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. 2025 Nov 4;10(45):54608–54619. doi: 10.1021/acsomega.5c07872

Synthesis and Performance Evaluation of Gemini Hydrophobically Associating Polyacrylamide

Li Liu †, Shuaishuai Zhao †,*, Zhiqiang Wang †, Ende Zhan †, Jinxin Liu ‡, Jinyun Wei †, Xuliang Fan †, Mengyi Xing †
PMCID: PMC12631663  PMID: 41280775

Abstract

To overcome the constraints posed by traditional acrylamide-based polymers in harsh reservoir environments, this study synthesized a novel Gemini-type hydrophobically associating polymer (PAAD) via micellar copolymerization, incorporating acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and a meticulously designed Gemini surfactant monomer. A comprehensive evaluation was conducted on PAAD’s solution behavior, resistance to temperature and salt, shear stability, and enhanced oil recovery performance, with a comparative analysis to partially hydrolyzed polyacrylamide (HPAM), which is currently utilized in oilfields. The experimental results revealed that the critical association concentration of the Gemini-type hydrophobically associating polymer fell within the range of 1.5 to 2.0 g/L. Under identical concentration conditions, PAAD demonstrated superior thermal stability, salt tolerance, and shear resistance when compared to HPAM, while effectively reducing the oil–water interfacial tension to 0.3 mN/m. This study provides both theoretical basis and technical support for the practical application of hydrophobically associating polymers in high-temperature, high-salinity reservoirs.

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1. Introduction

Chemical flooding is one of the important enhanced oil recovery (EOR) methods, playing a key role in the efficient development of oil fields. , This technology works by injecting functional chemical agents into reservoirs to improve rheological properties and interfacial behavior; it thereby significantly enhances both macroscopic sweep efficiency and microscopic displacement efficiency. , Based on the type of displacing medium, chemical flooding is primarily categorized into polymer flooding, surfactant flooding, and combination flooding. − As the most mature chemical EOR technique, polymer flooding optimizes the oil–water mobility ratio. This is achieved by leveraging the viscosity-enhancing effect of polymers to increase aqueous-phase viscosity and reduce water-phase permeability. This suppresses viscous fingering and expands sweep coverage. The key performance indicators lie in the long term stability of polymer solutions under reservoir conditions and their seepage characteristics. In contrast, surfactant flooding focuses on improving displacement efficiency by reducing oil–water interfacial tension, modifying rock wettability, and promoting crude oil emulsification. However, due to high chemical costs and complex formation adaptability requirements, surfactant flooding is often used as an auxiliary method in combination with other techniques. Combination flooding represents a cutting-edge approach in chemical EOR, primarily encompassing binary systems (polymer–surfactant) and ternary systems (alkali–surfactant–polymer). This technology effectively integrates the synergistic effects of different chemical agents to achieve dual objectives of mobility control and interfacial activity regulation. Extensive field applications have revealed several technical challenges: increased complexity in chemical compatibility, substantially higher chemical costs, and potential formation damage induced by alkali. These limitations necessitate comprehensive techno-economic evaluations based on reservoir geological characteristics prior to field implementation.

Most mature oilfields in China now face the dual challenges of high-temperature and high-salinity reservoir conditions during their middle-to-late development stages. Conventional polymers like HPAM suffer severe performance degradation under such harsh environments. Elevated temperatures accelerate the hydrolysis of amide groups and backbone degradation of HPAM, while high salinity induces undesirable cross-linking via interactions with multivalent cations. Together, these mechanisms lead to a substantial loss in viscosity. This fundamental limitation has driven oilfield chemistry research toward developing thermally stable and salt-resistant polymer alternatives. Hydrophobically associating polymers (HAWP) have emerged as particularly promising candidates for enhanced oil recovery. These polymers incorporate low concentrations of hydrophobic groups into a water-soluble backbone, creating dynamic network structures through intermolecular associations. , Compared to conventional HPAM solutions, HAWP systems demonstrate both superior viscosity enhancement and significantly improved thermal/salt resistance. The unique performance of HAWPs stems from their molecular self-assembly characteristics. Once exceeding a critical association concentration, their hydrophobic groups form microdomains that effectively thicken the aqueous phase while maintaining remarkable insensitivity to salinity variations. Recent advances in HAWP design have focused on optimizing hydrophobic group architecture. Yang synthesized a new class of HAWP through free radical copolymerization using acrylic acid, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and hydrophobic monomers with varying carbon chain lengths. Their results demonstrated that increasing the hydrophobic chain length significantly enhanced intermolecular association while substantially reducing the critical association concentration. Notably, this polymer series exhibited superior salt tolerance. Similarly, Zhu team developed dendritic HAWPs showing excellent shear recovery in porous media compared to HPAM’s irreversible degradation, attributed to their branched structure-enabled network reformation. Particularly noteworthy are Gemini-type HAWP designs developed by Mao, where N,N-di-n-dodecylacrylamide comonomers created polymers capable of maintaining 50 mPa·s viscosity even at 16,000 mg/L salinity. Zhu’s team prepared a Gemini-type polymer through micellar polymerization using AM, AMPS, and N,N-dodecylacrylamide monomers. The polymer incorporates hydrophilic groups (−CONH2 and –SO3H) for solubility enhancement and twin-tailed hydrophobic segments for association capability. This synergistic combination endows it with excellent aqueous solubility, temperature/salt resistance, and mechanical shear stability. These structural innovations highlight HAWPs’ potential for chemical flooding in challenging reservoirs, particularly their ability to maintain viscosity stability under extreme conditions. , The molecular design strategieswhether through chain length optimization, dendritic architectures, or twin-tailed monomers-share a common theme of creating robust yet dynamic associative networks. − However, extensive laboratory analyses have characterized the solution properties of these polymers. Significant gaps remain, however, regarding their actual oil displacement performance and compatibility with authentic reservoir conditions. Urgent systematic investigations are therefore required to bridge the gap between bench-scale studies and field applications and fully realize the potential of HAWPs in EOR operations.

