

Journal Pre-proof media and good water shut-off performance in the mature oil fields. Keywords: Hydrogel; crosslinker; water shut-off; permeability; rheology; sandpack flooding. 1. Introduction Oil and gas production is typically hindered by the formation of the water in the hydrocarbon-bearing reservoirs. This formation water can enter into the production zones through fractures already existing in the reservoirs which causes issues such as corrosion of the reservoir facilities. This along with reduced oil and gas production ultimately ends up in

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damage and earlier shut-off the wells. Billions of dollars are spent to minimize produced

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water because of environmental problems and provisions of environmental regulations. An approach to save on treatment costs of this produced water is to apply water shut-off

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treatments [1, 2]. Nowadays produced water management has become an important issue for

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hydrocarbon industries. There are three common methods to improve water shut-off

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performance, which are increasing the viscosity of the flooding fluid, reducing the permeability of high permeable streaks present in the reservoir and increasing the

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permeability of low-permeable zones [3]. Among all water control processes, chemical water

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shut-off treatment is one that has shown feasibility and practicality in oilfields for decades. The polymeric gel system, which is formed after the crosslinking reaction between polymer and crosslinker, is one of the most promising water shut-off agents because of its low price, easy method of preparation and effectiveness [4]. Field application confirmed that polymeric gel treatments can be applied successfully in heterogeneous reservoirs to minimize excessive water production [5]. Field studied also demonstrated that gel formulation, gelling fluid or gelant is supposed to form a 3D molecular network within the porous media of the reservoir. The effectiveness of water shut-off treatment depends on the compatibility of the gel formulation with harsh reservoir conditions as well as the appropriate placement of the gelant in excessive water zones because the gel network blindly blocks any zone, which is treated by the gelant [6,7]. Polymeric gel systems 2

Journal Pre-proof are usually composed of a water-soluble polymer or monomer crosslinked with an organic or inorganic crosslinker. Polymeric gels, which are crosslinked by inorganic crosslinker, as for example with zirconium and chromium crosslinker, become unsuccessful when reservoir temperature reaches above 80˚C. To overcome this issue, polymeric gel systems with organic crosslinkers are considered and preferred. Polyethyleneimine and phenol/formaldehyde have been largely used in high-temperature reservoirs. This selection is contributed by the existence of thermally stable bonds within the

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organic crosslinked polymeric gel system [8, 9, 10, 11]. Three different types of gel systems

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are used for water shut-off treatments in the reservoir. The first type of polymeric gel systems

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are in- situ gel systems, however, second and third types formed particle gel and foamed gels

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respectively [12, 13,14]. In 1989, Hoskin and Shu invented a PVA cross-linked with phenolic derivative and aldehyde. This crosslinked PVA gel system was an effective permeability

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control agent, which was found to be stable even at the high underground formation

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temperatures. PVA is a well-known commercially available polymer. This is prepared by substituting acetate groups of polyvinyl acetates with hydroxyl groups [15]. In 1997, Shu

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again developed a polymeric gel system using derivative of PVA i.e., polyvinyl alcohol-covinyl amide copolymer (PVOH-VAM), the crosslinking agent Maloaldehydebis (dimethyl acetal) and Maloaldehydebis (diethyl acetal) along with a strong base for adjustment of the desired pH of the system [16]. Victorius also developed PVA cross-linked with methylated melamine formaldehyde resin in 1991 [17]. Polymer blending is one of the most important recent methods for the development of new polymeric materials. The significant advantage of the polymer blend is that the properties of the final product sometimes can be more useful than that of an individual polymer [18]. Polyvinyl pyrrolidone (PVP) has a good reputation due to its outstanding absorption and complex forming abilities, whereas polyvinyl alcohol presents important features such as high hydrophilicity and good capacity of film formation

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Journal Pre-proof [19]. Generally, solution blending of different polymers is one of the methods used to get new material with a variety of properties. These properties mainly depend on the characteristics of the parent homopolymers and the blend composition [20]. Hence, the study of these systems is receiving increasing attention, since an adequate mixture of the polymers can be used to optimize the performance of polymer-based systems. In this case, whether the behavior of the blend is intermediate between the properties of the components or is

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considerably different, it will be determined by the miscibility and phase behavior of the

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blend. Miscibility at the molecular level is possible only when the different polymers are

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capable of establishing specific interactions between their chains. Polyvinylpyrrolidone

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(PVP) is a vinyl polymer possessing planer and polar side groups due to the peptide bond in the lactam ring [21]. It is an amorphous polymer and possesses high glass transition

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temperature (Tg) because of the presence of the rigid pyrrolidone group, which is strong in

