Literature Review
Enhanced oil recovery (EOR)
1.1.1 Introduction to Enhanced oil recovery
Hydrocarbon production is categorized into three terms based on the method and timing of extraction: primary oil recovery, secondary oil recovery, and tertiary (enhanced) oil recovery Primary oil recovery involves extracting hydrocarbons using the natural driving forces within the reservoir, without additional assistance from injected gas or water Secondary oil recovery employs conventional techniques like water injection and immiscible gas injection to enhance extraction Tertiary oil recovery, or enhanced oil recovery (EOR), aims to recover residual oil remaining after both primary and secondary methods have reached their economic limits These recovery categories illustrate the progression of oil extraction techniques in the industry.
Figure 1.1 Oil recovery categories 1.1.2 Mechanism of enhanced oil recovery
Improved oil recovery (IOR) encompasses various strategies to enhance oil extraction, including infill drilling and horizontal wells, while enhanced oil recovery (EOR) is a specific subset of IOR focused on reducing oil saturation below residual levels EOR targets differ based on hydrocarbon types; for light oil reservoirs, EOR typically follows secondary recovery, aiming for about 45% of the original oil in place (OOIP) In contrast, heavy oils and tar sands are less responsive to primary and secondary recovery methods, with most production derived from EOR techniques.
Figure 1.2 Target for different crude oil systems
The magnitude of the reduction and mobilization of residual oil saturation “S or ” by an EOR process is controlled by two major factors:
The capillary number is defined as the ratio of viscous force to interfacial tension force, or
Where μ = viscosity of the displacing fluid σ = interfacial tension (IFT) between the displacing fluid and the displaced fluid (oil) v = Darcy velocity θ = the contact angle
= porosity k 0 = effective permeability of the displaced fluid Δp/L = pressure gradient h
Figure 1.3 Effect of Nc on residual oil saturation
Figure 1.3 illustrates the relationship between the ratio of residual oil saturation before and after an Enhanced Oil Recovery (EOR) process and the capillary number The diagram demonstrates that as the capillary number increases, the residual oil saturation decreases According to Equation (1.2), the capillary number can be enhanced through various methods.
- Increasing the viscosity of the displacing fluid
- Decreasing the interfacial tension between the injection fluid and displaced fluid
A key concept in understanding the displacement mechanism of an Enhanced Oil Recovery (EOR) process is the mobility ratio (M) This ratio represents the relationship between the mobility of the displacing fluid and that of the displaced fluid.
Where “k” is the effective permeability and “μ” is the viscosity The mobility ratio influences the microscopic (pore-level) and macroscopic (areal and vertical sweep) displacement efficiencies
A value of M > 1 is considered unfavourable because it indicates that the displacing fluid flows more rapidly than the displaced fluid (oil) This unfavourable condition can h
7 cause channeling and bypassing of residual oil Improvement in mobility ratio can be achieved by increasing the viscosity of the injection fluid, polymer flood
All EOR methods that have been developed are designed to increase the N c
(capillary number) Generally, EOR technologies can be broadly grouped into the following four categories:
Each of the four categories contains an assortment of injection schemes and a different variety of injection fluids, as summarized in Figure 1.4: h
Polymers
Polymers are abundant on Earth, found both naturally and through artificial synthesis These macromolecules, characterized by high molecular weight, consist of repeating units known as monomers Monomers are covalently bonded in chains, often resembling links in a chain, though they can also exhibit branching and cross-linking Additionally, monomers may form two- or three-dimensional polymer networks.
EOR methods Chemical Flood Miscible Others
Polymers are categorized into two main types: homopolymers, made from a single type of monomer, and copolymers, which are composed of two or more different monomers The process of polymerization, which involves the chemical linking of these monomers, is essential for the formation of polymers.
Currently, polymerization is also carried out by ring-opening or complex mechanisms other than condensation or addition
Polymers play a crucial role in enhancing oil recovery efficiency, particularly in Enhanced Oil Recovery (EOR) technology Their high viscoelasticity, due to long-chain structures of high molecular weight, allows them to stretch oil droplets and films during flow, thereby increasing carrying capacity By increasing the viscosity of the water phase, polymers help minimize permeability differences between oil and water phases They can boost the viscosity of injected brine by up to 20 times at low concentrations Additionally, polymers must be resilient to high temperatures exceeding 70°C, high salt concentrations, and prolonged injection durations.
Every year, numerous Enhanced Oil Recovery (EOR) projects are initiated to extract residual oil Currently, two main types of polymers are utilized in EOR processes: synthetic polymers like polyacrylamide and biopolymers such as cellulose and xanthan gum.
Xanthan gum, also known as xanthan polysaccharide, is a biopolymer produced by pseudoxanthomonas Research by Nasr et al demonstrated that the viscosity of xanthan gum is largely unaffected by variations in temperature, pH, and salt content, with the solution retaining at least 80% of its original viscosity Furthermore, xanthan gum has been identified as an optimal solution for enhanced oil recovery (EOR) at temperatures reaching 120 ℃.
Xanthan gum is renowned for its exceptional properties, including excellent suspension ability, water solubility, thickening capacity, emulsification capabilities, and stability in varying pH levels Its versatility makes it a popular choice in polymer flooding technology Nonetheless, it does face significant limitations, particularly in terms of its relative hydrophobicity.