In this study, a novel polymerizable quaternary ammonium Gemini surfactant monomer, 2-(acryloyloxy)-N 1,N 1,N 3,N 3-tetramethyl-N 1-nonyl-N 3-octylpropane-1,3-diammonium bromide (DC8), was synthesized through rational molecular design. DC8 contains two hydrophilic ionic headgroups that exhibit excellent water solubility, providing favorable conditions for the synthesis of hydrophobically associating water-soluble polymers. Using AM, AMPS, and the newly synthesized DC8 as monomers, a hydrophobically associating terpolymer, P­(AM-AMPS-DC8) (designated as PAAD), was prepared via aqueous micellar polymerization. The resulting polymer was characterized by hydrogen nuclear magnetic resonance (1H NMR), Fourier transform infrared spectroscopy (FTIR), and microscopic morphology analysis. The solution properties of PAAD, including its temperature resistance, salt tolerance, shear stability, and oil displacement performance, were systematically investigated and compared with those of a conventional polymer currently used in oilfield applications. This study provides a theoretical basis for the molecular design and performance regulation of HAWP used in chemical flooding. It also offers valuable technical references for the efficient development of high-temperature and high-salinity oil reservoirs.

2. Experimental Section

2.1. Chemicals and Reagents

Acrylamide (AM, 98%) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS, 98%) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1-Bromooctane (>98%), acetonitrile (99.9%), potassium iodide (AR, 99%), ethanol (99.5%), and methyl tert-butyl ether (AR, 99%) were obtained from Macklin Biochemical Co., Ltd. OP-10 (AR), NaCl (AR), dimethylamine aqueous solution (AR, 40 wt %), epichlorohydrin (99%), trimethylamine (99.5%), hydroquinone, chloroform (AR), acryloyl chloride (98%), and 2,2′-azobis­(2-methylpropionamidine) dihydrochloride (V-44, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The HPAM (molecular weight: 12 × 106 Da, hydrolysis degree: 23%) and dehydrated crude oil used in this study were sourced from Block H of the Liaohe oilfield. The crude oil exhibited a viscosity of 8.65 mPa·s at the reservoir temperature of 73 °C. Simulated formation water with a salinity of approximately 8750 mg/L was prepared for experiments. Petroleum sulfonate, mass concentration 20%, produced by Daqing Wantong Chemical Co., Ltd. (Daqing, China).

2.2. Preparation of Gemini Cationic Monomers

The preparation process of Gemini cationic monomers (DC8) is as follows: preparation of Intermediate I: a 50 mL aqueous solution of dimethylamine (40 wt %) was added to a round-bottom flask. Under ice-bath cooling, 15 mL of epichlorohydrin was added dropwise, and the mixture was stirred at room temperature for 24 h, followed by reaction at 75 °C for 72 h. Solid NaOH was then added in portions under ice-bath conditions until phase separation occurred. The crude product was transferred to a separatory funnel, and the upper organic layer (pale yellow liquid) was collected. This collected layer was then dried over anhydrous MgSO4 overnight, filtered under reduced pressure, and ultimately afforded Intermediate I as a clear liquid in 68.4% yield. Synthesis of Intermediate II: Intermediate I (10 mL) was dissolved in chloroform (50 mL), followed by dropwise addition of triethylamine (10 mL) and hydroquinone (0.006 g) under stirring. Acryloyl chloride (5 mL) was added slowly under ice-bath cooling, and the reaction proceeded at 35 °C for 48 h. The solvent was removed by rotary evaporation. The residue was dissolved in excess dichloromethane, filtered, and washed with saturated NaCl solution. The organic phase was separated and retained to yield Intermediate II (yield: 51.4%). Final product (DC8) formation: Intermediate II (5 mL) and 1-bromooctane (12 mL) were dissolved in acetonitrile (50 mL) with hydroquinone (0.005 g) and KI (0.2 g). The reaction was carried out at 45 °C for 70 h. After solvent evaporation, the crude product was dissolved in ethanol and recrystallized with methyl tert-butyl ether at low temperature, yielding DC8 as a white solid powder (yield: 62.5%). The synthetic route of the Gemini cationic monomer is illustrated in Figure .