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drawing the group and is known to form various complexes with other polymers. On the other hand, PVA is a semi-crystalline polymer, studied extensively because of its many

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interesting physical properties, which arise from the presence of –OH group and the hydrogen bond formation. When these two polymers are mixed, the interaction between PVA and PVP are expected to occur through inter-chain hydrogen bonding between the carbonyl group of PVP and the hydroxyl group of PVA. Hence, these hydrogel blends are stable within the physiological environment because of physical crosslinks consisting of intermolecular hydrogen bonds [22]. The crystallinity of PVA/PVP blend decreases with increasing the level of PVP [23]. PVA has analogous properties like PVP but it varies in some properties like good tensile strength, elasticity [24, 25, 26]. Due to these special properties of PVA, it works well with another polymer as a blend [27]. PVA-PVP blends have been reported for many uses [23, 28, 29, 30].

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Journal Pre-proof The original research presented in this article began with systematic studies on in-situ polyvinyl alcohol, polyvinyl pyrrolidone and their blend (PVA-PVP) formed by crosslinking with resorcinol and formaldehyde. The first step was the selection of suitable polymers and crosslinkers. Then, research was carried out to determine the effects of PVA, PVP and PVAPVP blends and crosslinker concentration on gel performance (gelation time, the viscosity of in-situ bulk gels, storage, and loss modulus). Infrared spectroscopy (IR) analysis was conducted to confirm the hydrogen bonding of the gel systems. Field emission scanning

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electron microscopy (FESEM) was also used for further observations of the microstructure of

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gel. Differential scanning calorimetry (DSC) analysis was done to determine the thermal

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stability of the gels. To test the effectiveness of gel systems, the sandpack flooding

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experiments were carried out at a temperature of 65 °C.

2.1. Materials

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2. Experimental

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Polyvinyl alcohol (PVA) with an average molecular weight of 1,04,500 g/mol and hydrolysis degree of 99.0% and Polyvinylpyrrolidone (PVP) with molecular weight 44,000 g/mol were

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provided by Central Drug House Private Limited (India). The cross-linker was prepared using resorcinol and purchased from Fisher Scientific with the purity 99.0%. Sodium hydroxide and Sodium chloride all were obtained from Molychem, India. Sodium hydroxide (NaOH) solution was made using water to adjust the pH of the system between 8-9.

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Journal Pre-proof 2. Methods Preparation of bulk gel Bulk gels or polymeric gelant formulations (before being gelled) contain different polymers. Initially, stock solutions of different polymers and crosslinkers were prepared using 0.5 % (w/v) brine (NaCl) separately. Crosslinker and polymer solutions were prepared as follows: 1. The bulk gel solutions were prepared by mixing 2.0 % (w/v) of PVA polymer and

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2.0% (w/v) of PVP polymer in 0.5% brine (NaCl). PVA and PVP used for maki g the

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solutio were fou d to have 8.0% and 6.0 % moisture co te t respectively. Brookfield

room temperature and then aged for 24h.

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viscosity of PVA and PVP was observed 0.86 cP and 0.67 cP respectively. It was mixed at

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2. 2.0 % (w/v) resorcinol solution was prepared in 0.5% (w/v) brine (NaCl) and 37%

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formaldehyde (Resorcinol formaldehyde = RF) was used to prepare crosslinker. The resorcinol-formaldehyde (RF) solution was prepared at low temperature to avoid

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crosslinking, which is possible at high temperature. The solution of PVA was prepared using

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magnetic stirrer at room temperature and 800 rpm while resorcinol solution was prepared at 30 °C using 800 rpm because it was easily soluble at room temperature. 3. A stock solution of PVA and PVP polymer was taken separately and was mixed with a pre-defined concentration of crosslinker. After mixing of solution, pH adjustment was done using a sodium hydroxide solution to get the desired pH. Finally, after taking the pH of gelant and maintaining it as per requirement, about 25 mL of gel formulations were transferred into scintillation vials (30 mL, Riviera glass Pvt. Ltd.). The crosslinking reaction was started when the gelant formulation was placed in the oven at a preferred temperature of 90 °C. The gel strength was expressed according to Sydansk’s strength code [31] as given in Table 1. Table 1: Sydansk’s strength code 6

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Gel type

Gel behavior in bottle

A

No detectable gel

B

Highly flowing gel

The gel solution appears to have almost the same viscosity as an initial solution. The gel is comparatively more viscous.

C

Flowing gel

The gel flows up to the cap upon inversion.

