Development and performance evaluation of macropore intelligent plugging hydrogels

Abstract

Polymer hydrogels are the optimal hydrogels for water channeling control applications, and water-dominant channels in deep reservoirs are often blocked by them.A dual-network hydrogel system (DN-Gel) is innovatively proposed in this study. A chemical crosslinking network (ACL-HG) is constructed via glycidyl methacrylate (GMA), poly(ethylene glycol) methyl ether methacrylate (PEGMA), and dopamine methacrylamide (DMA), and is composited with a physical crosslinking network (GQDs-Gel) of functionalized graphene quantum dots (GQDs-GX)/ascorbic acid. A dense dual-network structure is formed through the synergistic effect of Michael addition reaction and hydrogen bonding.FTIR and SEM characterization results show that DN-Gel possesses both the functional group characteristics of the chemical crosslinking network and the nano-reinforcement effect of the physical crosslinking network, and its cross-sectional pore structure is significantly denser than that of single-network hydrogels.Under the optimal ratio and concentration, the viscosity reduction rates of DN-Gel both before and after three shearing cycles within 56 days are maintained below 20%.Under high-temperature and high-salinity conditions, a mass retention rate of over 80.6% is achieved after 60 days.Core simulation results show that DN-Gel has a breakthrough pressure gradient of 129.21 MPa/m and a permeability change rate as high as 99.21%, which is significantly improved compared with traditional plugging agents.This study provides a new material design method and theoretical basis for the green and efficient plugging of sandstone reservoirs.

Introduction

In the development process of sandstone reservoirs, water channeling is considered one of the constraining factors that limit high and stable production of the reservoir^1–3.The water channeling problem in high-permeability sandstone reservoirs is targeted.Hydrogel materials have been frequently applied to issues such as water plugging in high-permeability layers due to their properties like high water absorption and swelling capacity as well as self-healing ability^4–6.However, in the actual process of selecting hydrogels, emphasis is often placed solely on the physicochemical properties of the hydrogel itself, such as strength stability, humidity response time, and mechanical-adhesive performance^7–9.The physicochemical interactions between the gel and the rock interface are frequently overlooked. Therefore, hydrogels used to address water intrusion in high-permeability sandstone reservoirs should be endowed with strong tensile and compressive resistance, and possess good stability and effective permeability modification under high-temperature and high-salinity environments^10–12.Gel-based profile control agents, due to the high gel strength generated by their three-dimensional network structure, have been shown to have significant advantages in solving water channeling and have achieved good results, as shown in Table 1 below.

Table 1.

Application of gel-based adjustment agent in oil fields.

Composition of the adjustment agent	Effect evaluation
WPL−12 Gel (8000 ~ 10000)mg/L + (6000 ~ 8000)mg/L composite organic phenolic crosslinking agent13	Water content reduction: 3% increase in oil volume: 8374.1m3
0.6%~0.75% HY−30 polymer + 0.3%~0.35% crosslinking agent14	Average water content reduction: 4.05% Increase in daily oil production: 26.91m3
0.4% hydrophobic conjugated polymer + 0.3%~0.35% organic aluminum zirconium crosslinking agent15	The core permeability rate with permeability of 2000 × 10 − 3㎛2 is more than 80%
0.6% emulsion acrylamide + 0.3% phenolic resin composite crosslinking agent16	Average water content decrease: 5.3% increase in oil volume: 8765m3
HPAM+ crosslinking agent17	Water content decrease: 6.4% increase in oil volume: 6 t
HPAM + organic chromium18	Water content decrease: 9.35% Cumulative increase in oil volume: 243 m³

In practical application processes, acrylamide-based hydrogels have been widely used as water channeling treatment agents for high-permeability sandstone due to their high swelling rate. However, problems such as the attenuation of mechanical properties under dynamic stress, the failure of adhesion at gas-liquid interfaces, and chemical aging and structural degradation have been faced in its application under underground working conditions. In the use stage, the chemical degradation products of polyacrylamide hydrogels pose ecological toxicity, readily causing acute toxicity to aquatic organisms and suppressing soil microbial activity^19,20. Consequently, a novel hydrogel sealing system that is practical, reliable, and whose decomposed elements are pollution-free is urgently needed. The performance challenges faced by single crosslinked network hydrogels in practical applications can be addressed by introducing another hydrogel network to complement their properties. Network hydrogels enhance the performance of hydrogels through the synergistic effect of two networks, while preserving the unique properties of each individual network^21–23. Dual network hydrogels achieve a dynamic balance between mechanical strength and toughness through the collaborative interpenetrating structure of rigid and flexible networks, along with precise regulation of their proportions. They have emerged as the core development trend for the next generation of sealing materials. Their design concept stems from the structural mimicry of biological tissues such as cartilage and tendons in bionics, where the rigid covalent network bears the primary load, while the dynamic, reversible physical crosslinking network dissipates energy and inhibits crack propagation^24,25.