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Synthesis route of the DC8 monomer.

2.3. Preparation of Polymer PAAD

The hydrophobically associating polymer (PAAD) was prepared as follows: in a custom reaction vessel, 35 mL of deionized water, 15 g of AM, and 3 g of AMPS were combined and stirred until fully dissolved. Subsequently, 0.7 g of OP-10, 0.4 g of DC8, and 0.05 g of initiator V-44 were emulsified into the mixture. The pH of the polymerization system was adjusted by the addition of NaOH solution. Nitrogen was then introduced at room temperature for 50 min to eliminate oxygen. The reaction vessel was placed in a 45 °C oil bath for 5 h to facilitate the polymerization reaction. Afterward, the vessel was cooled to room temperature to yield colloidal polymer. The white granular PAAD was obtained through shearing, washing with absolute ethanol for 2 h, and subsequent drying and crushing.

2.4. Characterization of Structure and Properties of Polymer PAAD

1H NMR spectroscopy experiments were conducted using a Bruker AV 400 MHz NMR spectrometer. The DC8 monomer was dissolved in CDCl3, while the copolymer PAAD was dissolved in D2O.

Fourier-transform infrared spectroscopy was carried out on a Bruker Vertex-70 infrared spectrometer. Polymer powder was uniformly mixed with pure KBr and compacted into flake samples. Experimental parameters were set as follows: spectral resolution of 4 cm–1, wavenumber range from 400 to 400 cm–1.

The morphology of the polymer aqueous solution was analyzed using a Hitachi S480 scanning electron microscope and a JEOL-1011 transmission electron microscope. The polymer solution was deposited onto a silicon wafer surface and rapidly frozen in liquid nitrogen.

The interfacial tension of a PAAD polymer solution was assessed using a TX-500C rotary drop tensiometer under various conditions. The polarity of hydrophobic association microdomains in PAAD solutions was determined using a Hitachi F-450 fluorescence spectrophotometer with pyrene as the fluorescent probe. At low PAAD concentrations, the polymer solution primarily exhibits intramolecular associations. As the content of hydrophobic groups increases, the associations transition from intramolecular to intermolecular, leading to the formation of hydrophobic microdomains. The fluorescence spectrum of the pyrene solution (1 × 10–5 mol/L) shows emission peaks at wavelengths of 373, 379, 384, 390, and 410 nm, with the corresponding peak intensities denoted as I 1, I 2, I 3, I 4, and I 5, respectively. The intensity ratio of the first to the third emission peaks (I 1/I 3) serves as an indicator of the polarity of the polymer solution.

The apparent viscosity of PAAD polymer solutions was measured under different temperatures and salinities using a DV-II+ PRO viscometer. For the thermostability test: The apparent viscosity of a 2000 mg/L PAAD solution was determined in the temperature range of 20–90 °C using rotor No. 0 at a rotational speed of 6 rpm. For the salt tolerance test: PAAD solutions with a concentration of 2000 mg/L were prepared using simulated formation water with salinities ranging from 0 to 26,250 mg/L. The apparent viscosity of these solutions was measured at room temperature using a rotational viscometer. All the above experiments were conducted 3 times and the average values were taken.

Shear resistance experiment: PAAD and HPAM solutions (both at 2000 mg/L) were prepared using simulated formation water. Five artificial cores (1.2 m in length, permeability 300 mD) were connected in series. Polymer solutions were injected into the core sample at a constant flow rate of 0.6 mL/min using an ISCO syringe pump (model 260D) to systematically evaluate viscosity loss induced by shear degradation through porous media. All the aforementioned experiments were performed in triplicate, and the average values are reported. The experimental setup is shown in Figure .

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Schematic diagram of the custom-built shear property test apparatus.

Long-term stability testing: solutions of PAAD and HPAM at a concentration of 2000 mg/L were prepared using simulated formation water and aged in an oven at 70 °C for 14 days. Samples were taken at 24 h intervals, and their apparent viscosities were measured using a viscometer. The stability of the two polymer solutions was compared based on these measurements. Each experiment was performed in triplicate, and the results are presented as mean values.