D

Moderately flowing gel

E

Barely flowing gel

A small portion of gel does not flow upon inversion A small portion of gel slowly flows upon inversion.

F

Highly deformable non-flowing gel

No gel flow upon inversion.

G

Moderately deformable non-flowing gel

The gel flows approximately halfway upon inversion.

H

Slightly deformable nonflowing gel

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Ringing rigid gel

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There is no gel surface deformation on inversion.

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Rigid gel

No gel surface deformation upon inversion.

Strong gel- upon inversion.

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2.3. Measurement and analysis

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2.3.1. Determination of gelation time using Bottle testing method Gel formation monitoring and gel time determination were conducted through the bottle testing method. The bottle testing technique or bulk gelation study provides a semiquantitative measurement of gelation rate and gel strength. Bottle testing method is a faster and low-cost suitable technique to study the gelation kinetics. For that purpose, ampoules were placed in an oven at 90 ˚C temperature, which was further visually checked after every 1 hour. The gelling time was noted to observe the gel formulation of all three samples (PVA: RF, PVP: RF, and PVA/PVP: RF) with NaCl and CaCl2 brine. 2.3.2. Determination of gel strength The gel strength of any polymeric gel system can be controlled by varying the concentrations

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Journal Pre-proof of different components of that gel system and salinity. The gel systems prepared in distilled water showed very high gel strength and in brine exhibited comparable low gel strength. The gel strength of prepared gel systems was determined by the breakthrough vacuum method [32, 33]. The experimental set-up for this method is shown in Fig. 1 (a). For this experiment, firstly an ampoule tube containing 25 mL gel was taken. After that, a glass tube was placed into the ampoule until the end of the determination. Finally, the maximum pressure was recorded and it was noted as gel strength. All the experiments were repeated thrice to reduce

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the errors during experimentation and the final data reported below is the average of the three

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results obtained from experiments.

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2.3.3. FTIR measurements

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The Fourier transform infrared spectra of the samples were recorded by taking solid dried powder of gel samples. The Perkin Elmer Spectrum PIKE MIRacle single reflection

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horizontal attenuated total reflectance (ATR) accessory equipped with a ZnSe ATR Crystal

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was used for recording the spectra. FTIR spectrum is obtained from performing a mathematical Fourier transform on the interferogram. FTIR spectrometry is a type of

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absorption spectrometry in the infrared region of the electromagnetic spectrum at which chemical bonds vibrate in either a stretching or bending mode. The tool is used to determine functional groups present in samples. The spectra of prepared gel samples were recorded in the wavenumber range of 250-4000 cm-1. 2.3.4. FESEM analysis Field Emission Scanning Electron Microscope (FESEM) analysis was conducted to visualize the internal morphological structure of gel samples. FESEM operates in reflection mode to generate an image of the sample surface. This technique utilizes the low energy secondary electrons that are reflected off the sample surface upon the bombardment of the gel samples with a high-energy electron beam. FESEM requires a conductive sample surface; therefore,

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Journal Pre-proof insulating sample surfaces are often coated with a thin layer of a conductive surface. Gold coating for 105 seconds was used for the samples. In general, SEM resolution varies from 0.5µm to 20µm. The morphologies of samples were imaged by using FE-SEM Zeiss Supra 40 (accelerating voltage ranged 15 kV). 2.3.5. DSC analysis The differential scanning calorimetry (DSC) profiles of the samples were recorded on a DSC 4000 from Perkin Elmer under N2 atmosphere (20mL/min). Pre-weighed (~10mg) polymeric

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blend samples were taken in an aluminium pan for the measurement. The samples were

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heated from -20 to 200 ˚C at a rate of 3 ˚C/min with sample mass comprised of 10-20 mg in

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2.3.6. Rheological characterization

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the crucible. An empty sample pan was used as a reference

Rheological experiments of gel samples were performed for viscosity and moduli (G’ and

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G”). Rheological investigations are important and deal with the deformation and flow of the

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sample as a function of shear deformation. In viscoelastic materials, both viscous and elastic properties are present. These properties were evaluated using a cone plate sensing system

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using a compact rheometer (MCR-52, Anton Paar, Physica, Austria). Shear rate was varied from 1 to 100 S-1 to perform viscosity measurements. Moduli for the samples were investigated in the range of 1 to 100 rad/s of frequency sweep (at constant amplitude). All the experiments were conducted at room temperature. 2.3.7. Sandpack studies (Sandpack preparation and flooding experiments) For carrying out sandpack studies, sand used for the flooding experiments was collected from a commercial retail outlet, which was washed using toluene and dried in the oven for 48 hours at a temperature of 100 ºC. The sandpack was prepared manually in a cylindrical pipe (diameter: 3.81 cm and length 61.3 cm) made of stainless steel supplied by D-CAM Engineering, Ahmedabad, India through the washing of dry sand together with 2.0% NaCl