Xanthan gum exhibits superior shear and wear resistance compared to polyacrylamide due to its more rigid molecular structure, which enables it to withstand mechanical damage effectively However, its susceptibility to bacterial degradation poses a challenge, as this can lead to reservoir profile blockages To mitigate these issues, the application of bactericides and deoxidizers is essential for proper maintenance and cleaning.
Cellulose, the most abundant natural polymer, has affordable derivatives used across various applications One notable derivative, hydroxyethyl cellulose (HEC), is produced by reacting alkali cellulose with ethylene oxide and is utilized in flooding processes, such as those in the Romashkino oil field HEC is nonionic, making it compatible with both monovalent and divalent metals Research by Kjoniksen et al indicates that adding hydroxypropyl-β-cyclodextrin (HP-β-CD) monomers to an aqueous solution of charged hydrophobically modified hydroxyethylcellulose (HM-HEC(−)) enhances the polymer's viscosity across a wider range with increasing temperature This ability to chemically modify polymer solutions allows for the optimization of their characteristics, thereby improving the efficiency of flooding processes.
Polyacrylamide (PAM) was the first polymer utilized as a thickening agent in aqueous solutions Today, polyacrylamides (PAM) and partially hydrolyzed polyacrylamides (HPAM) are the most commonly used synthetic polymers in polymer flooding HPAM is a copolymer created through the partial hydrolysis of polyacrylamide.
In high-temperature and high-salinity conditions typical of many oil reservoirs, specifically at temperatures exceeding 90 °C and salinity levels above 10,000 mg/L, polyacrylamide (PAM) is prone to hydrolysis, especially in the presence of certain divalent cations.
The hydrolysis of the amide group (AM) to acrylic acid in reservoir brines containing Ca²⁺, Mg²⁺, and Sr²⁺ is significantly influenced by solution pH and the presence of divalent cations This interaction leads to notable reductions in the viscosity of hydrolyzed polyacrylamide solutions Additionally, the hydrolysis rate is primarily temperature-dependent, with rapid hydrolysis occurring at temperatures exceeding 90 °C, which also accelerates the precipitation of polyacrylamide in the presence of divalent ions in brine.
The demand for polymers capable of withstanding elevated temperatures (up to 120 °C) and higher salinity levels has increased, particularly for offshore oil-bearing formations like the White Tiger Miocene and Oligocene reservoirs These reservoirs are crucial for oil production in Vietnam and Southeast Asia.
To safeguard HPAM/PAM from hydrolysis due to high electrolysis ion content and temperature, extensive research has focused on modifying their structures with various monomers Li Qi et al discovered that a copolymer of acrylamide (AM) and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) exhibits excellent thermal stability, as the sulfo group in AMPS enhances the main chain's stability and controls AM degradation Tamsilian et al explored AM-based thermoassociative copolymers (TAP), which are designed to smartly modify the mobility ratio, effectively addressing the limitations of traditional methods; TAP demonstrated significant viscosity increase in high-temperature reservoirs Additionally, N-vinylpyrrolidone (NVP) has been shown to protect acrylamide groups from thermal hydrolysis, with stability tests indicating that P(AM-NVP) copolymers maintain long-term stability in seawater under reservoir conditions Mqasalmesh et al reported field test results showing that acrylamide-based polymers with a high content of acrylamide tertiary-butyl sulfonic acid (ATBS) (SAV 10) remain stable under Abu Dhabi reservoir conditions.
Free radical polymerization is the predominant method for synthesizing enhanced oil recovery (EOR) polymers This technique falls under the category of chain-growth polymerization, involving the addition of two molecules (monomers) with C=C double bonds In this process, homopolymerization uses a single type of monomer, while copolymerization involves multiple monomers in the reaction Suitable monomers for free radical polymerization typically contain unsaturated structures with C=C double bonds.
Figure 1.5 Suitable monomers for free radical polymerization
Free radical polymerization involves three key steps: initiation, propagation, and termination The process begins with the formation of radicals, which then react with vinyl monomers As the reaction progresses, monomers are rapidly added to the growing polymer chain, maintaining the active center The process concludes with termination, where the active growth center is destroyed, typically through coupling of two growing polymer chains or disproportionation Additionally, chain transfer can occur, transferring the active growth site to an inactive chain, a monomer, or a solvent molecule This method utilizes chemical initiators or radiation to generate free radicals.
GO–Polymer materials
Polyacrylamides, a type of synthetic polymer, play a crucial role in various applications, particularly in polymer flooding techniques for enhanced oil recovery The effectiveness of these polymers is influenced by their molecular weight and degree of hydrolysis High molecular-weight, water-soluble acrylamide-based polymers are utilized to thicken displacing fluids, reducing the mobility of the aqueous phase, increasing the swept volume, and ultimately improving oil recovery efficiency HPAM, known for its strong hydrophilicity, readily dissolves in water to form hydrogen bonds, while the electrical repulsion among molecular chains allows for significant expansion, resulting in a high hydrodynamic volume.