2.5. Enhanced Oil Recovery Experiment

A dual-layer heterogeneous core flooding model (core parameters in Table ) equipped with a saturation monitoring system was employed to simulate reservoir conditions for oil displacement experiments. Each layer was instrumented with 36 electrode pairs to dynamically track oil saturation changes through resistivity measurements. Additionally, five pressure transducers were installed to monitor pressure distribution across the model. The experimental setup is shown in Figure . The procedure was as follows: (1) the core sample was first vacuum-saturated with simulated formation water, with porosity calculated using the mass difference method; (2) Liaohe crude oil was then injected at a constant flow rate of 0.2 mL/min, and the core was aged in an 73 °C oven for 5 days to establish initial reservoir wettability; (3) following this, simulated formation water was injected at 0.6 mL/min until the water cut at the production end reached 98%; (4) subsequently, HPAM/PAAD polymer solutions (both at a concentration of 2000 mg/L) were injected at the same flow rate (0.6 mL/min) for a total of 0.6 pore volumes (PV). After polymer flooding, water flooding was continued until the water cut of the produced fluid returned to 98%. (5) During the experiment, the displacement pressure differential was recorded in real-time using high-precision pressure transducers. Oil and water production volumes were accurately measured at each stage. Based on these data, the recovery efficiency and water cut were calculated for different displacement phases. To further investigate the sweep characteristics of the oil displacement agent in heterogeneous reservoirs, a real-time saturation monitoring system was employed. This system dynamically recorded oil saturation changes across permeability layers at various displacement stages. The monitoring device was designed based on the principle of rock-electrical experiments (electrode method). It characterized the evolution of oil saturation by measuring changes in core resistivity (Supporting Information S1 for detailed experimental procedures). , Specifically, a high-sensitivity resistivity measurement device acquired real-time resistivity values at each monitoring point during different displacement phases. These values were converted using pre-established calibration curves that correlate resistivity with oil saturation for each permeability layer. Ultimately, this process generated saturation field distribution maps for the dual-layer heterogeneous core flood model. , This technique enables real-time visualization of spatiotemporal saturation evolution in heterogeneous reservoirs. It provides a robust experimental method for analyzing the fluid flow behavior of oil displacement agents in complex porous media.

1. Specific Parameters of the Double-Layer Heterogeneous Core Well Pattern Model.

core number L × W × H (cm) permeability (mD) porosity (%) oil saturation (%)
1 30 × 30 × 4.5 low: 100 27.38 72.58
    high: 300    
2 30 × 30 × 4.5 low: 100 27.65 72.41
    high: 300    
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Schematic of the double-layer heterogeneous core flooding setup integrated with a saturation monitoring system.

3. Results and Discussion

3.1. Structural Characterization of DC8 and PAAD

The structures of the DC8 monomer and PAAD polymer were confirmed through 1H NMR spectroscopy (Figure ). For the DC8 monomer, the signals observed in the range of 0.80 to 1.94 ppm correspond to the protons of the long hydrophobic chain derived from n-octane. Specifically, the peaks at 1.741 and 1.287 ppm were assigned to the methylene protons, while the signal at 0.803 ppm was attributed to the terminal methyl protons. The peak at approximately 3.141 ppm was assigned to the –CH3 protons adjacent to the –N+ group, while the peak at about 4.748 ppm was attributed to the deshielded methylene protons connected to the –N+ group. The characteristic signals of the olefinic protons were observed in the range of 5.85 to 6.45 ppm. Specifically, the peaks at 5.969 and 6.155 ppm were assigned to the protons of the terminal vinylidene group (CH2), while the signal at 6.383 ppm corresponded to the proton of the vinylene group (CH−). For the polymer PAAD, the peaks contributed by the hydrophobic chain protons, –CH3 protons, and the –CH2 protons next to –N+ remained the same as in the DC8 spectrum. The peaks corresponding to alkene protons disappeared, and new peaks appeared at 1.257 and 2.341 ppm, indicating the successful copolymerization of the monomers. The presence of singles at 3.332 ppm confirmed the introduction of AMPS. IR spectroscopy was used for structural characterization of the obtained samples (Figure ). The distinctive peaks observed at 3389 cm–1 and 1677 cm–1 were indicative of the –NH2 and –CO stretching vibrations within the amide group (−CONH2). Peaks at 2931 cm–1 and 2862 cm–1 were attributed to the stretching vibrations of methylene groups. The peak at 1173 cm–1 is associated with the –SO3 stretching vibration in the AMPS monomer. The peak at 875 cm–1 is characteristic of the quaternary ammonium salt –N+ stretching in the DC8 component. The successful synthesis of the target polymer was confirmed by both FTIR and NMR spectroscopic analyses. Although quantitative elemental analysis was not performed due to instrumental constraints, the spectroscopic evidence collectively supports the successful incorporation of DC8 into the polymer backbone.

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1H NMR structure characterization of monomer DC8 and the terpolymer PAAD.

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FT-IR spectra of the terpolymer PAAD.

3.2. Scanning Electron Microscopy and Transmission Electron Microscopy Tests

SEM and TEM techniques were employed to investigate the micromorphology of polymer solutions. Figure illustrates the formation of a spatial network aggregate in the polymer PAAD aqueous solution, attributed to the hydrophobic alkyl chains association, resulting in a densely packed network structure.

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Microscopic morphology of the PAAD solution revealed by (a) SEM and (b) TEM.