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Journal Pre-proof (brine). Sandpack holder consists of an electrical thermal jacket with a temperature controller to create the desired temperature during flooding experiments. The sandpack setup consists of a horizontal sandpack holder, a syringe pump (100DX, Teledyne ISCO, USA), fluid accumulators and a flask to collect the effluent. The flow rate was kept constant (1ml/min). Experiments start with the brine injection as preflush to insure the water saturation (100%) in sand packs. Absolute permeability of brine for each sandpack is determined using Darcy’s law. Brine injection was followed by the injection of diesel oil in

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the sand pack until there was no more water produced as effluent. This is called the condition

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of irreducible water saturation (Swi). The original oil in place (OOIP) was calculated using

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material balance calculations from the brine produced; the volume of brine produced = OOIP.

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Again sandpack was flooded with brine. All the solutions were injected in the sandpacks at a constant flow rate of 1ml/min was injected using syringe pump and pressure drop across the

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sandpack was recorded. After injection of brine, 3-4 pore volumes of gelant was injected in

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the sandpack and the pack was shut- in for desired time to ensure the gel formation. After gel formation, brine, diesel oil and brine was injected to see the variation in permeability.

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Toluene was used as a pushing fluid and temperature was 65 ºC [34 35]. Darcy’s law was used to calculate relative permeabilities of brine before and after gel treatments. After that percentage permeability and residual resistance factor (RRF) were calculated. The sandpack represents sandstone reservoirs of fair porosity and permeability. The schematic representation of the experimental set-up for the sandpack experiment is shown in Fig. 1 (b). After calculating pre and post gelation relative permeability of gel, residual resistance factor and percentage permeability reduction (PPR) were calculated using given formulae.

𝒌=

𝑸µ𝑳 𝟎.𝟕𝟖𝒅𝟐 ∆𝑷

………….. (i)

Where Q is the flow rate (cm3/s), µ is the viscosity of the brine (cP), L is the length of the 10

Journal Pre-proof sandpack holder (in cm), ΔP is the pressure drop (atm), and d is the sandpack diameter (cm) and k is the permeability of sandpack (Darcy).

𝑹𝑹𝑭 =

𝝀𝒘𝒊 𝝀𝒘𝒂

=

𝑷𝑷𝑹 = 𝟏 −

𝑲𝒘𝒊 𝝁𝒘𝒂 𝑲𝒘𝒂 𝝁𝒘𝒊

𝑲𝒘𝒂 𝑲𝒘𝒊

……………… (ii)

∗ 𝟏𝟎𝟎 ………… (iii)

Where λwi and λwa are water mobility ratio before and after gel treatment, however, K wi and

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Kwa are water absolute permeability before and after gel treatment.

Fig. 1 (a) the experimental set-up for the determination of gel strength (b) Schematic experimental set-up for sandpack flooding. 3. Results and discussion 3.1. Gelation time and gel strength

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Journal Pre-proof Gelation time and gel strength are two key factors for gel placement and plugging ability in the reservoir for any kind of gel. The gelation time should be sufficient to place the gel into the targeted zones of the reservoir [36]. To observe the gelation time, the gel solutions of different concentrations of polymer, crosslinker, and salinity were prepared. The prepared samples were placed into ampoules and put in the pre-heated oven at a temperature of 90 °C. These ampoules were observed after every 1 hour to determine the gelation time and gel strength of the samples. Polymer or blend and crosslinker ratio were fixed in the ratio 1: 0.25

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for all samples.

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3.2. Effect of salinity on gel time

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One of the main influencing factors on the gel is the presence of salt in the reservoir. The

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salinity effect was checked using NaCl and CaCl2 (5000-25000 ppm) at 90 ºC. With the increase of the concentration of the monovalent ion (sodium), it is assumed that sodium is

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assembled around the functional groups of the polymers. Due to the presence of both anions

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and cations, double layer formation takes place around the functional group of the polymers. This generated double layer causes the shrinkage of the polymer chains. The presence of

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divalent ion (calcium) has negative effects on the gel strength. Divalent ion may even cause the polymer to be deposited. But the presence of these divalent ions leads to gelation time increment [37].