Partially hydrolyzed polyacrylamide (HPAM) is a straight-chain polymer formed from acrylamide monomers, with some hydrolyzed components, resulting in a flexible random coil structure This polyelectrolyte interacts with ions in solution, making it highly effective for enhanced oil recovery (EOR) applications HPAM is favored due to its low cost, excellent viscosifying properties, and well-established physicochemical characteristics It is available in various molecular weights, reaching up to 30 million, and can withstand temperatures up to 99 °C, depending on the hardness of the brine.
Figure 1.11 Acrylamide powder (a), structure of acrylamide (b) and polyacrylamide
PAM undergoes hydrolysis under high temperature (> 90 °C) and high salinity (> 10,000 mg/L), as illustrated in Figure 1.12 The hydrolysis reaction can be catalyzed by both acids and bases, which reduce activation energies and facilitate the formation of a 6-membered ring transition state This process is aided by water or ammonia molecules, resulting in lower enthalpic energy and promoting the interaction of three molecular species Additionally, ammonia and ammonium, which are products of hydrolysis, further accelerate the reaction, leading to auto-accelerated amide hydrolysis kinetics In extreme conditions characterized by significant hydrolysis or high concentrations of divalent ions, phase separation may occur.
Figure 1.12 Reaction formula of hydrolysis of polyacrylamide
N-vinylpyrrolidone (NVP) has been shown to effectively protect AM groups from thermal hydrolysis, as reported by Xu et al [27] The incorporation of 5-membered lactam-pyrrolidone in copolymers limits hydrolysis due to several factors: the significant steric hindrance of NVP enhances the backbone rigidity, leading to improved glass transition temperatures (Tgs) of the copolymers; the carbonyl group of NVP readily forms hydrogen bonds with the amide in the PAM segment, thereby safeguarding the adjacent side group at lower temperatures; and NVP encourages small side group or intermolecular crosslinking.
Figure 1.14 Structure of graphene and graphene oxide
The conjugation of high-viscosity polymers on graphene oxide nanosheets (GONs) has been explored due to GO's excellent thermostability and compatibility with crude oil, highlighting its potential application in enhanced oil recovery (EOR) The hydrophobic basal interface of oil and water, along with the carboxylic groups on GO's edges, can create a highly elastic interfacial film, characterized as a two-dimensional (2D) material Research indicates that GO's 2D structure influences interfacial tension, emulsion-droplet radius, and contact angle, as described by Gibbs energy in Young's equation EOR tests demonstrate that GONs can enhance water viscosity, lower interfacial tension, and significantly emulsify oil droplets in water; however, GO exhibits low dispersion in brine solutions.
22 solution, and its surface needs to be modified to induce its fair distribution in brine and enable its use in EOR
1.3.3 The researches about GO–Polymers materials
GO is usually prepared by Hummer’s method [40] GO contains functional groups making it more dispersed in the polymeric solution or easy to make bonds with polymers
In 2014, Hongyi Qin and colleagues demonstrated the covalent functionalization of graphene oxide (GO) using the 1-(3-aminopropyl)pyrrole (APP) monomer This innovative product was synthesized via an esterification reaction, which involved the amine groups of APP reacting with the carboxyl groups present in GO.
Poly(GO–APP) exhibits enhanced thermal stability, surpassing that of GO by 80–100 °C The numerous amino groups in poly(GO–APP) facilitate strong intramolecular and intermolecular chelation with metal ions Additionally, its conductive properties broaden its potential applications, highlighting its significant promise across various fields.
In 2016, Huawen Hu et al developed a highly sensitive amphiphilic copolymer-based nanocomposite (mPEG-PCL-mPEG) integrated with graphene oxide (GO), demonstrating a unique low-intensity UV light-triggered sol-gel transition The incorporation of GO ensured that the composite gels remained non-cytotoxic, making them safe for use Notably, the spontaneous formation of microchannels within the GO-incorporated UV-gel facilitated sustained drug release, highlighting the potential of these composites for enhancing treatment options for cancer and other diseases.
In their 2023 study, Fan Zhang and colleagues developed a series of cross-linked sodium acrylate and acrylamide copolymer/graphene oxide (P(AANa-co-AM)/GO) hydrogels, varying the mass ratios of acrylic acid to acrylamide and the content of N,N-methylenebisacrylamide through dispersive polymerization The resulting P(AANa-co-AM)/GO hydrogel demonstrated excellent reusability and effective performance in fixed-bed column operations, indicating its potential as a promising adsorbent for the removal of heavy metal ions from wastewater.
1.3.4 The researches about GO–Popolymers materials in EOR
Recent studies indicate that graphene oxide (GO) sheets and polymer dispersion can be utilized in polymer flooding for enhanced oil recovery (EOR) under challenging conditions A novel approach has been developed to improve the stability of GO–polyacrylamide (PAM) nanocomposites in high-temperature, high-ionic-strength brine Furthermore, copolymers of acrylamide (COPAM) have been covalently bonded to partially reduced graphene oxide (rGO) through free-radical polymerization and subsequently reacted with 1,3-propane sultone to introduce zwitterionic groups This modification has resulted in the rGO and COPAM composite demonstrating enhanced dispersion stability in brine.