3.3. Performance Testing of Polymer Solutions

3.3.1. Critical Association Concentration

The polarity of hydrophobic association microdomains in a PAAD solution can be assessed using the pyrene fluorescence probe method. At low PAAD concentrations, the polymer solution primarily exhibits intramolecular association. As the PAAD content increases, the intramolecular association transitions to intermolecular association, leading to the formation of hydrophobic association microdomains. The ratio of the intensity of the first emission peak to the third emission peak (I 1/I 3) serves as an indicator of the solution’s polarity. The I 1/I 3 values of solutions with varying PAAD content are presented in Figure . It is evident from Figure that I 1/I 3 gradually decreases as the polymer content increases. This is attributed to a decrease in solution polarity with increasing polymer content, resulting in the formation of polymer micelles, enhanced hydrophobic association, and a decrease in I 1/I 3. When the PAAD content is below 1.5 g/L, molecular association predominates. In contrast, when the PAAD content ranges between 1.5 and 2 g/L, the I 1/I 3 ratio experiences a sharp decline, signifying a shift toward intermolecular association in the PAAD solution. This transition leads to the formation of more hydrophobic microdomains through the random entanglement of molecular chains. The low polarity of pyrene in hydrophobic microdomains indicates that PAAD exhibits pronounced hydrophobic characteristics and a strong propensity to form a spatial network structure. The I 1/I 3 ratio plateaus and remains relatively constant when the PAAD concentration exceeds approximately 2.0 g/L. This indicates that the polarity of the pyrene microenvironment no longer changes significantly with increasing polymer concentration. We hypothesize that this stabilization occurs because the intermolecular hydrophobic association has reached a saturation point, forming a stable and continuous three-dimensional network structure throughout the solution. Based on the pyrene fluorescence probe method, the CAC of PAAD was determined to be 1.5–2 g/L. Mao et al. and Zhu et al. prepared twin-tailed HAWP with a CAC of 2.5 g/L. By contrast, the polymer PAAD synthesized in this study exhibits a lower CAC value, indicating a stronger intermolecular association at lower concentrations. This phenomenon is primarily attributed to the shorter octane-based hydrophobic chains within PAAD, which reduce steric hindrance effects. Such a structural feature facilitates the aggregation of hydrophobic groups and promotes the formation of more compact associative structures even at relatively low concentration levels.

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Ratio of I 1/I 3 from pyrene fluorescence spectra as a function of PAAD concentration.

3.3.2. Temperature Resistance Performance

In the study, simulated water was utilized to prepare PAAD and HPAM solutions at a concentration of 2000 mg/L, and their viscosity changes under varying temperature conditions were examined. The findings, depicted in Figure , reveal a decrease in viscosity for both polymers as temperature rises. At 90 °C, the viscosity of the PAAD polymer solution reached 23.1 mPa·s, with a corresponding viscosity retention rate of 52.85%; in contrast, the HPAM polymer solution exhibited a viscosity of only 11.9 mPa·s and a retention rate of 32.47%. Compared with HPAM, PAAD always shows a higher viscosity at the same temperature, indicating that it has better temperature resistance. This distinction can be attributed to several factors: HPAM molecules are prone to hydrolysis at elevated temperatures due to the susceptibility of the amide group leading to chain fragmentation. Additionally, salt ions may precipitate by binding with hydrolyzed groups, further diminishing system viscosity. , Conversely, the enhanced temperature resistance of PAAD originates from the incorporation of AMPS in its molecular structure. AMPS contains robust hydratable sulfonic acid groups that effectively counteract hydrolysis under high-temperature conditions. Furthermore, the hydrophobic association structure in PAAD maintains a physical cross-linking network even at elevated temperatures, restraining excessive chain contraction and preserving solution viscosity within a stable range. , Yang synthesized a synthesized a twin-tailed HAWP (DQM2-PAM) that exhibited a viscosity retention rate of merely 38.5% at 85 °C. By contrast, the polymer PAAD utilized in this study demonstrated far more excellent thermal resistance, even under the conditions of the same polymer concentration and a higher temperature.

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Evaluation of the thermal resistance of PAAD and HPAM polymers, demonstrated by the evolution of solution viscosity under high-temperature aging.