Effect of inorganic salts (NaCl and CaCl2) on the gel strength and gelation time of gels is shown in Fig. 2 (a, b). In 5000 ppm NaCl salt solution, gel strengths for PVP: RF, PVA/PVP: RF and PVA: RF gel systems were observed to be 25000, 16000, and 12000 Pa respectively. In 25000 ppm NaCl salt solution, observed gel strengths were 11000, 9000 and 7000 Pa respectively. In 5000 ppm CaCl2 salt solution, gel strengths for PVP: RF, PVA/PVP: RF and PVA: RF gel systems were observed 23000, 14000 and 11000 Pa respectively. In 25000 ppm CaCl2 salt solution, observed gel strengths were 8000, 6000 and 6000 Pa

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Journal Pre-proof respectively. The very low gel strength of PVA: RF gel system. It may be due to the precipitation in the presence of NaCl and CaCl2 [38]. Therefore, the strength of PVA: RF gel

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system is highly affected by NaCl and CaCl2 ions.

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Fig. 2. Effect of different concentrations of (a) NaCl and (b) CaCl2 on the PVP: RF, PVP/PVA: RF and PVA: RF gel systems.

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3.3. Characterization of gels using ATR FTIR

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Functional groups of the polymeric gel systems and their chemical compositions could be checked by using FTIR spectroscopy. Infrared spectra of these materials differ conferring to

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their composition and may be able to indicate their occurrence of complexation and interaction among various constituents. The FTIR spectra for pure PVA, PVP and PVA/PVP: RF gel samples are shown in Fig. 3. The mutual interaction between them produces modifications in their vibrational mode of the atoms or molecules in the sample, which leads to a change in the physical and chemical properties of the constituents of the complex. The spectrum exhibits bands characteristics of stretching bending vibrations of O-H, C-H, and C=O and CH2.

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Fig. 3. FTIR curves (a) PVA: RF (b) PVP: RF (c) PVA/PVP: RF.

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A strong band observed at 3500 cm-1 has been assigned to bending mode vibration

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corresponding to the free –OH group. While another strong band observed around 1660 cm-1 described the stretching vibration of C=O attributed to PVP. The band corresponding to CH2

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asymmetric stretching vibration occurs at about 2977, 2968 cm-1. The peaks of –OH groups

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due to hydrogen bonding are attributed to 3265, 3285 and 3269 cm-1. A C-C stretching mode is observed at 946 cm-1. A characteristic alcohol band at 1152 cm-1 was assigned to the stretching of C-O of PVA, which is affected by hydrogen bonding along with C-H and O-H bending. The band observed at 1390 cm-1 has been attributed to combination frequencies of CH and –OH groups. Characteristics C-N stretching vibration of PVP was observed at 1256 cm-1. 3.4. FESEM analysis Field Emission Scanning Electron Microscope (FESEM) was used to investigate the morphological structures of synthesized PVP: RF, PVA: RF polymeric gels and PVP/PVA: RF polymeric blend gels. It has been observed that PVP: RF (Fig. 4a) shows a rough surface of the gel system. Polymeric blend gel system, PVA/PVP: RF (Fig. 4b) confirms that the 14

Journal Pre-proof granular appearance of PVA/RF and rough appearance of PVP/RF disappeared and changed to a 3-D network gel system. This observation suggests the formation of a new type of gel network. PVA: RF (Fig. 4c) gel system is having granular morphology. This polymeric gel system has a structure with several smaller agglomerates joining. In all figures, the gel systems containing different polymers are having a special network structure because of

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crosslinker, which helps in connecting the polymeric chains more strongly.

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Fig. 4. FESEM micrographs of (a) PVP: RF (b) PVP/PVA: RF and (c) PVA: RF at 10,000 resolution.

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3.5. Differential scanning calorimetry (DSC) Analysis An important factor in the development of the new materials based on polymeric blend is

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miscibility between the polymers in the mixture because the degree of miscibility is directly related to the final properties of polymeric blend gel. There are many studies related to the miscibility of PVA and PVP. In the present study, the miscibility of PVP/PVA: RF blend was confirmed by DSC [39, 40]. Fig. 5 (b-d) shows the DSC thermograms for the thermal behavior of PVP: RF, PVP/PVA: RF and PVA: RF polymeric gel systems. Polymer and the cross-linker ratio were the same in all samples i.e., 1: 0.25. The scanning temperature ranges were from -20 to 200 ºC and the heating rate was 3 ºC/min. Gel samples were prepared at 90 ºC. The bond strength of the interaction between polymer and cross-linker can be determined by how much energy is required to break the bonds between polymer and cross-linker. This