This study investigates the synthesis of a novel material combining graphene oxide (GO) and poly(acrylamide-co-N-vinylpyrrolidone) (P(AM-NVP)), designed for enhanced oil recovery applications The material exhibits excellent thermal and chemical stability, crucial for the oil industry Gamma-ray irradiation was utilized to initiate copolymerization of acrylamide (AM) and N-vinylpyrrolidone (NVP) through a radical mechanism, with careful analysis of monomer ratios and concentrations to optimize viscosity The resulting copolymers were covalently bonded with GO, synthesized from natural graphite via a modified Hummer’s method The synthesized products were tested in a brine environment mimicking the conditions of the White Tiger reservoir, with experiments assessing their potential in enhanced oil recovery through high-temperature annealing, visual inspection, and viscosity evaluation.
Experimental
Chemical and materials
Table 2.1 Chemical and materials used for experiments
No Name of chemical/material Formulation/Abbreviations
2 Graphite (Powder) C Solid/Pure Fisher -
3 Sulfuric acid H2SO4 Liquid/95% China
4 Potassium permanganate KMnO4 Solid/99% China
5 Ammonium persulfate (NH4)2S2O8 Solid/98% China
7 Sodium nitrate NaNO3 Solid/98% China
4 Acetone CH3COCH3 Liquid/95% Vietnam
For synthesis GO–P(AM-NVP)
3 Acetone CH3COCH3 Liquid/95% Vietnam
Lab (preparing as brine in White Tiger reservoir)
Table 2.2.The ingredient and properties of brine pH
Na + K + Ca 2+ Mg 2+ HCO 3 Cl SO 4 2
Table 2.3 Characteristics of WT Miocene and Oligocene Reservoirs [45]
Equipment, instruments, software
The Iscledavachel gamma source was the primary irradiation device used for synthesizing copolymers, utilizing Gamma – 60 Co as its radiation source with a radioactivity level of 2000 Ci This source has a half-life of 5.26 years and emits two rays corresponding to energy levels E1 = 1.17 MeV and E2 = 1.33 MeV, resulting in a total energy output of 2.50 MeV The power of the source is measured at 0.0148 W/Ci, and the device features a projection chamber with a volume of 4.4 L, ensuring an evenly distributed projection field.
Table 2.4 Equipment, instruments, and software used for characterizing/researching the obtained materials
No Using for Equipment/Instrument Condition in thesis
For synthesis, characterization and evaluation of products
1 Stirring Mechanical stirrer Maximum 900 rpm USA
Digital Temperature Control Hotplate with Magnetic Stirrer
7 Measuring pH pH/ORP Mettler Hana
Annealing at high temperature, high pressure
Fourier transform infrared spectroscopy (FT-IR) PerkinElmer frontier
Characterizing chemical structure in GO’s structures
Determining morphology and atoms distribution on surface
Agilent 1260 Infinity gel permeation chromatography
Software for finding the appropriate condition for copolymerization reaction/plotting the graphs h
1 StatGraphics 18–X64 for finding the optimal condition of copolymerization and plotting 3D response surface
2 Origin 8.5 for plotting Raman spectra, FT-IR spectra and graph of viscosity change
3 Mendeley for citing the references Mendeyley –
4 Chemdraw Professional 16.0 for drawing chemical formulas PerkinElmer –
5 Google scholar for find the reference documents USA
6 Microsoft Visio 2016 for delineating graphical abstract Microsoft –
7 MathType 7 for typing equations USA
Procedure
Graphene oxide was synthesized using a modified Hummers’ method, involving the oxidation of graphite powder The process began by mixing 2.0 g of graphite with 50 mL of sulfuric acid in a three-neck round-bottom flask, stirring the mixture at 80 °C for 60 minutes, followed by sonication for another hour Afterward, 1.0 g of sodium nitrate was added while maintaining the temperature in an ice bath (0–10 °C), and 6.0 g of potassium permanganate was gradually introduced over 120 minutes Once the ice bath was removed, the mixture was stirred at 30 °C for three days Subsequently, 160 mL of deionized water was added, causing the temperature to rise to 60 °C and changing the color from dark to light brown To complete the oxidation process, 200 mL of deionized water and 5 mL of hydrogen peroxide were incorporated, and the resulting mixture was dispersed in deionized water and centrifuged at 10,000 rpm to eliminate impurities.
31 the unreacted/unexfoliated graphite residue Finally, freeze-drying was used to obtain the brown dried GO product
Figure 2.1 Synthesis procedure of GO
2.3.2 Gamma-rays irradiation-induced copolymerization of AM and NVP monomers
Solutions of AM, NVP, and N-methylpyrrolidone (NMP) as a thermal-stability enhancer at different monomer ratios and reactant concentrations were placed into
Polyethylene (PE) containers were purged with nitrogen and securely sealed before being placed in an irradiation chamber They underwent irradiation at various doses to identify the optimal level for polymerization The results of the polymerization at different doses are presented in Table 2.5.
Table 2.5 Result of P(AM-NVP) polymerization at different irradiation dose
Irradiation doses between five and ten kGy make copolymers difficult to dissolve, while doses under five kGy result in highly soluble copolymers Considering these results and cost estimations, a five kGy irradiation dose was selected for polymerization The resulting product was then thoroughly washed and purified using Soxhlet extraction with acetone to eliminate impurities.