As a natural organic polymer material, lignin can not only improve the water absorption, hydrophilicity and mechanical properties of hydrogel, but also improve the thermal stability and biodegradability.Lignin hydrogels are rich, environmentally friendly, biocompatible and biodegradable²⁶. As a water channeling inhibitor, after being used for a period of time, if the hydrogel needs to be discarded, it can be decomposed by microorganisms in the formation into harmless small molecular substances and will not cause long-term pollution to the formation and the surrounding environment²⁷.The crosslinking properties of water molecules rapidly induce the rearrangement of amphiphilic polymers at interfaces. This rearrangement, facilitated by reversible electrostatic interactions and hydrogen bonding, enhances the structural stability of the hydrogel, allowing it to maintain excellent performance even in water-intruded environments^28,29.The catechol structure, as an effective structure for the strong underwater adhesion of mussels, is capable of forming non-covalent interactions such as hydrogen bonds, metal coordination bonds, and π-π bonds with multiple surfaces. It can exhibit good adhesion to rocks underwater, and this structure can realize intelligent and selective viscosity increase when in contact with water^30,31.In this paper, considering the aforementioned factors, lignin monomers, amphiphilic polymer monomers, and catechol monomers are selected to form multivalent bonding, resulting in a highly stable and multifunctional “cross-linking” effect.Incorporating nanomaterials into hydrogels for physical crosslinking is a commonly used and effective method to enhance their mechanical strength. Graphene quantum dots (GQDs) exhibit structural stability, a high specific surface area, and multiple active sites, conferring structural advantages in functional modifications.Hydrogels prepared using GQDs can be dispersed within other gels, bolstering the overall structural stability of double-network hydrogels^32,33.A double-network structural design composed of chemical-physical-chemical crosslinking is formed.The crosslinking density of the hydrogel network can be increased.The mechanical properties of the system are effectively enhanced see fig 1.

Fig. 1.

Spontaneous underwater adhesion mechanism.

This study innovatively proposes a chemical crosslinking network constructed by glycidyl methacrylate (GMA), poly(ethylene glycol) methyl ether methacrylate (PEGMA), and dopamine methacrylamide (DMA).The physical crosslinking network constructed by graphene quantum dots (GQDs-GX) and ascorbic acid (L(+)) is combined with the aforementioned chemical crosslinking network.A water-thickening dual-network hydrogel is constructed.In this system, DMA and GMA are used in the chemical crosslinking part to carry out Michael addition, and a branched structure is formed.The bifunctional characteristic of PEGMA is utilized to enable it to act as a “linker arm”.Different branched chains are bridged via two Michael addition reactions, and a crosslinking network with a specific crosslinking density is formed.In the physical crosslinking part, a physical crosslinking network is formed through intermolecular forces between graphene quantum dots (GQDs-GX) and ascorbic acid (L(+)).The structure of the synthesized product is characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM).Shear resistance experiments, high-temperature stability experiments, high-temperature and salt tolerance experiments, and core permeability change experiments are carried out in this study.Characteristics including shear stability, high-temperature stability, salt tolerance, spontaneous underwater adhesion, and water-thickening property are exhibited by this hydrogel.It demonstrates great potential in plugging large pores of high-permeability reservoirs.

Results and discussion

Structural characterization of ACL-HG and GQDs-GX

The structure of the prepared ACL-HG is characterized by the liquid film method. Figure 2 shows the Fourier transform infrared (FTIR) spectrum.Amino groups, carbon-carbon double bonds in acrylamide, amide bonds, aromatic groups, and benzene ring hydroxyl groups are included in the molecular structure of DMA.Broad peaks at 3291 cm⁻¹ and 3017 cm⁻¹ can be observed, which are regarded as the superposition of the stretching vibration broad peaks of amino groups (-NH) and phenolic hydroxyl groups (-OH).C-N stretching vibration absorption peak at 1285 cm⁻¹ is also observed, and the existence of amino groups (-NH) is confirmed thereby.

Fig. 2.

ACL-HG infrared spectrum and infrared contrast curve of monomer.

Functional groups including epoxy groups, carbon-carbon double bonds, and carbon-oxygen double bonds are contained in the molecular structure of GMA.A stretching vibration peak at 1724 cm⁻¹ can be observed, and the presence of C = O bonds is inferred therefrom.

Functional groups including carbon-carbon double bonds, carbon-oxygen double bonds, carbon-oxygen single bonds, and hydroxyl groups are contained in the molecular structure of PEGMA.1723 cm⁻¹ corresponds to the stretching vibration of the C = O bond in the PEGMA structure, and 1095 cm⁻¹ corresponds to the stretching vibration of the C-O-C bond.

Catechol characteristic groups (phenolic hydroxyl groups and amino groups) from DMA, carbonyl groups from GMA, and carbonyl groups from PEGMA are included in the molecular structure of ACL-HG.