3.3.3. Salt Resistance Performance

PAAD and HPAM solutions at a concentration of 2000 mg/L were prepared in simulated water to examine the impact of varying salinity levels on polymer viscosity at room temperature. The findings, depicted in Figure , illustrate a notable decrease in the apparent viscosity of both polymer solutions as salinity levels increase. Under the condition of a salinity of 26,250 mg/L, the viscosity of the PAAD polymer solution reached 25.1 mPa·s, with a corresponding viscosity retention rate of 38.03%; in contrast, the HPAM polymer solution exhibited a viscosity of only 10.3 mPa·s and a retention rate of 15.37%. This decline can be attributed to the ions generated through the dissociation of electrolyte salt, which shield the charged groups along the polymer molecular chain. Consequently, the electrostatic repulsion among the molecular chains weakens, leading to a transition in conformation from an extended to a coiled state and ultimately resulting in reduced solution viscosity. , Furthermore, it is noteworthy that salt ions have the capacity to enhance the polarity of the solution, thereby promoting the association between hydrophobic groups within the molecular chains. This phenomenon contributes to a decrease in viscosity for HAWP. Additionally, the robust hydration of sulfonic acid groups in the AMPS monomer imparts a degree of salt tolerance to the polymers, enabling them to retain certain viscosity characteristics even under elevated salinity conditions. Liu employed a HAWP that merely reached a viscosity of 15 mPa·s under a salinity condition of 20,000 mg/L. By contrast, the polymer PAAD developed in this study exhibited significantly superior salt tolerance even when subjected to the same polymer concentration and a higher salinity environment.

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Salt tolerance of PAAD and HPAM polymers, evaluated by measuring the viscosity retention in brines with different salinities.

3.3.4. Shear Resistance of Polymer Solutions

The shear stability of polymers generally refers to their ability to resist high-stress conditions in high-flow-rate regions. In this study, core flooding experiments were conducted using 6 m-long artificial cores to simulate reservoir conditions and evaluate polymer shear resistance through porous media flow. The results (Figure ) demonstrate that as polymer solutions propagate further through the core medium with prolonged shear exposure, their viscosity retention decreases. After being sheared by the porous medium of the long core, the viscosity of the PAAD polymer solution reached 8.6 mPa·s, with a corresponding viscosity retention rate of 18.92%; in contrast, the HPAM polymer solution exhibited a viscosity of only 1.5 mPa·s and a retention rate of 3.57%. Mechanistic analysis reveals that HPAM suffers significant viscosity loss due to molecular structure degradation under shear. In contrast, the physically cross-linked three-dimensional network of HAWP can reform through associative interactions after shear disruption, granting them partial shear recovery capability and higher retained viscosity. These findings collectively demonstrate that the synthesized hydrophobically associating polymer exhibits better temperature tolerance, salt resistance, and shear stability compared to conventional HPAM.

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Transport behavior of the PAAD solution through porous media, shown by the variation of solution viscosity with migration distance.

3.3.5. Long-Term Stability

The long-term viscosity stability of polymer solutions is a critical parameter for characterizing the durability of oil displacement systems within the porous media of oil reservoirs. Under reservoir conditions, the long-term stability of polymer solutions directly affects their effective viscosity during propagation through the porous medium. The stability of two types of polymers was evaluated by comparing the evolution of their viscosities over aging time under identical experimental conditions. This study investigated the thermal stability of PAAD and HPAM at 70 °C over various aging durations, as summarized in Figure . As shown in Figure , the viscosities of both the hydrophobically associating polymer PAAD and HPAM decreased gradually with prolonged aging time. After 14 days of aging, the viscosity of PAAD remained at 21.6 mPa·s, corresponding to a retention rate of 80.89%, whereas that of HPAM was only 10.3 mPa·s, with a retention rate of 65.19%. Notably, the viscosity of PAAD was consistently higher than that of HPAM throughout the entire aging process. These results demonstrate that the PAAD-type hydrophobically associating polymer exhibits superior thermal stability.

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Long-term stability of the PAAD solution, assessed by the viscosity retention over time.

3.3.6. The Surface Tension of the Polymer Solution

The interfacial tension between oil and water plays a crucial role in chemical flooding for enhanced oil recovery. In this study, the interfacial tension of several solutions was measured using an interfacial tensiometer. The solutions tested included HPAM, PAAD, and a combination of HPAM with petroleum sulfonate (WPA, mass fraction: 0.2%), all at a concentration of 2000 mg/L and a temperature of 25 °C, as shown in Figure . The results indicate that the PAAD exhibits a lower oil–water IFT than HPAM, reaching an order of magnitude of 10–1 mN/m, demonstrating its ability to reduce surface tension. This behavior can be attributed to the hydrophobic chains in PAAD, which tend to escape from the aqueous phase due to their hydrophobicity, forming an adsorption layer at the water–oil interface and thereby reducing IFT. However, compared to small-molecule surfactants, hydrophobically associating polymers exhibit limited IFT reduction capability. This is because, when adsorbed at the interface, the bulky polymer chains occupy significant space, resulting in a low surface concentration of adsorbed molecules and thus a weaker IFT-lowering effect. It should be noted that the hydrophobicity of PAAD was inferred through pyrene fluorescence probing and interfacial tension measurements. Although these are established indirect methods, direct quantification of the hydrophobic–hydrophilic balance (e.g., via contact angle, HLB, or solubility parameter analysis) was not feasible under current experimental conditions and could be considered in future studies.

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Surface tension of PAAD, HPAM, and surfactant solutions as a function of concentration.