The unreacted monomers and PAM were removed, and the resulting P(AM-NVP) copolymers were collected and dried in an oven at 50 °C A schematic diagram illustrating the potential polymerization reaction between acrylamide (AM) and N-vinylpyrrolidone (NVP) is presented in Figure 2.2.
Figure 2.2 Shortened steps of the γ-ray induced free radical P(AM-NVP) copolymerization
Copolymers are influenced by various factors, including irradiation dose, monomer concentration, and the mole ratio of the two monomers Among these, the concentration of monomers and the molar ratio between them are the most critical parameters impacting copolymerization outcomes.
The optimization of polymer viscosity involved two key input factors: the concentrations of monomers and their molar ratios Utilizing response surface methodology (RSM), a regression equation was developed to analyze this relationship The experimental design, comprising 12 runs with three center point replicates, was executed using Statgraphics Centurion software XVIII The study examined molar ratios of acrylamide (AM) to N-vinylpyrrolidone (NVP) ranging from 1:1 to 2:1, while the concentration of monomers was varied between 15% and 25% by weight.
Experimental conditions (molar ratio and monomers concentrations) used for optimization are summarized in Table 2.6. h
Table 2.6.Experimental conditions for optimization of polymerization
No The number of experiments Molar ratio (AM/NVP) Concentration of monomers (wt.%)
Nine experiments with varied mole ratio and monomer’s concentration
Three experiments at the center 3/2 20
Following the identification of the ideal polymerization conditions, the reaction was executed under these parameters and compared to the predicted outcomes Subsequently, these results were utilized to synthesize the GO–P(AM-NVP) nanocomposite.
2.3.4 Synthesis of GO–P(AM-NVP) nanocomposite
Approximately 0.03 g of GO was dispersed in 30 mL of distilled water To obtain partial rGO, 15.1 mg of L-ascorbic acid was added to the solution and then gently stirred at 60 °C for 60 min The solution was cooled down and used immediately in the next stage Meanwhile, 0.03 g of optimized P(AM-N VP) copolymers were dissolved in 50 mL of water in a separate container and stirred for h
The synthesis of rGO–P(AM-NVP) nanocomposites involves a series of steps starting with the slow addition of rGO dispersion to the P(AM-NVP) copolymer solution, followed by 30 minutes of stirring The resulting mixture is then sonicated for another 30 minutes and stirred overnight at 60 °C before being reduced in vacuo After adding acetone, the solution is centrifuged at 5000 rpm for 15 minutes to promote flocculation Finally, the product is freeze-dried to yield solid rGO–P(AM-NVP) nanocomposites, as depicted in Figure 2.3.
Figure 2.3 Synthesis procedure of GO–P(AM-NVP) nanocomposite
Determination of the effect of the monomer composition and concentratio n
After purifying, the copolymers products were weighed to determine the yield of reactions
Yield % was calculated as follow:
Yield % = [m P(AM-NVP) /(m AM + m NVP )]x100% (2.1) where m P(AM-NVP) , m AM , and m NVP are the mass of P(AM-NVP) after purifying, AM monomer and NVP monomer (for irradiation), respectively h
Characterization measurements
The functional group of copolymers and GO–copolymer nanocomposite was analyzed by Fourier transform infrared spectroscopy (FTIR) using a Bruker Equinox
The study utilized a 55 spectrometer to analyze the spectral range of 4000–500 cm⁻¹, focusing on the sp² or sp³ hybridization and functional groups in graphene oxide nanocomposites (GONs) through Raman spectral analysis using the Horiba Xplora One Scanning electron microscopy (SEM) with a HITACHI S-4800 was employed to examine the morphology of the composites and the elemental mapping of copolymers on the graphene oxide surface Additionally, molecular weight determination of the polymer was conducted using Agilent Technologies Infinity gel permeation chromatography (GPC), with deionized water as the mobile phase at a flow rate of 2.0 mL/min Prior to injection, samples were filtered through a 0.2 μm pore size GVWP hydrophilic membrane The thermal decomposition characteristics of the copolymers and nanocomposite materials were analyzed using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) with a Mettler Toledo Stare system, operated under nitrogen gas at a flow rate of 50 mL/min across a temperature range of 35–900 °C, with increments of 10 °C/min.
Thermal and chemical stabilities of P(AM-NVP) and GO–P(AM-N VP)
The thermal and chemical stabilities of copolymers and GO–P(AM-NVP) dispersions were evaluated through annealing experiments Copolymers and GO–P(AM-NVP) nanocomposites were dispersed in brine to achieve a concentration of 1.0 wt.%, which was then diluted to 0.1, 0.3, 0.5, and 0.7 wt.% The viscosity of these dispersions was measured using a Brookfield DV III+ Rheometer, and the samples were placed in heat-resistant ampules To prevent oxidative degradation during annealing, the ampules were rendered inert with nitrogen gas The copolymer solutions were heated in an oven to 123 °C, simulating the Miocene reservoir temperature at WT Oilfield, while the GO–copolymer nanocomposite dispersions were heated to 135 °C, corresponding to the Oligocene reservoir temperature at the same location.