The Fourier transform infrared (FTIR) spectra of the prepared functionally modified graphene quantum dots (GQDs-GX) and graphene quantum dot hydrogels (GQDs-Gel) are shown in Fig. 3.The peaks at 1461 cm⁻¹, 1405 cm⁻¹, and 1264 cm⁻¹ in the GQDs-GX sample are assigned to the C-N stretching vibration, O-H stretching vibration, and C-OH stretching vibration, respectively.The presence of -OH and N-H is indicated by the broad bands centered at 3405 cm⁻¹ and 3168 cm⁻¹, respectively.

Fig. 3.

Infrared spectral curves of GQDs-GX and GQDs-Gel.

The presence of -NH₂ and -OH functional groups on the surface of the synthesized GQDs-GX is indicated by the FTIR results.The characteristic absorption peaks at 1638 cm⁻¹, 2842 cm⁻¹, and 1080 cm⁻¹ are assigned to the stretching vibrations of C-C, C = N, and sulfonic acid groups, respectively, which demonstrates that GQDs-GX has been successfully prepared.It is indicated by the characteristic absorption peaks at 1264 cm⁻¹ and 1080 cm⁻¹ in the GQDs-Gel sample that the C-OH and sulfonate groups of GQDs-GX are involved in the gel network, and the target functional groups are contained in the structure of the prepared GQDs-Gel.

Figure 4 shows the comparative Fourier transform infrared (FTIR) spectra of GQDs Gel, ACL-HG, and DN-Gel.It can be found by comparing the three curves that the characteristic absorption peaks of the stretching vibrations of hydroxyl groups and sulfonate groups from the GQDs Gel structure (1262 cm⁻¹ and 1076 cm⁻¹), along with the C-N stretching vibration absorption peak from the ACL-HG structure (1286 cm⁻¹), are contained in the infrared spectrum of DN-Gel.The characteristic absorption peak of the symmetric stretching vibration of the S = O bond in the -SO₃ group is particularly noted to appear near 1080 cm⁻¹.It could be indicated by the experimental results that the target functional groups were contained in the graphene hydrogel (DN-Gel).Consistency is shown with the double-network structural design constructed by GQDs Gel and ACL-HG.

Fig. 4.

FTIR Spectra of GQDs Gel, ACL-HG, and DN-Gel.

SEM analysis

Figure 5 shows the cross-sectional SEM images of the freeze-dried GQDs Gel, ACL-HG, and DN-Gel samples.

Fig. 5.

Cross-section SEM images of samples.

Figure 5 reveals that the freeze-dried ACL-HG features a typical honeycomb pore structure corresponding to a single-layer ordinary crosslinked gel network (Fig. 5a).The freeze-dried GQDs Gel features a partially protruded pore-like structure (Fig. 5b).The freeze-dried DN-Gel exhibits an extremely dense structure with finer pores (Fig. 5c).

This may be explained by the fact that, during the process in which the double-network structure of DN-Gel is formed.GQDs Gel is dispersed within the ACL-HG network, by which the original free volume space of the ACL-HG network is occupied.The mobility of molecular chain segments is reduced, and the formation of micropores is suppressed.The hydroxyl and carboxyl groups present on the surface of graphene quantum dots in GQDs Gel may be involved in the formation of hydrogen bonds or electrostatic interactions with the polymer chains of ACL-HG.The interfacial bonding is enhanced, by which a denser network is resulted in.

DN-Gel dual-network hydrogel ratio optimization

Optimize the system ratio

The effective substance concentration of the hydrogel is fixed at 1500 mg/L before crosslinking.The influences of different mass ratios (ACL-HG to GQDs Gel) on system viscosity and gelation time are studied, and the experimental results are depicted in Fig. 6.The experimental results reveal that the system viscosity increases remarkably with the increasing mass proportion of ACL-HG, peaking at 3922 mPa·s at a mass ratio of 1.5:1.ACL-HG is a chemical crosslinked network that forms a rigid and dense structure through covalent bonding.GQDs-Gel is a physical crosslinked network with lower structural stability than chemical crosslinking.An increased proportion of ACL-HG leads to a significant rise in the number and density of chemical crosslinking points, strengthening the confinement of molecular chains by covalent bonds. This enhances intramolecular friction in the system, resulting in a macroscopic increase in viscosity.At a mass ratio below 1.5:1, the crosslinking points in ACL-HG’s chemical crosslinked network are unsaturated.The physical crosslinking of GQDs-Gel efficiently fills the pores of the chemical network, and their interpenetration forms a double network structure. This rapidly boosts crosslinking density and network entanglement, resulting in a remarkable viscosity increment.At a mass ratio above 1.5:1, the crosslinking points of ACL-HG’s chemical crosslinked network are saturated, and the low proportion of GQDs-Gel significantly weakens the pore-filling effect of physical crosslinking on the double network, resulting in a decreasing trend of system viscosity.The double network system takes over 14 days to attain final gelation, with no notable changes in the gel thereafter.Hence, the mass ratio of 1.5:1 is selected as the optimal experimental formulation for subsequent performance assessments.