3.4. Enhanced Oil Recovery Testing

To investigate the differential oil displacement performance between HPAM and PAAD, core flooding experiments were conducted under controlled laboratory conditions. As illustrated in Figure , during the initial water flooding stage, both displacement pressure and oil recovery factor exhibited increasing trends with continuous brine injection. However, as preferential flow channels developed, the displacement pressure gradually stabilized while the recovery rate increment significantly diminished. Water flooding was terminated at a produced fluid water cut of 98%, achieving ultimate water flooding recoveries of 31.8% and 31.6% respectively. Subsequent polymer flooding using brine-formulated polymer solutions. The higher viscosity of polymer solutions induced a substantial pressure increase during injection, accompanied by a pronounced reduction in produced water cut. Polymer flooding results revealed final recovery factors of 45.7% and 55.8% for HPAM and PAAD solutions respectively, representing significant enhancements of 13.9% and 24.2% compared to water flooding. Mechanistic analysis indicates that PAAD’s superior performance stems from its pronounced hydrophobic association effects in aqueous solutions. This unique characteristic enables the polymer to maintain elevated solution viscosity under high-temperature and high-salinity conditions, thereby effectively improving the oil–water mobility ratio. Furthermore, direct interaction between PAAD’s hydrophobic groups and crude oil induces partial emulsification, forming oil-in-water emulsions that reduce crude oil viscosity and enhance flow ability. Through these synergistic mechanisms, the displacement fluid achieves more efficient mobilization of residual oil, ultimately yielding significantly improved recovery efficiency. Guangfan synthesized a twin-tailed hydrophobically associating polymer (designated as PASSDD) using didecyl methallyl ammonium chloride as the hydrophobic monomer. At a polymer concentration of 2000 mg/L, oil displacement with PASSDD achieved an incremental oil recovery of 15.40%. Notably, under the same concentration and even higher temperature conditions, the polymer PAAD prepared in this study exhibited a more pronounced enhancement in oil recovery efficiency.

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Evolution of pressure differential and oil recovery with cumulative injected pore volume during the core flooding process for (a) HPAM and (b) PAAD solutions.

This research conducted core flooding experiments incorporating real-time oil saturation monitoring technology. The experimental apparatus, based on petrophysical response principles, enabled dynamic monitoring of oil–water distribution patterns during the displacement process. A systematic investigation was performed to evaluate the flooding performance of two polymers: HPAM and PAAD. Figures and present the oil saturation distribution characteristics at different stages of polymer flooding for both polymers. Analysis of Figure a–c reveals that in high-permeability reservoirs following water flooding, crude oil was primarily displaced along the main flow channels, while residual oil accumulated in peripheral regions. After polymer injection, the sweep area expanded significantly, leading to further reduction of residual oil in edge zones. Grid-based saturation calculations indicated that the high-permeability layer achieved a recovery factor of 42.2% during water flooding, with an additional 11.4% improvement after HPAM polymer flooding. Figure a–c demonstrate similar displacement characteristics, showing a water flooding recovery of 42.2% and a 17.5% enhancement after PAAD polymer injection. Comparative analysis of displacement effects in high- and low-permeability layers revealed that water flooding mainly mobilized oil in high-permeability zones, while residual oil in low-permeability layers concentrated along the flanks of main flow channels Figure d–f. Polymer injection significantly improved sweep efficiency in low-permeability regions due to enhanced mobility ratios, effectively mobilizing residual oil in these peripheral areas. Grid calculations showed the low-permeability layer achieved 25.4% recovery during water flooding, with HPAM providing a 9.5% incremental recovery. Figure d–f demonstrate PAAD’s superior performance in low-permeability zones, with water flooding recovery of 25.2% and a remarkable 24.3% improvement after polymer injection. Systematic grid-based calculations (Table ) indicated minimal differences between the two polymers in terms of displacement efficiency and sweep coefficient in high-permeability layers. However, for low-permeability layers, PAAD exhibited significantly better performance in both displacement efficiency and sweep coefficient compared to HPAM. This superior performance can be attributed to PAAD’s unique hydrophobic associative structure, which proves particularly effective in mobilizing residual oil from low-permeability zones. The results demonstrate that the hydrophobically associating polymer PAAD possesses excellent oil displacement properties, showing particular advantages in recovering residual oil from low-permeability formations. These findings suggest strong potential for PAAD application in tertiary oil recovery operations in oilfields.

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Oil saturation distribution during the HPAM flooding process. (a–c) High-permeability layer: (a) initial state, (b) after water flooding, (c) after polymer flooding. (d–f) Low-permeability layer: (d) initial state, (e) after water flooding, (f) after polymer flooding. (Triangles and stars denote injectors and producers, respectively).

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Oil saturation distribution during the HPAM flooding process. (a–c) High-permeability layer: (a) initial state, (b) after water flooding, (c) after polymer flooding. (d–f) Low-permeability layer: (d) initial state, (e) after water flooding, (f) after polymer flooding. (Triangles and stars denote injectors and producers, respectively).