36 observed, and the viscosity of mixtures was measured after 1, 3, 7, 14, 21, and 31 days h
Results and Discussion
Effect of monomer composition and concentration on product yield, molecular weight, and product viscosity
molecular weight, and product viscosity
Figure 3.1.P(AM-NVP) copolymers after purifying (a) and P(AM-PVP) copolymers disperse in brine (b)
Figure 3.1 illustrates the impact of the mole ratio (AM/NVP) and monomer concentration on the yield and molecular weight of P(AM-NVP) copolymers The data indicates that both product yield and molecular weight increase with higher monomer concentrations at a constant mole ratio Notably, these parameters stabilize at a mole ratio of 2/1 and a concentration of 20–25 wt.% Additionally, Table 3.2 demonstrates the influence of the mole ratio and monomer concentration on product viscosity.
Table 3.1 Effect of monomer composition and concentration on product yield and molecular weight
Table 3.2.Effect of monomer composition and concentration on product viscosity
Most polymerization reactions traditionally rely on chemical initiation or catalysis combined with thermal activation In contrast, radiation-induced free-radical polymerization features a temperature-independent initiation step and significantly lower overall activation energies compared to chemically initiated processes This method yields high-quality copolymer products that are clean and easy to manage throughout the investigation stages.
Optimization
The analysis of Table 3.2 reveals that 0.5 wt.% polymer solutions exhibit an average viscosity between 4.6 and 4.9 cP, making them suitable for the specific reservoir type of crude oil This viscosity range was utilized as a key input for the optimization process Additionally, the experimental outcomes for various input factors of the 0.5 wt.% polymer solution viscosity, presented in Table 3.3, led to optimal results detailed in Table 3.4.
Table 3.3.The viscosity value of 0.5 wt.% polymer solution at different polymerization conditions
Molar ratio (AM/NVP) Concentration of monomers (wt.%) Viscosity 0.5 wt.% (cP)
Note: a, b, and y are concentration of monomers (wt.%), mole ratio, and optimal viscosity of 0.5 wt.% copolymers solution, respectively
Figure 3.2 Response surface of viscosity of 0.5 wt.% copolymers solution
The obtained coefficient of correlation r = 0.954 (> 95%) and coefficient of determination R 2 = 0.91 (> 90%) are relatively high These high values of r and R 2 indicate the similarity between the predicted and experimental results They also h
42 show that the regression equation used for optimization has accurately described the experimental data [47]
The response surface analysis conducted using StatGraphics 18–X64 software revealed optimal values for the monomer mole ratio (AM/NVP), monomer concentration, and viscosity of 0.5 wt.% copolymer solutions, which were determined to be 1.71, 23.19 wt.%, and 5.02 cP, respectively Following this analysis, polymerization was performed under these optimal conditions, with the resulting viscosities at various concentrations presented in Table 3.1.
Table 3.5.Viscosity of optimal-condition polymer solutions at different concentrations
The result indicates that viscosity of 0.5 wt.% optimal-condition copolymers solution, which is 5.01 cP, is similar to optimization results.
Characterization of P(AM-NVP) and GO–P(AM-NVP)
3.3.1 FTIR of GO, P(AM-NVP) and GO–P(AM-NVP)
FT-IR analysis confirmed the presence of NVP and AM in the copolymers, with characteristic bands at 1320–1293 cm−1 indicating the successful incorporation of NVP through the asymmetric stretching of the C–N–C group Additionally, the P(AM-NVP) spectrum displayed absorption bands for C=O stretching at 1660–1650 cm−1, characteristic of amides, aligning with previous studies The spectrum also revealed bands at 3361–3298 cm−1 corresponding to the asymmetric –NH– stretching vibrations of primary amides Notably, the olefinic stretching bands at 1612 and 1629 cm−1, associated with acrylamide and N-vinylpyrrolidone units, were absent in the P(AM-NVP) sample.
43 spectrum The absence of these units indicates that the monomers have been entirely consumed after the reaction in copolymers [46]
Figure 3.3 FTIR spectra of (a) P(AM-NVP), (b) NVP and (c) AM
The peaks and corresponding bonds are shown in the Table 3.6
Table 3.6 FTIR results of (a) P(AM-NVP), (b) NVP and (c) AM
N o Material Peak (cm -1 ) Bond Note
4 NVP, P(AM-NVP) 1320–1293 C–N–C NVP ring
The FTIR spectra analysis of the GO–P(AM-NVP) nanocomposite, P(AM-NVP) copolymers, and GO, as illustrated in Figure 3.4, confirms the successful integration of P(AM-NVP) onto GO Notably, the bands observed at 3361–3298 cm −1 in the P(AM-NVP) FTIR spectrum indicate the asymmetric –NH– stretching vibrations of primary amides (NH2) In contrast, the FTIR spectrum of the GO–P(AM-NVP) nanocomposite shows bands at 3450–3350 cm −1, which can be attributed to O–H stretching, highlighting the interaction between the components.