Fig. 6.

Effect of Different Mass Ratios on the Viscosity of the Plugging System.

Optimization of the total mass concentration of the system

With the mass ratio of ACL-HG to GQDs Gel fixed at 1.5:1, the influences of different system concentrations on viscosity and gelation time are investigated, and the experimental results are depicted in Fig. 7.Increasing system concentration elevates the number of chemical and physical crosslinking points, enhancing molecular chain entanglement and collision frequency, which substantially boosts internal friction resistance.Hence, the system viscosity increases with increasing concentration.At 500 mg/L, the low system concentration results in a relatively large intermolecular distance, and the weak chemical and physical crosslinking strength fails to construct a dense double network structure.At concentrations above 500 mg/L, the system viscosity increases sharply within 1–7 days. Rapid synergy between chemical and physical crosslinking forms a dense double network, leading to a rapid viscosity rise with network formation.From 8 to 25 days, the double network structure stabilizes, with intermolecular interactions and network compactness reaching equilibrium, resulting in negligible changes in system viscosity.From 26 to 40 days, the system exhibits slight swelling. The reversibility of physical crosslinking leads to a marginal decrease in viscosity, yet the overall level remains high.The viscosity increment from 1500 to 2000 mg/L is notably smaller than that from 500 to 1000 mg/L.Once the concentration reaches a certain threshold, the viscosity-promoting effect of further increasing concentration diminishes gradually. Considering cost-performance balance, 1500 mg/L is selected as the optimal concentration for subsequent performance evaluations.

Fig. 7.

Effect of different concentrations on the viscosity of the plugging agent system.

For all subsequent experiments, the experimental materials are used with a mass ratio of m(ACL-HG): m(GQDs Gel) = 1.5:1 and a total system mass concentration of 1500 mg/L.

Shear stability performance of hydrogels

Figure 8 depicts the shear stability test of DN-Gel, utilizing the hydrogel with m(ACL-HG): m(GQDs Gel) = 1.5:1 and a concentration of 1500 mg/L. Figure 9 shows the shear resistance test of the single-network ACL-HG at 1500 mg/L.At the same shear point, viscosity decreases significantly with increasing shear rate.The physical crosslinking network of GQDs Gel features damage-reversible weak intermolecular forces. Shear force application induces rapid weak bond cleavage, resulting in macroscopic viscosity reduction.The chemical crosslinking network of ACL-HG exhibits damage-irreversible strong intermolecular forces. It retains the basic network skeleton even at high shear rates, granting the gel system a low viscosity damage rate.After initial shearing, both DN-Gel and ACL-HG exhibit a viscosity retention rate exceeding 80% at 500 r/min.At 1500 r/min (high shear rate), ACL-HG exhibits a viscosity retention rate of merely 67%, notably lower than that of DN-Gel.Three high-speed shearing tests are carried out within 56 days at 14-day intervals.After each shearing, GQDs-GX active sites in the physical crosslinking network rapidly bind non-covalently to ACL-HG long chains.Meanwhile, only a small fraction of the crosslinked skeleton suffers irreversible cleavage.Thus, DN-Gel shows a viscosity reduction of less than 20% before each of the three shearings and 14 days post-shearing, while ACL-HG exhibits lower viscosity recovery than DN-Gel under the same conditions.Viscosity testing after 180 days shows that at 500 r/min, DN-Gel’s viscosity further decreases compared with 56 days but remains above 60% of the initial value.In contrast, ACL-HG retains merely 34% of its initial viscosity.The dynamic equilibrium of physical crosslinking is crucial: upon long-term storage, the chemical crosslinking network suffers reduced physical entanglement points between molecular chains and chain fracture, while sulfonate/hydroxyl groups in GQDs-GX re-form hydrogen bonds with the catechol structure of ACL-HG, sustaining high network density.

Fig. 8.

Shear stability of DN-Gel at a concentration of 1500 mg/L.

Fig. 9.

Shear stability of ACL-HG at a concentration of 1500 mg/L.

Analysis of the thermal decomposition

TGA tests are performed to investigate the weight loss of ACL-HG and DN-Gel, heating from 25 °C (room temperature) to 500 °C. Figure 10 depicts the weight% changes of the two hydrogel samples upon heating.As shown in Fig. 10, the weight loss of the two samples upon heating from 25 °C to 200 °C is due to water evaporation.No precipitous weight loss occurs when heating up to 200 °C, indicating its structural stability at elevated temperatures.Heating DN-Gel to 200 °C induces oxidative decomposition of ascorbic acid, resulting in a marked increase in weight loss.The amide bonds and PEG segments in ACL-HG’s main chain undergo thermal decomposition at 340 °C.DN-Gel retains a residual mass of 18.03% after heating to 500 °C.ACL-HG maintains a consistent mass loss rate up to 360 °C.After ACL-HG is heated to 500 °C, a pyrolysis residue of 32.04% is obtained.

