2.1.1. Optimization of High-Temperature-Resistant Resin Substrates
In this study, the curing performance characteristics of five typical thermosetting resins (urea-formaldehyde resin, epoxy resin, phenolic resin, polyvinyl alcohol resin, and melamine resin) were systematically investigated. Controlled experiments were conducted at three temperature gradients (60 °C, 80 °C, and 100 °C) by adding an appropriate amount of curing agent to observe their curing behavior. The results (
Table 1) indicate that none of the tested resins exhibited curing behavior at an ambient temperature of 40 °C. As the temperature increased, urea-formaldehyde resin, phenolic resin, polyvinyl alcohol resin, and epoxy resin all demonstrated varying degrees of curing tendency, while melamine resin failed to achieve effective curing within the 40–100 °C temperature range. Notably, under 100 °C curing conditions, urea-formaldehyde resin and phenolic resin exhibited excellent curing strength and mechanical properties. In contrast, the curing degree of polyvinyl alcohol resin and epoxy resin was significantly reduced, and their mechanical performance failed to reach the desired levels.
The curing profiles of five thermosetting resins were systematically evaluated under isothermal conditions (60 °C). As illustrated in
Figure 1, the urea-formaldehyde resin and phenolic resin showed the best curing properties, and the cured products had a complete network structure and good mechanical properties. These variations strongly correlate with molecular architecture and temperature-dependent crosslinking efficiency. Conversely, epoxy, polyvinyl alcohol, and melamine resins showed incomplete crosslinking under identical thermal activation, indicating restricted network formation. The results demonstrate temperature-mediated control over resin curing dynamics while establishing selection criteria for thermosetting systems in moderate-temperature applications.
Further optimization was conducted by selecting urea-formaldehyde resin, epoxy resin, and phenolic resin as the primary candidates. Each resin matrix was added to water and uniformly dispersed by stirring at 600 rpm for 5–15 min. Subsequently, thiourea, lithium-based bentonite, curing agent, crosslinking agent, nano-silica, walnut shell, and quartz sand were incorporated. The choice of curing agent was tailored to the specific resin: triethanolamine for epoxy resin, sodium hydroxide for phenolic resin, and a latent curing agent composed of sulfonic acid and ammonium compounds for urea-formaldehyde resin. The mixture was further stirred for 5–15 min until uniformly dispersed, then poured into cylindrical high-temperature-resistant molds. The molds were placed in a variable-frequency high-temperature roller heating furnace and subjected to rolling heating at 100–140 °C for a specified duration. After removal, the curing status was observed.
From the curing point of view, under the same experimental conditions (100–140 °C), epoxy resin and phenolic resin failed to meet the molding requirements, and the strength requirements required for pressure sealing, while urea-formaldehyde resin has the best performance among the three thermosetting resins, with good molding and certain strength, combined with its good settling stability. Finally, the urea-formaldehyde resin was selected as the resin matrix for this experiment.
2.1.2. Preparation of Water-Soluble Resin
In order to meet the application requirements of an underground in situ cross-linked plugging agent, a water-soluble resin with controllable curing properties was prepared by using urea-formaldehyde resin as the target resin matrix through innovative synthesis technology. Compared with traditional resins, which have uncontrollable curing times and high costs at high temperatures, water-soluble resins not only have good water solubility but also have better curing process adaptability.
The synthesis process of the water-soluble resin comprises three critical steps:
Preparation of Prepolymer. Urea, formaldehyde, and additives such as resorcinol and furfural are mixed at specific molar ratios, followed by pH adjustment to initiate the condensation polymerization reaction;
Hydrophilic Modification. Utilizing an internal emulsification method, hydrophilic groups or chain extenders containing hydrophilic groups are introduced into the prepolymer to achieve chemical modification;
Crosslinking and Curing. Multifunctional organosilicon compounds are selected as crosslinking agents. Their unique structure enables the formation of covalent bonds with both the active groups of the resin (hydroxymethyl groups, amide bonds, etc.) and the surface groups of inorganic materials, thereby constructing a crosslinked network.