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Journal Pre-proof energy is referred to as degradation enthalpy. The stronger the interaction, the more endothermic energy is required to break down the bonds in gel systems. A higher degree of enthalpy would suggest the formation of a strong bond between polymer and cross-linker thus producing a gel with high gel strength. In addition, the temperature at which the crosslinked chains of the gel systems start to breakdown is known as the degradation temperature. At this temperature, the mobility of PVA and PVP chains increases. There are many possible direct and indirect interactions, which are responsible for strong and weak gel network. These

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both type interactions take place due to the hydrophilic nature of the polymer and cross-linker

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which could form hydrogen bonds with water molecules. Indirect interactions take place as

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free or bound water. The free water is referred to as the absorbed water that is, water

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molecules attached to other water molecules via hydrogen bond. Meanwhile, the bound water refers to water that is chemically bound to the surface as depicted in Fig. 5.a [41].

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In other words, the presence of bound water decreases the availability of direct

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interactions. It leads to a weak gel network. Therefore, in the presence of higher bound water, less endothermic energy is required to break the bond between polymer and cross-linker. The

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bound water and free water can be calculated from the endothermic peak from the 0 to 7 ºC through the following equation found in the literature [41, 42, 43]:

𝑾𝒃 = (𝜟𝑯⁄ 𝜟𝑯ᵒ ) ……. (iv) 𝑾𝒇 = 𝟏 − 𝑾𝒃 ………. (v)

Where Wb is the fraction of bound water, Wf is the fraction of free water, ΔH is the enthalpy required for heating the free water in the gel, and ΔHº is the 333.5 J/g standard degradation enthalpy for free water. Two peaks were detected in the DSC curves of gel samples. The first peak representing the presence of free water in the gel systems and the second peak represents the 16

Journal Pre-proof degradation temperature (Tdeg) of the gel systems. It was observed that PVP: RF gel system

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had higher free water, however, PVA: RF gel system had less free water.

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Fig. 5. (a)Water bonding types in gel and representation of free and bound water [adapted from 40]. DSC thermographs for all gel systems including water and degradation peak (b) PVP: RF (c) PVP/PVA: RF (d) PVA: RF. From Fig. 5 (b-d) samples, it can be seen that when PVP concentration decreases, degradation temperature decreases. From DSC results, it can be seen that PVP: RF and PVP/PVA: RF samples show degradation at a higher temperature than PVA: RF. This could be due to the restriction of chain mobility of PVP and PVP/PVA polymer and blend due to strong hydrogen bonding [18, 44, 45, 46, 47]. 3.6. Rheological and viscoelastic measurements 3.6.1. Shear rate dependent viscosity test Fig. 6 (a) demonstrates the viscosity Vs shear rate plot for all samples. The flow curves for PVA/PVP: RF blend was similar to that of the PVA: RF and PVP: RF gels. The results prove that the shear viscosity of all the blends decreases with increasing shear rate, which is typical 17

Journal Pre-proof of viscoelastic non-Newtonian or shear-thinning viscoelastic. Polymeric blend showed shear viscosity, which is lower than PVP: RF and higher than PVA: RF gel systems. However, PVP: RF showed higher viscosity than PVA: RF and PVA/PVP: RF gel [48]. The lower viscosity of the PVA: RF gel system could be attributed to a decrease in the semi-crystalline property of PVA due to the presence of salt. The decrease in the shear viscosity could be attributed to the alignment of chain segments of all the samples in the direction of applied stress. Fig. 6 (a) again shows that the shear viscosity is constantly

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decreasing and it can be predicted that all the blends showed a Newtonian molten flow. It

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indicates the same behavior of polymer gels and polymeric blend gel. The dependence of

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shear viscosity on the shear rate at all shear rate regime is more pronounced as the amount of

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gel component varied. The increase in viscosity was probably due to increased interaction

3.6.2. Frequency sweep test

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between hydroxyl groups or hydrogen bonding.

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The frequency (ω) or rate of deformation dependence on elastic (G’) and viscous (G”) moduli of the gel samples were further tested by sweeping the frequency (1 Hz to 100 Hz) at a

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constant Strain. To determine the mixing of two polymers on the stability of gels. As given in Fig. 6 (b), it can be seen that within the frequency range tested, the storage modulus (G’) increased with increasing frequency. The curves of G’ versus ω followed almost a linear mixing rule [49].