Fig. 10.

Thermogravimetric analysis of DN-Gel and ACL-HG.

Temperature and salt resistance analysis of the hydrogel

The stability of DN-Gel under high-temperature (60 °C, 75 °C, 90 °C) and high-salinity (6000–10000 mg/L) conditions is evaluated by measuring its weight changes.As shown in Fig. 11: within 60 days, the mass retention rate of DN-Gel is higher than 80.6%.Under the same salinity, ACL-HG exhibits a much lower mass retention rate than DN-Gel, with the modified graphene quantum dots in GQDs Gel enhancing DN-Gel’s temperature and salt resistance.

Fig. 11.

Mass retention rates of DN-Gel and ACL-HG: (a) Mass retention rates of gels with different salinities at 60 °C; (b) Mass retention rates of gels with various salinities at 75 °C; (c) Mass retention rates of gels with different salinities at 90 °C.

Core water flooding permeability changes

Experimental results are shown in Table 2. The breakthrough pressure, defined as the pressure when the first drop of liquid exits the core outlet, ranges from 6.9 MPa to 12.9 MPa.During the process in which the water shutoff agent is injected, the injection pressure is consistently maintained at a relatively low level.During the process in which fresh water is injected after gelation, the injection pressure is continuously increased.This is likely due to DN-Gel’s water absorption and swelling capacity; the catechol structure of DMA in the hydrogel oxidizes to quinone structures upon water contact, forming strong coordination bonds with metal ions on the rock surface and significantly improving contact interface adhesion.Upon the hydrogel reaching swelling equilibrium, DN-Gel exhibits a breakthrough pressure gradient of 69.54–129.21 MPa/m and a permeability change rate of 98.43%–99.21%, both exceeding those of conventional gel-type water shutoff agents.Thus, the developed water shutoff agent demonstrates superior permeability modification efficiency.

Table 2.

Breakthrough pressure and change rate of DN-Gel.

Core number	Water permeability measurement(K)		Break through the pressure(MPa)	Break through the pressure gradient(MPa/m)	Permeability rate of change(%)
	K1	K2
1	2003.61	31.46	6.9	69.54	98.43
2	1021.68	12.77	7.4	74.68	98.75
3	498.09	4.23	10.5	105.42	99.15
4	102.33	0.8	12.9	129.21	99.21

Conclusion

1. The optimal synthesis conditions for DN-Gel (predominantly consisting of ACL-HG and GQDs Gel) are a mass ratio of m(ACL-HG): m(GQDs Gel) = 1.5:1 and a total system mass concentration of 1500 mg/L.Under these conditions, the system exhibits a gelation time of ~ 14 days and a maximum viscosity of 3922 mPa·s, forming a stable double network structure.The chemical crosslinking network (ACL-HG) offers a rigid backbone via covalent bonds, while the physical crosslinking network (GQDs-Gel) fills the chemical network pores through weak interactions; their interpenetration and entanglement enhance the crosslinking density.Below a mass ratio of 1.5:1, physical crosslinking filling is inadequate; above this ratio, chemical crosslinking saturation induces network redundancy. At 1500 mg/L, the system exhibits high molecular chain entanglement alongside economic feasibility.

2. FTIR spectra confirm the presence of GQDs-Gel-derived hydroxyl (1262 cm⁻¹) and sulfonate (1076 cm⁻¹) characteristic peaks, as well as the ACL-HG-derived C-N stretching vibration peak (1286 cm⁻¹) in DN-Gel.SEM images reveal that DN-Gel exhibits a dense fine-pore cross-sectional structure, distinctly differing from the honeycomb pores of ACL-HG and the local protrusions of GQDs-Gel.As nanofillers, graphene quantum dots (GQDs-GX) disperse in the ACL-HG network. Their surface hydroxyl and amino groups form hydrogen bonds and electrostatic interactions with ACL-HG molecular chains, suppressing micropore formation, enhancing interface bonding.and ultimately a dense network structure characterized by “chemical-physical crosslinking” is formed.

3. At 1500 r/min (high shear rate), DN-Gel exhibits a viscosity reduction of < 20% after three shear cycles and retains > 60% of its initial viscosity after 180 days, whereas the single-network ACL-HG retains only 34%.ACL-HG’s covalent bonds sustain the basic network skeleton and resist high-shear destruction. GQDs-Gel’s reversible weak physical crosslinking allows rapid hydrogen bond reconstruction between GQDs-GX active sites and ACL-HG long chains post-shearing, realizing damage self-healing. This dynamic equilibrium enhances overall mechanical performance and stability.