The reaction process and microstructure of the water-soluble resin preparation are illustrated in
Figure 2. Through precise control of the prepolymerization reaction, the introduction of hydrophilic groups, and the use of multifunctional crosslinking agents, a water-soluble resin gel system with both excellent fluidity and controllable curing properties was successfully synthesized. This achievement provides a reliable material foundation for the practical application of in situ leakage control agents in subsurface environments.
An orthogonal experimental design was employed to systematically assess the effects of chemical component ratios on the synthesis of water-soluble resins. Through rigorous multi-factor analysis, the optimal formulation parameters (summarized in
Table 2) were determined as follows: resorcinol at 0.5 wt%, furfural at 1.0 wt%, sodium dodecyl sulfate at 0.6 wt%, and organosilicon crosslinking agent at 0.3 wt%. This optimized combination maximizes intermolecular interactions between constituents, significantly enhancing the resin’s structural uniformity, solution stability, and adhesion properties. The systematic screening approach further reveals dose-dependent relationships between additives and functional performance, providing quantitative guidance for scalable production while maintaining targeted application specifications.
2.1.3. Optimization of the Type and Dosage of Curing Agent
To prevent premature curing of the leakage control system during the pumping process, the curing agent was optimized to extend the curing time of the resin gel plugging system, thereby ensuring operational safety. Three types of curing agents—ammonium-based curing agents, latent curing agents, and mixed ammonium-based curing agents—were selected and compared for their effectiveness in curing the resin aqueous solution. The curing temperature was maintained at 130 °C, and the curing time was set at 180 min for all tests.
The curing performance of resin-mortar composites under thermal compression (130 °C, 180 min) was systematically analyzed as a function of ammonium-based hardener concentration (3–10 wt%). As depicted in
Figure 3, lower hardener dosage (3 wt%) produced structurally heterogeneous consolidations with compromised mechanical strength, indicative of insufficient crosslinking density. Elevated concentrations (5–10 wt%) facilitated uniform network formation, yielding cohesive composites with enhanced structural integrity and load-bearing capacity. This concentration-dependent behavior aligns with thermally activated crosslinking kinetics, where adequate hardener availability promotes complete covalent bonding between polymeric chains. The findings establish critical thresholds for additive optimization in high-temperature resin processing and industrial composite fabrication.
The impact of different ratios of mixed curing agents (ammonium persulfate and diammonium hydrogen phosphate) on the curing effectiveness of the resin gel plugging system was studied under the conditions of 130 °C and 180 min of thermal rolling. The ratios of ammonium persulfate to diammonium hydrogen phosphate in the mixed curing agents were set at 1:1, 1:2, and 2:1, respectively. The experimental results, as depicted in
Figure 4, reveal that a mixed curing agent ratio of 1:1 resulted in the formation of a complete resin mortar consolidation with high strength. In contrast, ratios of 1:2 and 2:1 produced consolidations that exhibited brittleness.
Ammonium persulfate (APS) served dual roles as catalyst and initiator for the urea-formaldehyde resin curing process, mediated by its unique redox-active coordination chemistry. Thermal activation facilitated APS decomposition into sulfate radicals that abstracted hydrogen atoms from the resin’s hydroxymethyl/methylene moieties, inducing radical chain reactions. These reactive intermediates promoted covalent crosslinking via ether/methylene bridge formation between polymeric chains, driven by radical recombination mechanisms. Concomitantly, APS’s acidic dissociation products lowered the system pH, catalytically accelerating polycondensation through the protonation of hydroxyl groups. The synergistic action of radical-mediated crosslinking and acid-catalyzed polycondensation generated a densely crosslinked, interpenetrated network architecture with enhanced thermomechanical stability. This dual-functional mechanism underscores APS’s efficacy in controlling both reaction kinetics and final network topology during thermosetting resin matrix consolidation. The findings elucidate redox initiator design principles for tailoring covalent adaptable networks in functional polymer composites.
