Against the backdrop of a rapidly changing international geopolitical and economic landscape, China’s dependence on foreign oil and natural gas remains high. In 2023, the external dependence rates for crude oil and natural gas reached 71.2% and 40.14% respectively, posing challenges to national energy security. Approximately half of China’s oil and gas production comes from mature oilfields. With limited proven reserves, enhancing recovery from these existing fields represents a practical and efficient approach to increasing production.
The recovery factor of many domestic mature oilfields, such as Daqing and Yumen, is generally below 50%, whereas some international mature oilfields achieve recovery factors around 70%. This indicates significant potential for enhanced recovery in domestic fields. High-temperature and high-salinity formations account for about 10% of these, a proportion expected to grow. Consequently, efficient and composite Enhanced Oil Recovery (EOR) technologies are a major focus of innovation and development.
To address issues like high water cut and reservoir heterogeneity in mature fields, researchers have developed nearly a hundred plugging agents across eight categories. These include precipitating inorganic salts, particulate agents, foam agents, rock wettability alteration agents, resin agents, microbial agents, cement agents, and polymer gel agents. Polymer gel agents are among the most widely used and promising profile control and water shutoff technologies due to their controllable gelation time and strength, along with relatively low cost. When injected into formations, these agents preferentially enter high-permeability zones, where the solution transforms into a high-viscosity gel, blocking large pores or high-permeability channels, reducing heterogeneity, and improving sweep efficiency.
As the petroleum industry advances, reservoir conditions become increasingly complex, demanding higher performance from profile control and water shutoff technologies. Conventional polymer gel systems using phenolic resin or chromium-based crosslinkers often struggle to meet the requirements of high-temperature and high-salinity formations. Elevated temperatures can shorten gelation time, preventing deep penetration into the reservoir and reducing water control efficiency. Furthermore, excessively high temperatures can break chemical bonds between polymer molecules and crosslinkers, weakening gel strength and compromising the seal in high-permeability zones. This article summarizes laboratory research and application progress regarding polymer gel systems designed for complex high-temperature and high-salinity formations, based on a review of extensive domestic and international literature.
1. Modified Polyacrylamide Gel Systems
In increasingly complex reservoir environments, particularly those with high temperature (>90°C) and high salinity (>50,000 mg/L), conventional polymer gels like Polyacrylamide (PAM) and Xanthan gum can degrade or break down rapidly.
Researchers have improved temperature and salt tolerance by grafting copolymerization, incorporating functional monomers such as 2-Acrylamido-2-methylpropanesulfonic acid (AMPS), vinyl sulfonate, sodium styrene sulfonate, vinylpyrrolidone, vinyl alcohol, dimethylsiloxane, and methyl methacrylate onto the polyacrylamide polymer chain.
For instance, Dong Shuyang et al. prepared a three-dimensional network modified polyacrylamide polymer using Polyvinyl Alcohol (PVA), crosslinker, acrylamide, and AMPS, formulating a gel system using formation brine from the Tahe Oilfield (Tarim Basin, Northwest China). Increasing AMPS content within a certain range introduces sulfonic acid groups that create electrostatic repulsion. However, these groups can also cause steric hindrance, potentially hindering free radical polymerization between monomers and affecting gel strength. At 5% AMPS content, the modified polyacrylamide gel achieved maximum strength, reaching Grade I under high temperature (130°C) and high salinity (210,000 mg/L) conditions.
To enhance the stability of AM/AMPS gel systems, Liao Yuemin et al. added hydrophilic PA fiber stabilizers, which increased the density of the gel network structure, improving water retention capacity and extending stability at high temperatures. At 140°C and 220,000 mg/L salinity, the gelation time exceeded 15 hours, dehydration was below 2% after 120 days, gel strength remained at Grade G, water shutoff efficiency exceeded 99.70%, and oil plugging was less than 6.00%, demonstrating promising field application potential.
To delay the gelation time of AM/AMPS systems, Pu Wanfen et al. introduced N-Vinylpyrrolidone (NVP) and the hydrophobic monomer DHT (containing benzene rings and aliphatic hydrophobic chains). The pyrrolidone rings, benzene rings, and hydrophobic chains significantly increase the steric hindrance of the polymer chains, delaying gelation. Under conditions of 120°C and 365,000 mg/L salinity, delayed expansion for up to 3 days was observed.