4. TGA results reveal no obvious mass change for single-network ACL-HG and double-network DN-Gel upon heating to 200 °C, confirming their high-temperature structural stability. DN-Gel exhibits a residual mass of 18.03% and ACL-HG 32.04% after heating to 500 °C, with both demonstrating excellent thermal stability under formation temperatures.

5. Temperature and salt resistance tests demonstrate that DN-Gel retains over 80.6% of its mass after 60 days under harsh conditions (90 °C, 10000 mg/L high salt), significantly outperforming ACL-HG. GQDs-GX’s nano-reinforcement effect suppresses molecular chain thermal motion, and its surface sulfonate/hydroxyl groups form multiple hydrogen bonds with ACL-HG’s catechol structure, resisting high-temperature segment degradation and high-salt ion erosion to enhance system structural stability under extreme conditions.

6. Core water flooding simulation results reveal that DN-Gel achieves a breakthrough pressure gradient of 69.54–129.21 MPa/m and a permeability change rate of 98.43%–99.21% across cores with varying permeabilities, outperforming conventional gel-type water shutoff agents. Water-induced oxidation of DMA’s catechol structure to quinones forms strong coordination bonds with rock surface metal ions, enhancing adhesion. Combined with water absorption swelling to fill core pores, the double network’s compactness and water-triggered viscosity increase synergistically enable efficient plugging of large pores in high-permeability reservoirs.

Materials and methods

Experimental materials

The materials and instruments utilized in the synthesis and characterization of GQDs Gel, ACL-HG, and DN-Gel are listed in Tables 3 and 4.

Table 3.

List of specific parameters of main experimental materials.

NO	Name of chemical agent	Physical parameters	Supplier	Purpose
1	Dopamine methacrylamide(DMA)	Solid, 98%	Shanghai Jixiang Biotechnology Co., Ltd	To prepare monomers of ACL-HG hydrogels
2	Glycidyl methacrylate(GMA)	Solid, ≥ 97%	Shanghai Aladdin Biochemical Technology Co., Ltd	To prepare monomers of ACL-HG hydrogels
3	Poly(ethylene glycol) methacrylate(PEGMA)	Solid, ≥ 60%	Lab homemade	To prepare monomers of ACL-HG hydrogels
4	Triethylamine	Liquid, ≥ 99%	Shanghai Aladdin Biochemical Technology Co., Ltd	Alkaline catalyst for the preparation of ACL-HG hydrogels in the future
5	Azobisisobutyronitrile(AIBN)	Solid, ≥ 95%	Shanghai Aladdin Biochemical Technology Co., Ltd	To prepare initiators for ACL-HG hydrogels
6	1,5-Dinitronaphthalene	Solid, ≥ 97%	Sinopharm Chemical Reagent Co., Ltd	Raw materials for the preparation of GQDs
7	Aqueous ammonia	Liquid, ≥ 25%	Shanghai Aladdin Biochemical Technology Co., Ltd	Raw materials for the preparation of GQDs
8	Glyoxal	Liquid, ≥ 25%	Sinopharm Chemical Reagent Co., Ltd	Graft GQDs modified
9	Sodium 4-aminobenzenesulfonate	Solid, ≥ 97%	Sinopharm Chemical Reagent Co., Ltd	Graft GQDs modified
10	Ascorbic Acid	Solid, ≥ 99%	Shanghai Aladdin Biochemical Technology Co., Ltd	Raw materials for the preparation of GQDs Gel

Table 4.

List of specific parameters of main experimental instruments.

NO	Name of Instrument	Model Specifications	Supplier	Purpose
1	Fourier transform infrared spectroscopy	Nicolet iS50	Thermo Fisher Scientific Molecular Spectroscopy Company, Shanghai, China	Structural characterization of synthetic products
2	Scanning electron microscopy	Evo Ma 15	Zeiss, Germany	Structural characterization of synthetic products
3	Rheometer	HAAKE MARS III	HAAKE, Germany	Evaluate gel strength and gel formation time
4	Thermogravimetric analyzer	Q500	American TA Instruments Waters Inc	Thermogravimetric analysis of hydrogels
5	Oil displacement simulation device	Thermostat, core holder, ISCO pump, constant speed and pressure pump, intermediate container	Designed and assembled by the laboratory	Core permeability change experiment

Synthesis of dual-network hydrogels

Synthesis of ACL-HG

Branched polymer (ACL) is synthesized via Michael addition with glycidyl methacrylate (GMA), poly(ethylene glycol) methyl ether methacrylate (PEGMA), and dopamine methacrylamide (DMA) as monomers. The monomer molar ratio is PEGMA: DMA: GMA = 2:2:1 (10 mmol PEGMA, 10 mmol DMA, 5 mmol GMA), and the synthetic route is illustrated in Fig. 12.