The influence of different ratios of latent curing agents on the curing effectiveness of the resin gel plugging system was investigated under the conditions of 130 °C and 180 min of thermal rolling. The latent curing agents were composed of p-toluenesulfonic acid, hexamethylenetetramine, diethanolamine, and ammonium persulfate, with the ratios set at 1:1:1:1, 1:1:1:2, 1:2:1:1, and 2:1:1:1, respectively. The experimental results, as illustrated in
Figure 5, demonstrate that the resin gel plugging system achieved the highest strength when the latent curing agent ratio (p-toluenesulfonic acid: hexamethylenetetramine: diethanolamine: ammonium persulfate) was 1:1:1:2. While the resin gel plugging system also solidified at ratios of 1:1:1:1, 1:2:1:1, and 2:1:1:1, the resulting consolidations exhibited lower strength.
The superior mechanical performance at the 1:1:1:2 stoichiometric ratio arises from balanced acid-base dynamics, combining reduced p-toluenesulfonic acid (PTSA) content with elevated ammonium persulfate (APS) levels. This formulation leverages the latent curing behavior of APS, which maintains resin-curing agent metastability at ambient conditions. Upon thermal or photonic stimulation, APS undergoes heterolytic cleavage, activating reactive sulfate radicals that abstract hydrogen atoms from the resin’s hydroxymethyl/methylene functionalities. These radical intermediates drive covalent crosslinking via radical chain-transfer mechanisms, while residual p-toluenesulfonic acid catalyzes methylene bridge formation through Friedel–Crafts alkylation. The synergistic interplay between radical-mediated polymerization and acid-catalyzed polycondensation induces stereochemically directed 3D network assembly, converting linear oligomers into a glassy thermoset matrix. The optimized curing kinetics minimizes stress-induced microcracks, producing homogeneous architectures with superior interfacial adhesion. These structure–property relationships demonstrate rational formulation design for stimuli–responsive thermoset systems requiring delayed activation and high mechanical fidelity.
The compressive strength of the resin varies depending on the materials and processes used. To evaluate the compressive strength, samples of the resin gel plugging system with the highest strength were selected from those formed using three types of curing agents: ammonium chloride curing agent, latent curing agent, and a mixed curing agent of ammonium persulfate and diammonium hydrogen phosphate. The experimental results, as presented in
Table 3, indicate that the latent curing agent yielded the best compressive strength, reaching up to 6.26 MPa. Consequently, the latent curing agent was chosen for subsequent experiments due to its superior performance in enhancing the mechanical properties of the resin gel plugging system.
The curing agent concentration critically influenced the apparent viscosity, bearing capacity, and curing time of the leakage control slurry. By adjusting its dosage, the slurry’s comprehensive performance was optimized. Injections were performed into a steel fracture (7 mm inlet, 5 mm outlet), allowing in situ curing. Post-curing, the bearing capacity was evaluated using a leakage control instrument with incremental pressure application. Compressive strength, bearing strength, and apparent viscosity were selected as key metrics for assessment. A single-factor analysis method was employed to determine the optimal curing agent concentration, as summarized in
Table 4. Experimental outcomes are visualized in
Figure 6, demonstrating the critical role of precise stoichiometric control in achieving balanced rheological and mechanical properties for fracture-sealing applications.
The compressive strength of the consolidated body increased proportionally with curing agent dosage. A critical enhancement in bearing strength was observed when the dosage rose from 8% to 10%, increasing significantly from 6.3 MPa to 9.2 MPa. Comparative analysis revealed nearly identical compressive strengths at 10% and 12% dosages, with both formulations demonstrating equivalent high-performance mechanical resistance under compressive loads.
As depicted in
Figure 7 and
Figure 8, the apparent viscosity of the leakage control slurry increased with curing agent dosage, while the curing time decreased. This relationship correlated with enhanced homogeneity of the consolidated body and improved bearing capacity. At 10% curing agent dosage, a curing time of 2 h and a pressure-bearing sealing strength of 8.0 MPa were achieved. This formulation notably balanced rapid gelation kinetics (to minimize fluid loss during injection) with sufficient strength development for sealing efficacy. Consequently, the optimal curing agent dosage was determined as 10%, striking an equilibrium between workability and structural integrity for fracture remediation.