2. Silica-Polymer Gel Systems
To further enhance the temperature and salt resistance of polymer gel systems, incorporating nano-inorganic particles like nano-silica has been proposed. Liu et al. prepared a reinforced polymer gel system using Hydrolyzed Polyacrylamide (HPAM), hydroquinone (HQ), hexamethylenetetramine (HMTA), and nano-silica. Studies found that adding nano-silica significantly shortened gelation time and improved gel strength, elasticity, and viscosity. The maximum tolerable temperature increased from 137.8°C to 155.5°C with nano-silica addition. Silanol groups on nano-silica form hydrogen bonds with polar groups like amine and hydroxyl groups on the polymer molecules, leading to physical crosslinking. This increases the hydrodynamic radius of the polymer chains and enhances the conversion of free water to bound water in the system, resulting in a tighter, more stable three-dimensional network with smaller pores, greater strength, and better temperature resistance. However, excessively high nano-silica concentrations can lead to over-crosslinking or self-aggregation, weakening gel strength or preventing gelation. Moreover, gelation time shortens with increasing nano-silica concentration, potentially reducing polymer propagation distance and sweep efficiency, thus lowering plugging effectiveness. Fadil et al. developed an HPAM/Cr³+/nano-silica gel system where gelation time decreased with increasing temperature and nano-silica concentration. At 3% nano-silica, gelation time was 9 hours with Grade H strength, but this decreased to Grade G after 25 hours. Therefore, nano-silica should be used in appropriate amounts as a stabilizer.
3. Slow-Gelling Polymer Gel Systems
3.1 Chromium-Based Slow-Gelling Systems
Chromium-based crosslinkers are commonly used, but the crosslinking reaction between Cr³+ and HPAM is often rapid and difficult to control. Two primary approaches address this:
3.1.1 Using Redox Systems
Cr⁶+ is inert towards HPAM. Adding a reducing agent converts Cr⁶+ to Cr³+, allowing gelation time control by adjusting the reducer amount. Sodium sulfite and thiourea have proven effective. Dai Caili et al. developed a slow-gelling chromium gel system using HPAM, sodium dichromate, and the organic reducer thiourea. Due to thiourea’s weak reducing ability, gelation time reached 8-10 days. Within certain concentration ranges, reducing crosslinker and thiourea content prolonged gelation time and increased strength. Excess thiourea accelerates the reduction of Cr⁶+ to Cr³+, speeding up network formation and shortening gelation time. Excess crosslinker can cause over-crosslinking, local dehydration, and disrupt network continuity, reducing strength. Liu Wenjing et al. constructed a slow-gelling system with HPAM, potassium dichromate, sodium sulfite, and a weak reducer (HN). Adding HN extended gelation time from 5 hours to 17 hours. The system exhibited good temperature and salt tolerance, and thermal stability, suitable for reservoirs with salinity below 50 g/L and temperatures of 50-90°C.
3.1.2 Introducing Complexing Agents
Complexing agents compete with HPAM for chromium ions, controlling gelation rate. Low molecular weight organic acids containing carboxyl groups, such as acetic acid, propionic acid, malonic acid, lactic acid, and salicylic acid, are commonly used. Research by Albonio, Bartosek, and others found that glycolic acid, salicylic acid, and malonic acid delayed gelation more effectively than acetic acid, achieving 12 to 33 times longer gelation times. Gao Zhiyong et al. synthesized slow-gelling crosslinkers from high-valence metal ions and organic acids, achieving adjustable gelation times from 1 to 15 days.
3.2 Polyethylenimine (PEI) Slow-Gelling Systems
Chromium-based slow-gelling systems typically have temperature resistance not exceeding 90°C. In high-temperature, high-salinity formations, gelation time may shorten excessively or crosslinking may not occur, hindering deep propagation. Polyethylenimine (PEI) is an environmentally friendlier, low-toxicity polymer containing primary, secondary, and tertiary amine groups, giving it high reactivity and one of the highest known charge densities among cationic polymers.
Due to its structure, PEI has been investigated as a crosslinker for HPAM and its derivatives in well conformance control. PEI crosslinks with amide groups on PAM via nucleophilic substitution, forming gel systems often used in medium-low temperature (40-80°C) formations. Jia Hu et al. systematically analyzed the gelation mechanism and core pore plugging effectiveness of PAM/PEI systems at 40-60°C, finding controllable gelation times from 15 hours to 9 days, significantly longer than conventional metal ion systems.
However, the PEI-PAM crosslinking process is temperature-sensitive, with higher temperatures shortening gelation time. Current methods to delay gelation in high-temperature formations include pH adjustment, introducing metal ions, and incorporating large side-chain groups.