Fig. 12.

Synthetic route of polymer ACL.

The Michael addition reaction is carried out under alkaline conditions. Herein, 2 mmol triethylamine serves as the alkaline catalyst and nucleophile, and 50 g dimethyl sulfoxide (DMSO) is used as the reaction solvent. The reaction monomers and catalyst (weighed in batches) are added to a sealed, corrosion-resistant glass reactor with magnetic stirring. After stirring at 300 rpm for 30 min, nitrogen is purged to create an oxygen-free environment. Triethylamine is added slowly, and stirring is continued for 15 min. The reaction is conducted isothermally at 60 °C for 24 h, followed by the addition of initiator azobisisobutyronitrile (AIBN) under continuous stirring and another 24 h of isothermal reaction. Post-reaction, the mixture is transferred to a fritted funnel and extracted with diethyl ether multiple times to remove impurities. The crude product is vacuum-evaporated at 40 °C to remove diethyl ether, dissolved in THF to form a homogeneous solution, and slowly dropped into n-hexane for precipitation. After stirring at 300 rpm for 30 min, the precipitate is filtered and dried in a vacuum oven to obtain the target covalently crosslinked hydrogel ACL-HG.

Synthesis of GQDs gel

GQDs-Gel was prepared via a three-step method. Firstly, sulfonate-grafted functional monomers were synthesized by dispersing glyoxal and sodium p-aminobenzenesulfonate in deionized water at a molar ratio of 1.1:1, reacting at 25 °C for 8 h to form a functional modified solution. Secondly, amino-functionalized GQDs were synthesized via a bottom-up approach: 1,5-dinitronaphthalene and ammonia water (mass ratio 1:6) were dispersed in deionized water (reaction solvent), ultrasonicated at 140 W for 30 min, then transferred to a high-pressure reactor for reaction at 200 °C for 18 h. After cooling to room temperature, the product was centrifuged at 10,000 rpm for 20 min to remove unreacted 1,5-dinitronaphthalene, and dialyzed with a dialysis bag (molecular weight cutoff: 1000) for 2 days to eliminate unreacted small molecules, yielding twice-purified amino-functionalized GQDs. Thirdly, functional graphene quantum dots (GQDs-GX) were synthesized by magnetically stirring amino-functionalized GQDs and the functional modified solution (molar ratio 1:10) at 25 °C for 6 h, followed by freeze-drying to obtain sulfonated GQDs-GX. The synthetic route is illustrated in Fig. 13. Subsequently, 20 mg of GQDs-GX was dissolved in deionized water and ultrasonically dispersed for 15 min. An appropriate concentration of reducing agent L(+)-ascorbic acid solution was added (mass ratio of GQDs-GX to L(+)-ascorbic acid = 1:2). The mixed dispersion was ultrasonically dissolved for 15 min, transferred to a polytetrafluoroethylene reactor, and reacted at 80 °C for 10 h. After the reactor cooled down post-reaction, the product was rinsed with deionized water and separated by suction filtration to obtain purified GQDs Gel.

Fig. 13.

Synthetic route of GQDs-GX.

Synthesis of DN - gel

The GQDs Gel and ACL-HG were mixed at a mass ratio of 1:1.5. Urea was added as a gelation retardant, and the mixture was sonicated for 30 min, by which a uniform and stable dispersion was formed.The double-network graphene hydrogel (DN-Gel) was thereby obtained.The synthetic route and network structure of DN-Gel are illustrated in Fig. 14.

Fig. 14.

Synthetic route of DN-Gel.

Structural characterization of hydrogels

The structures of GQDs Gel, ACL-HG, and DN-Gel were characterized by Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). FT-IR spectroscopy was used to characterize the molecular structure and compare changes in chemical bonds before and after the reaction. The wavelength range of infrared light is 2.5–25 μm (or 4800–400 cm⁻¹), which can be absorbed, transmitted, reflected, scattered, or induce fluorescence (also known as the Raman effect) when interacting with samples. After absorbing mid-infrared light, molecules undergo vibrational and rotational changes to form an infrared absorption spectrum, with a dispersion system of 2 cm⁻¹, a scanning range of 400–4000 cm⁻¹, 32 scanning times, and a resolution of 4. The coating method was adopted for sample testing in this study, which involved applying or coating liquid samples onto salt plates or window slides to form liquid films for FT-IR analysis.

Scanning electron microscopy (SEM) emits high-energy electron beams via an electron gun, which are focused into a nanoscale probe by electromagnetic lenses and scan the sample surface point-by-point under the control of scanning coils. With high resolution (up to 0.1–10 nm), it can clearly display the three-dimensional morphology of the sample surface, making it particularly suitable for microscopic observation of rough or complex structures. A FEI Quanta 450FEG SEM was used for sample scanning under the following test conditions: high vacuum mode and an accelerating voltage of 15 kV.