2.1.4. Construction of Basic Formula for Plugging System
In this study, an orthogonal experimental approach was employed to optimize the formulation of the resin-based leakage control system, aiming to reduce resin concentration and costs. Under the conditions of a curing temperature of 130 °C and a curing time of 180 min, the effects of urea-formaldehyde resin dosage (10%, 15%, 20%, 25%, 30%) on the curing performance were systematically investigated. The orthogonal experimental design with varying additive dosages is detailed in
Table 5, and the experimental results are presented in
Figure 9. The results indicate that when the urea-formaldehyde resin dosage was 10% and 15%, the resin underwent initial curing but exhibited insufficient strength, failing to meet practical application requirements. When the dosage was increased to 20% and 25%, the resin successfully cured into intact cylindrical consolidated bodies, demonstrating excellent curing performance and high strength. However, when the dosage was further increased to 30%, the cured samples exhibited fracturing, and the curing strength decreased. Comprehensive analysis revealed that a urea-formaldehyde resin dosage of 25% not only ensured adequate curing strength but also effectively reduced resin usage, achieving the dual objectives of optimizing the formulation and lowering costs.
This study evaluated the impact of resin concentration on curing strength and temperature on curing kinetics. As illustrated in
Figure 10, compressive strength exhibited a bell-shaped dependence on urea-formaldehyde resin concentration. At 20%, 25%, and 30% resin concentrations, compressive strengths measured 8.5 MPa, 9.6 MPa (peak performance), and 8.3 MPa, respectively, indicating an optimal concentration threshold at 25%.
Concurrently, elevated temperatures significantly accelerated curing kinetics, as demonstrated in
Figure 11. Curing time decreased progressively from 230 min at 100 °C to 153 min (120 °C) and 125 min (140 °C), underscoring the thermally activated nature of the curing process. These findings highlight the synergistic interplay between resin formulation and thermal conditions in tailoring cure kinetics and mechanical performance.
The integrated findings establish that a 25% urea-formaldehyde resin concentration, coupled with tailored thermal activation, achieves synergistic optimization of curing performance. This formulation balances rapid curing kinetics (minimizing operational delays) with maximal mechanical strength retention, critical for durable sealing applications. The identified parameters (resin composition and curing temperature) govern the interplay between crosslinking density and network formation dynamics, ensuring robust mechanical integrity under stress. These insights provide a foundational framework for refining resin-based leakage control systems, prioritizing both structural resilience and field-deployable curing efficiency in subsurface engineering applications.
Orthogonal experimental analysis optimized the resin-based leakage control formulation to 25% water-soluble resin + 10% curing agent, balancing cost and performance. As shown in
Figure 12, modulating resin/curing agent concentrations enabled precise control of curing parameters: temperature (100–140 °C), time (125–230 min), and strength (9–13 MPa), with 25% resin achieving peak strength (9.6 MPa). The formulation optimizes crosslinking kinetics—resin concentration governs polymer network density while curing agent concentration regulates reaction initiation. At 140 °C, curing was completed in 125 min, demonstrating rapid field-applicable polymerization. This synergy of stoichiometric precision and thermal activation provides a low-cost, high-performance solution for engineering applications requiring durable leak prevention.
2.1.5. Dosage Optimization of Flow Regulator
The regulation of the flow behavior of the leakage control agent during its movement is one of the critical technical challenges in ensuring effective leakage control. When the leakage control agent flows through surface piping, it requires low apparent viscosity to ensure pumpability, thereby reducing construction difficulty and energy consumption. Conversely, when flowing within formation fractures, it needs to exhibit a high apparent viscosity to resist the scouring effect of formation water, ensuring the agent can effectively reside and solidify. Therefore, regulating the flow behavior of the leakage control agent holds significant engineering value.