3.2.1 pH Adjustment for Delayed Gelation
PEI molecules contain numerous positively chargeable amine groups and are inherently basic. Zou Zhao found that under acidic conditions, protonation of PEI due to H⁺ prevents PAM/PEI gel formation. Qin Yi et al. observed that at 130°C, the PAM/PEI system gelled in 15 minutes under weak alkaline conditions (pH=9), but delayed to 30 minutes in a neutral environment. Mohammed et al. modeled the effect of pH on HPAM/PEI gelation at 70-90°C, finding fastest and most stable gelation at pH=10.5. Under strongly alkaline conditions (pH>10.5), higher pH values lead to longer gelation times, lower viscosity, or even prevent gelation.
3.2.2 Introducing Metal Ions for Delayed Gelation
Metal ions (Na⁺, K⁺, Ca²⁺, etc.) can shield charges on PEI molecules, reducing their reactivity and acting as retarders. Salts like NaCl and Na₂CO₃ have been used as retarders in the field. Li Qiang et al. developed a nano-SiO₂/HPAM/PEI gel system using CTAB-modified SiO₂ as a reinforcer. When NaCl concentration increased from 0 mg/L to 100,000 mg/L, gelation time delayed from 3 hours to 5 days, but strength decreased from Grade I to Grade G.
3.2.3 Introducing Large Side-Chain Groups for Delayed Gelation
Large side-chain groups (e.g., sulfonate groups) increase polymer chain steric hindrance, improving salt/temperature resistance and delaying gelation. Lü Junxian et al. developed a high-temperature delayed crosslinking polymer plugging agent PM-1 using acrylamide, AMPS, V-50 as initiator, and PEI and MBA as crosslinkers, combining free radical polymerization and polymer crosslinking. This agent offers high strength and controllable gelation time at high temperatures. Increasing sulfonate group content in PM-1 improves temperature resistance and delays gelation.
While PEI itself has low toxicity, its traditional synthesis process can involve highly toxic intermediates like ethyleneimine and generate significant acidic/alkaline wastewater, causing environmental concerns. BASF has developed an alternative synthesis using ethanolamine or ethylene glycol and ethylenediamine condensation, avoiding ethyleneimine. However, this method requires transition metal catalysts, increasing cost, and offers less control over PEI molecular structure, potentially leading to inconsistent products.
4. Bio-Based Gel Systems
Replacing traditional oilfield chemicals with greener alternatives is a growing trend. Chitosan (CS), the second most abundant natural polymer, is a linear polysaccharide derived from deacetylated chitin, chemically known as β-(1,4)-2-amino-2-deoxy-D-glucose. Its molecular structure contains numerous hydroxyl and primary amino groups, enabling reactions like alkylation and acylation.
Zhao Shicheng et al. prepared CS/P(AM-AMPS) microspheres using 7 white oil as the oil phase, AM and AMPS as monomers, and CS as crosslinker via inverse microemulsion. The average particle size was around 100 nm, with temperature resistance exceeding 200°C. Yang Yang et al. prepared an organically crosslinked chitosan gel using CS, MBA, and HPAM. At 120°C and 30,000 mg/L salinity, gelation time was 16 hours with a strength of 170 Pa. Within a pH range of 3.2-5.6, lower pH slowed the crosslinking reaction but also reduced gel strength.
Chitosan’s unique structure makes it a research focus across fields. However, chitin is notoriously difficult to dissolve, and the deacetylation process generates substantial alkaline wastewater, necessitating process improvements to reduce environmental impact.
Summary
While nearly a hundred plugging agents exist, few are suitable for high-temperature and high-salinity formations. Polymer gel agents, with their controllable gelation time/strength and relatively low cost, show promise for these challenging conditions.
Modified polyacrylamide systems incorporating hydrophobic or large rigid side-chain groups enhance temperature and salt resistance.
Introducing nano-silica particles can further improve temperature and salt tolerance, but excessive concentrations cause agglomeration, shortening gelation time and weakening strength.
Conventional phenolic and chromium-based crosslinkers can be toxic. The high molecular weight PEI crosslinker offers very low toxicity and slow-gelling properties, presenting broad application prospects for high-temperature, high-salinity reservoirs. However, its production process can be complex, involve toxic intermediates, and generate wastewater, requiring optimization of synthesis routes.
The trend towards green oilfield chemicals is clear. Chitosan, a abundant natural polymer with numerous reactive hydroxyl and amino groups, can potentially replace traditional crosslinkers. Nonetheless, the deacetylation process needs improvement to reduce the environmental footprint associated with alkaline wastewater generation.
Post time: Nov-25-2025