2. Sweep Coefficients of Each Permeability Layer and the Oil Displacement Efficiency after Polymer Flooding.

    layer position
polymer type experimental parameters high permeability layer low permeability layer
HPAM displacement efficiency (%) 62.7 50.8
  sweep efficiency (%) 85.4 68.6
  individual layer recovery (%) 53.5 34.8
  total recovery rate (%) 45.7
PAAD displacement efficiency (%) 65.6 61.9
  sweep efficiency (%) 91.1 80.4
  individual layer recovery (%) 59.8 49.8
  total recovery rate (%) 55.8
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4. Conclusions

This study successfully synthesized a novel Gemini-type hydrophobically associating polymer through free radical copolymerization of AM, AMPS, and a specially designed Gemini surfactant monomer. Comprehensive structural characterizationincluding 1H NMR, FTIR, SEM, and TEMconfirmed the successful formation of the target polymer. Systematic performance evaluations further demonstrated its outstanding potential for EOR, particularly under harsh reservoir conditions. The key findings of this work offer promising solutions to technical challenges associated with developing high-temperature, high-salinity oilfields, especially those with heterogeneous, low-permeability formations. In comparison with conventional HPAM, PAAD exhibited significantly enhanced thermal stability, salt tolerance, and shear resistance. This improvement effectively alleviates the viscosity loss and structural degradation commonly observed in traditional polymers under high-temperature and high-salinity environments. More importantly, heterogeneous core flooding experiments revealed that PAAD can effectively mobilize residual oil from low-permeability zones. This ability addresses a major limitation of conventional EOR polymers. The mechanism involves maintaining stable solution viscosity and inducing mild emulsification of crude oil. These contributions enhance both macroscopic sweep efficiency and microscopic displacement efficiency, offering a practical technical pathway for tertiary oil recovery in mature fields with declining production. Overall, these findings establish a solid theoretical and experimental basis for the industrial application of HAWP in high-temperature, high-salinity reservoirs. This approach holds the potential to reduce reliance on more complex and costly combination flooding techniques.

Despite these promising results, several limitations of this study should be noted. First, the synthesis of the Gemini surfactant monomer currently demands stringent reaction conditions. These include precise temperature control and extended reaction times, which lead to relatively low yield. Consequently, the process may involve elevated production costs. Such limitations could impede large-scale industrial application. Second, the performance evaluations were conducted under laboratory-simulated reservoir conditions. Thus, long-term stability studies (e.g., aging over several months) and compatibility tests with actual reservoir fluids, particularly those with high asphaltene content, are still needed.

Future work will focus on the following directions to address these limitations and enhance the application potential of PAAD. First, the synthesis route of the Gemini monomer will be optimized to increase yield and reduce cost. Specific strategies may include identifying more efficient catalysts, simplifying reaction steps, or adjusting reactant ratios. Second, molecular dynamics simulations will be combined with experimental analyses to elucidate the structure–property relationships of PAAD. This will involve investigating aspects such as the effect of Gemini tail length or hydrophobic group density on its association behavior and EOR performance. Such insights will facilitate the rational design of tailored polymer systems. Third, extended experimental validations will be conducted under more field-relevant conditions. These will include long-term aging tests, compatibility assessments with real reservoir fluids, and pilot-scale flooding experiments in geological models that closely mimic realistic formation heterogeneity. Additionally, exploring synergistic effects between PAAD and low-dose surfactants or nanomaterials will be prioritized. This aims to enhance oil–water interfacial activity and improve residual oil mobilization efficiency, while maintaining manageable chemical costs.

Supplementary Material

ao5c07872_si_001.docx (650.7KB, docx)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 52174023).

Glossary

Abbreviations

EOR

enhanced oil recovery

HPAM

partially hydrolyzed polyacrylamide

CAC

critical association concentration

HAWP

hydrophobically associating polymers

1H NMR

hydrogen nuclear magnetic resonance

All data generated or analyzed during this study are included in this published article.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c07872.

  • A brief description of the principle and experimental steps of the oil saturation detection device. Experiment on the relationship between saturation and electrical resistance: (1) experimental materials, (2) experimental apparatus, (3) experimental procedure. Figure S1: Schematic diagram of the experimental setup for establishing the resistance–oil saturation relationship. Figure S2: Connection diagram of the experimental setup for saturation monitoring (DOCX)

Li Liu, Shuaishuai Zhao and Zhiqiang Wang conceptualized and designed the experiments and contributed to the first manuscript draft; Li Liu procured funding and supervision; Ende Zhan, Jinxin Liu, Jinyun Wei, Xuliang Fan and Mengyi Xing analyzed the data, wrote and edited the manuscript. All authors edited and revised the manuscript critically, and agreed to submit it to this journal.

The authors declare no competing financial interest.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao5c07872_si_001.docx (650.7KB, docx)

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Articles from ACS Omega are provided here courtesy of American Chemical Society

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