This study utilized a flow behavior regulator developed by the team of Professor Bai Yingrui at China University of Petroleum (East China) [
26]. This regulator is a lithium-modified bentonite material with excellent suspension properties, high swelling capacity, and good adsorption performance. Its core advantage lies in the ability to precisely adjust the apparent viscosity of the leakage control agent through simple dosage regulation, thereby meeting the flow behavior requirements under different working conditions. To further investigate the impact of the rheological regulator on the performance of the resin gel plugging system, this study systematically examined the influence of rheological regulator concentration on the system’s shear thixotropy.
The experimental results (
Figure 13) reveal that incorporating the rheological regulator imparts distinct “shear-thinning, static-thickening” thixotropic behavior to the resin-gel plugging system. Under high shear rates, the system’s apparent viscosity decreases significantly, enhancing pumpability through surface piping. Conversely, under low shear or static conditions, viscosity rapidly recovers and increases, improving retention within fractures. Critically, the regulator minimally impacts the cured strength of the gel system, ensuring stable leakage control.
The optimal rheological regulator concentration was determined as 0.5%, balancing pumpability (excellent fluidity during injection) with resistance to formation water dilution. This concentration achieves operational efficiency without compromising long-term stability in reservoirs.
2.1.6. Optimal Selection of Filling Materials
In the process of optimizing the leakage control agent formulation, this study systematically screened ten high-water-loss filler materials with different physicochemical properties, including diatomaceous earth, sepiolite, asbestos powder, and others. The screening of these materials was primarily based on their performance in field leakage control operations, focusing on their impact on the water-loss performance, curing strength, and rheological properties of the leakage control agent.
High-water-loss filler materials exhibit significant curing and expansion characteristics under dehydration conditions. Introducing these materials into the resin mortar leakage control system enables rapid dehydration and expansion curing under pressure differentials, effectively sealing the target leakage zones. Single-factor optimization experiments revealed that the type of high-water-loss filler material significantly influences the homogeneity and strength of the mudcake. Among them, the leakage control slurry containing high-water-loss filler material C formed a mudcake with significantly improved density, uniform distribution, and excellent toughness, showing no fracturing under bending conditions. The sealing layer exhibited good strength (
Figure 14). Therefore, high-water-loss filler material C was selected as the key component of the leakage control slurry system.
Leakage control slurries formulated with high-water-loss filler materials at varying dosages were evaluated for apparent viscosity, with the experimental matrix outlined in
Table 6. As shown in
Figure 15, increasing the filler content enhanced consolidated body strength and reduced porosity, improving structural density. Concurrently, the slurries exhibited a progressive rise in apparent viscosity. At 10% and 15% filler dosages, viscosities reached 64 mPa·s and 65 mPa·s, respectively, showing comparable values.
Results demonstrate that filler dosage critically governs slurry rheology (e.g., viscosity modulation) and consolidated performance (e.g., strength–density trade-off). The near-identical viscosities at 10–15% suggest a threshold effect, where further filler addition minimally improves flow behavior while maintaining mechanical enhancement. This highlights the importance of optimizing filler content to balance injectability and plugging efficacy.
The leakage control slurry was injected into a steel-simulated fracture and cured under temperature-controlled conditions. A high-temperature/high-pressure evaluation apparatus performed sequential pressure-loading tests to assess pressure-bearing capacity. As shown in
Figure 16, the slurry’s pressure resistance markedly increased with higher high-water-loss filler content. At 10% and 15% filler dosage, the pressure-bearing capacities reached 10.1 MPa and 10.2 MPa, respectively, exhibiting negligible divergence. Post-injection observations confirmed complete sealing at the fracture entrance, with a dense internal structure forming a continuous sealing zone within the fracture.
Considering apparent viscosity (64 mPa·s at 10% vs. 65 mPa·s at 15%), pressure resistance, and cost-effectiveness, the optimal filler dosage was determined as 10%. This dosage balances mechanical robustness (<1% difference in pressure resistance between 10% and 15%) with operational efficiency, achieving both effective fracture sealing and economic feasibility.