It was found that the value of G’ of the polymeric blend was in between PVP/RF gel and PVA: RF polymeric gel. That indicates the formation of a new structure in this blend. It was believed that the interaction has occurred among PVA, PVP, and RF. It can be attributed that new copolymer formations took place at the interface of the blend. The interaction can stabilize the interface by reducing the coalescence and interfacial adhesion and viscosity. On the other hand, it should be emphasized that PVP: RF and PVA: RF gels have high viscosity

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Journal Pre-proof and low viscosity respectively. However, the viscosity of PVP/PVA: RF blend gel is in between of these two gel systems having desired viscosity. This phenomenon of blend could be attributed to set the desired relaxation time due to the decrease in semi-crystalline property of PVA due to the blending of PVP in gels. Fig. 6 (b) again represents the response of loss modulus G” against frequencies of different gel samples. As far as G” is concerned, PVP: RF produced a higher loss modulus and PVA: RF produced a lower loss modulus. That indicates higher energy dissipation compared to PVA: RF with PVP: RF. This indicates that the

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viscous components of the blends could be changing the concentrations of PVA, PVP, and

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RF.

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3.6.3. Complex viscosity dependent frequency graph

The complex viscosity dependence on the frequency of gels and polymeric gel blend is

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shown in Fig. 6 (c). From the figure, it is clear that PVP/RF shows higher complex viscosity

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than PVA: PVP/RF and PVA/RF gel systems. Therefore, the data indicated that the

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viscosities of different gel systems can be changed by changing polymers i.e., PVA, PVP and blend of these two. The blending of polymer gel resulted in significant changes in their

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rheological properties. In general, the polymeric blend gel and pure polymeric gels showed a typical behaviour, exhibiting a shear-thinning regime at all frequencies studied. The material exhibits solid-like behaviour if G's more than G”, or liquid-like behaviour if G” is more than G’. Therefore, from results, it could be attributed that gel systems showed a solid-like behaviour and can be categorized as viscoelastic materials.

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Fig. 6. Rheological properties of PVP: RF, PVA/PVP: RF and PVA: RF (a) Shear thinning

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behavior graph (b) Frequency-dependent storage modulus and loss modulus graph (c) complex viscosity versus frequency graph.

3.7. Gelation performance study of gels after flowing through porous media Sandpack experiments were used to study the water shut-off capacity of the PVP, PVA based gel systems. The result indicates that the PVP: RF, PVP/PVA: RF and PVA: RF gel systems have good effectiveness in porous media. Furthermore, the stronger the gel strength, the higher the effectiveness in terms of PPR and RRF. Higher PPR and PVP can be seen for PVP: RF gel system in comparison to PVP/PVA: RF and PVA: RF gel systems. The experimental results are shown in Table 2.

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Journal Pre-proof Gel systems

Gel strength (Pa)

Relative permeability (md)

PPR

RRF

Fresh gel

Effluent gel

Before

After

PVP: RF

26,000

22,500

594

27.14

95.44

21.88

PVP/PVA: RF

21,500

18,000

586

35.08

94.01

16.71

PVA: RF

18,000

15,000

575

41.04

92.86

14.01

The gelant solutions were injected into the sandpacks. After 24 hours, more than predefined

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gelation time, gels were crosslinked and there was retention in the sandpacks. Through

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retention in the large channels, bridging across the pore throats and adsorption on the sand

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surface, these gels systems reduced the permeability into the sandpacks.

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Porosities of the sandpacks were found between 28-30%. It can be seen that the percentage permeability reduction (PPR) and residual resistance factor (RRF) at 65 ºC are

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more than 90% and 14 respectively. All the gel samples have shown subsequent plugging

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effectiveness of the sandpack. PVA: RF gel system shows little low effectiveness than PVA: PVP/RF and PVP: RF. It may be due to the decrease in the crystalline properties of the PVA

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and salt effect. PVA: PVP/RF gel systems exhibited moderate effectiveness, while PVP: RF system showed greater effectiveness than the former two gel systems. The gel system used in porous media showed the plugging ability and suitability of these gel systems for water shutoff treatments.

The gel strength of fresh and effluent gel samples is shown in Fig. 7 (a). Results show that the PVP RF gel system has a higher gel strength in comparison to PVP/PVA RF and PVA: RF gel systems. It can also be depicted in Fig. 7 (b) that residual resistance factor and percentage permeability reduction of blended polymer i.e., PVP: PVA/RF lies in between its parent’s polymer system i.e., PVA: RF and PVP: RF. PVA: RF has a lower viscosity. The crosslinking mechanism of gel in bottles for fresh gel samples and effluent gel samples (after

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Journal Pre-proof sandpack flooding) and in porous media is depicted in Fig. 7 (c-e). Original gel samples and effluent gel samples (PVP: RF, PVP/ PVA: RF and PVA: RF) after sandpack flooding

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experiments were kept in a hot air oven at 65 ℃ and aged for 4, 5 and 7 days respectively.

