# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 4 Improving Salt Resistance of Polymer Gels
In a subsequent study, the researchers prepared a temperature and salt-resistant gel system for profile control and displacement by crosslinking the above polymer with a high-temperature-resistant crosslinker. This gel had a viscosity that reached 43,500 mPa * after 180 days at a temperature of 126 \({}^{\circ}\)C and a salinity of 117,000 mg/L. In addition, composite imbibition technology combined this gel with amphoteric surfactants, and it was tested in high-temperature and high-salinity reservoirs, which showed that cumulative crude oil production increased by 3660 tons. [PERSON] et al. found that intermolecular and intramolecular entanglements and curling were reduced through mutual repulsion between the lipophilic and hydrophilic groups on the polymer molecular chains [68]. For example, a comb-shaped arrangement of polymer molecular chains could increase the rigidity of the molecular chains and the hydrodynamic radius of the polymer chains, which significantly improved the salt resistance properties of the gel. Based on the above principle, [PERSON] et al. prepared a gel system with a comb polymer and organic chromium crosslinker, and this system remained stable after 60 days under high-salinity (NaCl 80,000 mg/L) and high-temperature (85 \({}^{\circ}\)C) conditions (Figure 10) [69]. Figure 10: Microstructures of star/comb polymer gels and variations in their properties at different salinities. (**a**) Microscopic image of a star polymer; (**b**) effect of salinity on star polymer viscosity; (**c**) microscopic image of a comb polymer gel; (**d**) crosslinking time and strength of the comb polymer gel at different salinities (Reprinted/adapted with permission from Refs. [66; 68]. 2016/2019, [PERSON], [PERSON], et al.). Gels prepared directly by the polymerization or crosslinking of monomers with salt-resistant functional groups typically have better salt resistance due to the denser 3D network structure and more uniform distribution of the salt-resistant functional groups in the gel backbone. [PERSON] et al. prepared a PPG using four polymer monomers--namely, N,N-dimethylacrylamide (MBA), AMPS sodium salt (AMPS-Na), AM, and NVP--with MBA as the crosslinker and sodium-based montmorillonite introduced as the filler. AMPS-Na contains sulfonic acid groups with strong hydration ability, strong polarity, and can complex with multivalent cations, thus protecting the gels [69]. NVP has a five-membered ring structure and large steric hindrance, and the introduction of NVP inhibits the hydrolysis of the amide group of the polymer side chain. Furthermore, nanoparticles can significantly increase the specific surface area of the gel skeleton and enhance the salt resistance of the gel. The synergistic effect of the combined above application enabled the system to remain stable for 120 days at a high temperature of 145 \({}^{\circ}\)C and high salinity of 225,000 mg/L. Although metal ion-based crosslinked gels are prone to hydrolysis at high temperatures, resulting in gel syneresis, they are not as stable as organic crosslinked gels under high-temperature conditions. However, studies have found that the salt resistance of gels formed by the crosslinking reaction between the polymer and organic crosslinker could be significantly improved by pre-embedding the metal ions into the polymer molecules. [PERSON] et al. synthesized a salt-insensitive microgel by free-radical suspension polymerization, in which Cr\({}^{3+}\) was pre-embedded in AM and crosslinked with MBAA [70]. The embedded Cr\({}^{3+}\) ligand pre-introduced the charged electrons into the gel network to inhibit the penetration of salt into the microgel, which reduced the ion shielding effect of salt on the carboxylic acid groups, thus substantially lowering the sensitivity of the gel to salt (Figure 11).