Performance evaluation

Apparent Viscosity and Gelation Time: The viscosity of the gel system in the experiment was determined using a rheometer, and the gelation time was defined as the time required for the measured viscosity of the system to reach 90% of the plateau value. The solvent water was laboratory-prepared simulated formation water with the formula shown in Table 5 (6778 mg/L), which was used for all subsequent system preparations. The viscosity of the gel system changes significantly during curing, so the appropriate rotor model must be selected based on the specific viscosity values obtained from the tests. To investigate the shear resistance of the adaptive gel system, a stirrer was used for pre-shearing treatment of pre-gelled gels with different concentrations.

Table 5.

Formula of simulated formation water.

Sample	KCl	NaCl	Na2SO4	MgCl2·6H2O	CaCl2	NaHCO3
Content/(mg⋅L− 1)	20	3 488	114	564	64	2 828

Shear Stability Test: Shear tests of the adaptive gel systems after gelation were conducted on ACL-HG and DN-Gel. The selected shear rates were 500, 1000, and 1500 r/min. The first shear was performed 14 days after gelation, with a shear duration of 23 s (corresponding to the field fluid migration rate: 0.9 m/day).

High-Temperature Stability Test: Thermogravimetric analysis (TGA) was performed on the freeze-dried samples of ACL-HG and DN-Gel. The weight loss was measured while heating from 25 °C to 500 °C at a heating rate of 10 °C/min.

Temperature and Salt Resistance Test: DN-Gel samples were cut into cylinders using a standard compression mold. The mass of each sample was weighed and recorded, and the diameter and height were documented by photography. Each sample was placed in a high-temperature and high-pressure reactor, and completely immersed in brine with salinities of 8000, 10,000, and 12,000 mg/L (note: supplemented unit “mg/L” for clarity, consistent with previous simulated formation water specifications) prepared with sodium chloride. The reactor was then placed in an oven at 60 °C, 75 °C, and 90 °C respectively. Every 10 days, the samples were taken out to measure and record their mass, and their diameter and height were photographed for documentation.

Core Permeability Change Test: The core data used in the experiment are shown in Table 6. The core was loaded into a core holder, and the confining pressure of the core holder was recorded. DN-Gel prepolymer was injected into the core at a constant flow rate of 1 mL/min to fill the core pores completely. The core was maintained at 60 °C for 350 h to ensure sufficient gelation. Then, clean water was injected for displacement at a rate of 10 mL/min, and the pressure change during displacement was recorded. The injection pressure when the first drop of liquid flowed out from the core outlet was defined as the breakthrough pressure (Pmax). Displacement was continued for a period of time until the pressure stabilized, and the stable pressure was recorded. The breakthrough pressure gradient (Pb) was calculated using the following formula (1):

1

Table 6.

Basic parameters of the core.

Core number	length (cm)	Cross-sectional area(cm2)	Core pore volume(cm3)	Core porosity(%)
1	10.05	4.91	14.85	30.11
2	10.04	4.91	14.12	28.64
3	10.04	4.91	11.86	24.11
4	10.03	4.91	9.57	19.43

P_max—Maximum breakthrough pressure of hydrogel plugging agent in the core(MPa).

P_b—Breakthrough pressure gradient(MPa).

L—Core length(m).

Continue injecting deionized water at a constant rate of 10 mL/min for displacement.After the injection pressure stabilized, the injected displacement volume was recorded.The permeability of the core after the injection process was calculated.The variation rate of water permeability in the core after the injection process was measured.The calculation formula is as follows (2):

2

η --Permeability variation rate(%).

K₁—Core permeability of fractures before plugging(10⁻³µm²).

K₂—Core permeability of fractures after plugging(10⁻³µm²).

Prospect

In the future, more direct characterization methods will be employed to clarify the interrelationship between the two gels in this study.Auxiliary chemical agents suitable for the double-network gel in this study are screened so that its on-site industrial application can be enhanced.Cyclic rheological tests are introduced to quantitatively demonstrate the self-healing behavior after shearing, and the self-healing capability of the system is verified from multiple perspectives.

Author contributions

Formal analysis, F.S.and W.J.; funding acquisition, F.S.; data curation, W.J. and B.Z.; investigation, J.L.and C.Z.; methodology, J.L. and J.W.; project administration, F.S.,J.W. and Bauyrzhan Sarsenbekuly.; resources, F.S.and W.J.; software, Bauyrzhan Sarsenbekuly. And W.J. All authors have read and agreed to the published version of the manuscript.

Funding

1. National Natural Science Foundation of China, Project number: 52304026. 2. Heilongjiang Provincial Natural Science Foundation, Project number: LH2024E007. 3. Daqing Science and Technology Innovation Guidance Project, Project number: YZ-XS-202312-10.

Data availability

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

Declarations

Competing interests

The authors declare no competing interests.