GO Furthermore, in GO–P(AM-NVP) spectrum N–O bonds formed between copolymers and GO make primary amide removed, so the signals of NH2 in FTIR h
The FTIR spectrum reveals significant absorption bands, with peaks at 2900–2800 cm −1 and 1090–1110 cm −1 attributed to Csp 3 –H (–CH2–) stretching and C–O stretching, respectively Additionally, a characteristic carbonyl group (C=O) stretching band is observed at 1660–1650 cm −1, typically appearing as a doublet in solid-state amides The bands at 1320–1293 cm −1 in the GO–P(AM-NVP) and P(AM-NVP) spectra indicate the asymmetric stretching of the C–N–C group from NVP rings, confirming the successful incorporation of NVP into the materials Furthermore, the bands between 1550–1500 cm −1 in the GO–P(AM-NVP) FTIR spectrum correspond to N–O stretching vibrations, signifying the covalent coupling of P(AM-NVP) with GO.
Figure 3.4 FTIR spectra of (a) GO–P(AM-NVP), (b) P(AM-NVP) and (c) GO
The peaks and corresponding bonds are shown in the Table 3.7 h
Table 3.7 FTIR results of (a) GO–P(AM-NVP), (b) P(AM-NVP) and (c) GO
N o Material Peak (cm -1 ) Bond Note
7 NVP, P(AM-NVP) 1320–1293 C–N–C NVP ring
The bonds that are between GO and P(AM-NVP)
3.3.2 Raman spectra of GO and GO–P(AM-NVP)
The Raman spectra of GO and the GO–P(AM-NVP) nanocomposite reveal two characteristic peaks associated with graphene: the D band at 1361 cm −1 for GO and 1336 cm −1 for GO–P(AM-NVP), indicating structural defects linked to sp² hybridization primarily due to oxygen functional groups, and the G band at 1592 cm −1 for GO and 1582 cm −1 for GO–P(AM-NVP), signifying sp³ hybridization or C–C bonds within the GO structure This analysis confirms effective oxygen functionalization of GO, enhancing its stability in aqueous environments during the EOR process The presence of oxygen groups contributes to the stability of GONs, while a lower concentration of these groups may result in pore plugging Although the Raman spectra show no significant differences, the ID/IG ratios of 0.92 for GO and 1.13 for GO–P(AM-NVP) suggest that the grafted copolymer molecules increase local defects and disorder in the GO structure.
Figure 3.5 Raman spectra of (a) GO and (b) GO–P(AM-NVP)
3.3.3 SEM analysis of P(AM-NVP) and GO–P(AM-NVP)
The SEM images presented in Figure 3.6 illustrate the morphology of P(AM-NVP) copolymers and GO–P(AM-NVP) composites, revealing distinct rod-like (Figure 3.6a) and flake-like (Figure 3.6b) structures formed by tightly bound polymer chains measuring approximately 100-200 mm Furthermore, Figure 3.6c showcases a stable dispersion of white polymer fibers on the surface of GO sheets, confirming the successful grafting of the polymer onto the GO sheets, as supported by the preceding Raman analysis.
Figure 3.6 SEM images of P(AM-NVP) copolymers [(a) and (b)] and GO–P(AM-
Figure 3.7 Scanning electron microscopy energy-dispersive X-ray spectroscopy
(SEM-EDX) of GO-copolymer
The SEM-EDX analysis presented in Figure 3.7 indicates that nitrogen atoms are absent in GO–P(AM-NVP), despite their presence in the NVP and AM monomers This absence can be attributed to the low mass percentage of nitrogen in these monomers, which falls below the detection limit Additionally, these monomers are conjugated with graphene oxide (GO), which is rich in carbon and oxygen Consequently, while nitrogen atoms may exist in GO–P(AM-NVP), their minimal quantity compared to carbon and oxygen renders them undetectable in the SEM-EDX analysis.
Figure 3.8 SEM mapping pictures of fracture surfaces of (a) GO–P(AM-NVP) and elemental mapping images of GO–P(AM-NVP) for (b) oxygen, (c) carbon, (d) nitrogen
Elemental mapping images of GO–P(AM-NVP) reveal a homogeneous and abundant distribution of nitrogen atoms on the GO surface, as illustrated in Figure 3.8 This SEM elemental mapping confirms the successful bonding of copolymers onto the GO surface.
3.4 Evaluation of thermal and chemical stabilities of P(AM -NVP) copolymers and GO-P(AM-NVP) dispersions
Figure 3.9 GO–P(AM-NVP) nanocomposite after free-drying (a) GO–P(AM-NVP) disperse in brine (b) h
In oilfield applications, particularly in enhanced oil recovery (EOR), it is essential for polymers to maintain their presence in reservoirs over extended periods Therefore, assessing the thermal stability of these polymers through the annealing method is crucial for their effective application.
The pore size of Miocene and Oligocene WT reservoir rocks was analyzed, leading to the selection of nanocomposites P(AM-NVP) and GO–P(AM-NVP) at concentrations of 0.5 wt.% and 1.0 wt.% for the annealing test, as detailed in Table 3.8.