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Fig. 7. (a) Gel strengths of the fresh and effluent gel samples (b) Percentage permeability reduction (PPR) and residual resistance factor (RRF) of (i) PVP/RF (ii) PVA: PVP/RF (iii) PVA: RF. Schematics of crosslinking reaction types between polymer and crosslinker (c) In the bottle as fresh sample (d) In the porous media (e) In the bottle as effluent sample (after flowing through porous media). In this study, the gelation performance of all three gel samples was continually observed through the bottle test to understand the difference of the gelation performance between the original gel samples and the samples after flowing through porous media. In Fig. 8 (a-c) left bottles of gels show that the original gel samples have an obvious strong 3-D structure. For original gel samples, it is demonstrated that the preliminary net structure of gels is formed. It was also confirmed by SEM images Fig. 8 (d-f). In contrast, the effluents of gel samples after flowing through porous media (as shown in the right bottles of Fig. 8 (a-c) did not show obvious network structure as observed in original samples. It is

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Journal Pre-proof shown in Fig. 8 (g-i). The presence of the high amount of salt can be seen in SEM images of effluent gel samples in comparison to original gel samples. A comparison diagram of all three gel samples confirms that the gel strength of gel samples apparently reduced after flowing through porous media as shown right side bottles Fig. 8 (a-c) and SEM images 8 (g-i). The addition of brine in the effluent of gel samples may be the reason for the reduction in

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viscosity.

Fig. 8. Comparison diagrams of the gelation performance of the fresh and effluent (flowing through porous media) gel samples (a) PVP: RF (b) PVA/PVP: RF (c) PVA: RF and FESEM images of the same (d, g) PVP: RF (e, h) PVA: PVP/RF (f, i) PVA/RF respectively.

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Journal Pre-proof Conclusions The following conclusions were drawn from the present research work: 

Three polymeric gel systems (PVP/RF, PVA/RF, and PVA: PVP/RF) were prepared and screened for water shut-off treatment using bulk gelation studies and sandpack flooding experiments. On increasing salt concentration, gel strength decreases but gelation time increases. FTIR analysis manifested the conclusion about the specific interaction between

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different polymers and crosslinker and the results showed hydrogen bonding as –OH

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stretching frequency decreased.

SEM result revealed the formation of 3D molecular structures of gel.



DSC data exhibited the thermal stability of all three gel systems and it may be

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confirmed that PVP/RF and PVA: PVP/RF gel systems showed good stability in

Viscosities of gel systems were determined as a function of shear rate and results

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comparison to PVA/RF.

demonstrated shear thinning behavior of gels. At a fixed frequency, the complex viscosity increased for PVP: RF system. 

The storage or elastic modulus(G’) and loss and viscous modulus (G”) were determined as a function of frequency and the result illustrated that elastic modulus is higher than the viscous modulus. It indicates the viscoelastic nature of gel systems.



Compared with the result obtained from the fresh and effluent gel sample, the gel strength of effluent obviously found to be decreased and the gelation time of effluent gel increased.



Remarkable permeability reduction was found in the case of all the gels systems i.e., 24

Journal Pre-proof PVA: RF, PVP: PVP/RF and PVP/RF but PVP: RF gel systems which show highestprofile control performance. 

All three gel samples showed good water controlling effectiveness in porous media.

Acknowledgement The authors are thankful to the Indian Institute of Technology (Indian School of Mines) Dhanbad, India for the necessary laboratory facilities to perform sandpack studies to test the

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effectiveness of prepared gel under in-situ condition. The authors also acknowledge the

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support of the Advanced Center for Materials Science (ACMS) facility of IIT- Kanpur, India for providing support during FESEM analysis. The authors also like to gratefully

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acknowledge Rajiv Gandhi Institute of Petroleum Technology, Jais, India for providing

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financial support and necessary laboratory facilities to carry out this work.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Journal Pre-proof Authors statement

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A. K. Choubey: Supervision, Project administration, Reviewing and Editing. Vikas Mahto: Supervision, Reviewing and Editing. Reena: Conceptualization, Visualization, Investigation, Methodology, WritingOriginal draft preparation. Abhinav Kumar: Methodology, Investigation, Validation

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  

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Journal Pre-proof Graphical abstract

Highlights  Novel hydrogels were developed using polyvinyl alcohol, polyvinyl pyrrolidone a d their ble d. 

Rigid 3-D network structures were formed in the hydrogel system.



Good gel strength and gelation time were found.



Rheology curves highlighted the shear-thinning behavior of the gelant solutions.



Significant permeability reduction was found during the sandpack study of all three

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developed hydrogel systems.

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