# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 4 Improving Salt Resistance of Polymer Gels  
Oil-gas field formation water contains salt ions with concentrations ranging from a few thousand to several hundred thousand ppm. For polymer gels, high metal ion concentrations (especially high-valent metal ions) will cause the gel structure to warp and undergo syneresis by compressing the hydrophilic groups on the polymer molecular chains, thus significantly reducing gel strength. When water salinity is high, -COOH will tend to complex with calcium and magnesium ions in the water, forming a precipitate, which will affect the gel strength, and the presence of salts will cause gel syneresis and weaken its performance. Salinity is often high in some oilfields, which will affect the application results of the gel. Therefore, research on the salt resistance of polymer gelling agents is often conducted to optimize the salt resistance of gels in terms of the polymer structure, polymer molecular weight, and the introduction of salt-resistant monomers/inorganic fillers [64]. The unsatisfactory salt resistance of conventional high molecular weight PAM (HPAM) gels is mainly because high ionic concentrations can severely compress the double diffusion layer on the carboxyl and amide group surfaces on the HPAM molecular chain, forcing the molecular chains to curl and shrink. In addition, high-valent cations (e.g., Ca\({}^{2+}\) and Fe\({}^{3+}\)) are prone to complexing with carboxyl groups, reducing crosslinking sites, and decreasing the stability of the gel [15]. Low molecular weight PAM has better deformability and solubility than high molecular weight PAM (generally greater than 10,000 kDa) and can maintain a suitable shape of the molecular chain without excessive curling under  
\begin{table}
\begin{tabular}{c c c c} \hline
**Strategy** & **Principle** & **Measure** & **Molecular Structure** \\ \hline Optimize polymer molecular structure & Introduce functional monomers (large side groups/rigid side groups, and hydrolysis-resistant/inhibiting amide monomers) & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (NVP) (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (AMPS) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g., NVP and AMPS) & (Resorcinol) \\  & Introduce functional monomers & Introduce functional monomers and terpolymers (e.g. high-salinity conditions. In addition, low molecular weight PAM has more intermolecular crosslinking with the crosslinker, increasing the crosslinking density. [PERSON] et al. prepared a high-salt-resistant gel system using low molecular weight HPAM (molecular weight was 3800 kDa) as the polymer and HMTA/HQ as the crosslinker, and the gel had a plugging rate of over 90% for high-permeability cores even, at a high salinity of 19.8 \(\times\) 104 mg/L [65]. The molecular structures of polymers have an important influence on gel properties. Conventional linear polymers are prone to curling under high-salinity conditions. Star polymers consist of a star-shaped core and multiple supramolecular polymer chains. This polymer has a special arm structure that enhances intermolecular chain entanglement and is prone to intramolecular associations in aqueous solutions, making it has higher salt resistance than linear polymers [66]. [PERSON] et al. prepared a salt-resistant star polymer, Star-PAM, through copolymerization of \(\beta\)-cyclodextrin-grafted functional monomer with AM, followed by hydrolysis [67].