Table 3.8 Viscosity of GO–P(AM-NVP) nanocomposite dispersed in brine at different concentrations
3.4.1 Observing the appearance after being annealing
Table 3.9.Appearance of P(AM-NVP) 0.5 wt.% and GO–P(AM-NVP) 1.0 wt.% dispersions after annealing
The picture of Table 3.9 showed the stability of P(AM-NVP) copolymers (0.5 wt.%) and GO–P(AM-NVP) (1.0 wt.%) in brine after being annealed at 123 °C and 135 h
In the initial week, the P(AM-NVP) 0.5 wt.% dispersion maintained a consistent appearance, but subsequently darkened, signaling a change in viscosity In contrast, the GO–P(AM-NVP) nanocomposite at 1.0 wt.% in brine, when annealed at 135 °C over 31 days, displayed a stable appearance, indicating that its viscosity remained largely unchanged.
3.4.2 The viscosity of annealed samples
The viscosity of dispersions was observed to change, indicating significant alterations in the P(AM-NVP) copolymers at a concentration of 0.5 wt.% The annealing test results, illustrated in Figure 3.10a, demonstrate the behavior of these copolymers when subjected to brine, highlighting the effects of the annealing process on their properties.
After 14 days at 123 °C, the viscosity of P(AM-NVP) copolymers remained relatively stable However, significant viscosity losses of approximately 9.30% and 10.63% were observed after 21 and 31 days, respectively Despite this decline, the viscosity of the 0.5 wt.% P(AM-NVP) copolymers after 31 days of annealing suggests that they remain viable for enhanced oil recovery (EOR) applications It is important to note that higher temperatures, such as those found in the Oligocene WT oilfield, exacerbate viscosity loss, a trend supported by previous research To address this issue, the GO–P(AM-NVP) nanocomposite has been proposed as a potential solution.
The annealing test results for the GO–P(AM-NVP) nanocomposite (1.0 wt.%) indicate that after 31 days in brine at 135 °C, the viscosity remained nearly unchanged This demonstrates the thermal and chemical stability of the dispersions under static conditions.
The GO–P(AM-NVP) dispersion demonstrates significantly greater thermal stability than the P(AM-NVP) copolymer solution, with stability temperatures of 135 °C and 123 °C, respectively These enhanced properties align well with the needs for enhanced oil recovery (EOR) applications in various oilfields, particularly in Miocene and Oligocene WT oilfields.
Figure 3.10 Viscosity of (a) 0.5 wt.% P(AM-NVP) copolymers in brine annealed at
123 °C and (b) 1.0 wt.% GO–P(AM-NVP) nanocomposite in brine annealed at 135 °C during 31 days h
Conclusions and Recommendations
Conclusions
The present study reported the completed procedure to obtain the copolymers and GO–copolymers nanocomposite, which can work for offshore reservoirs under high-temperature and high-salinity conditions
The successful γ-ray induced copolymerization of optimized acrylamide and N-vinylpyrrolidone mixtures resulted in high-efficiency production of thermostable P(AM-NVP) copolymers These copolymers offer easily customizable properties, are cost-effective, and are environmentally friendly.
The optimal conditions for copolymerization were obtained at 1.7 for AM/NVP monomer ratios and 23.2 wt.% for monomers concentration
The obtained copolymers were then covalently coupled with graphene oxide (GO) synthesized from natural graphite using the modified Hummer’s method
The properties of the copolymers are well-suited for reservoir conditions, enabling safe scalability for enhanced oil recovery (EOR) at high temperatures (up to 123 °C) and in high-salinity brine Notably, a 1.0 wt.% brine dispersion of P(AM-NVP) covalently bonded to partial rGO demonstrated remarkable stability in both viscosity and appearance during a 31-day annealing period at 135 °C.
As a result, the produced GO–P(AM-NVP) nanocomposite had excellent h
The P(AM-NVP) copolymers and GO–P(AM-NVP) nanocomposite exhibit exceptional dispersion stability under high-salinity and high-temperature conditions These advantageous properties make them promising candidates for enhanced oil recovery (EOR) applications in offshore reservoirs characterized by elevated temperatures.
Recommendations
A closer study of how these nanocomposites alter the wettability of a rock surface and contact angle should be executed before and after annealing
Measuring the interfacial tension (IFT) of the copolymers and GO–copolymers dispersions should be executed before and after annealing
Modifying PAM copolymers structures with another monomer (not N- vinylpyrrolidone) should be studied h
LIST OF PUBLISHED PAPERS RELATED TO LEARNER
In their 2021 study published in *Colloids and Surfaces A: Physicochemical and Engineering Aspects*, Nguyen et al investigate the stable dispersion of graphene oxide-copolymer nanocomposite aimed at enhancing oil recovery in high-temperature offshore reservoirs The research highlights the potential of this innovative material to improve the efficiency of oil extraction processes, addressing challenges faced in extreme environmental conditions.
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APPENDIX A.1 Pictures of the equipments and instruments
Figure A.3 Digital Temperature Control Hotplate with Magnetic Stirrer h f
Figure A.5 FreeZone 6L Benchtop Freeze Dry Systems
Figure A.6 pH/ORP Mettler Hana HI 3220 h g
Figure A.7 Brookfield DV-III+ Programmable Rheometer
Figure A.8 Stuart RE300 Rotary Evaporator
Figure A.9 Fourier transform infrared spectroscopy (FT-IR) PerkinElmer frontier h h
Figure A.10 DSC-3/TGA Mettler Toledo
Figure A.11 Raman Horiba Xplora One h i