# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 3 Improving Temperature Resistance of Polymer Gels
The amide groups (-CONH\({}_{2}\)) on the terpolymer molecular chain were crosslinked with the hydroxyl groups (-CH\({}_{2}\)OH) on the phenolic ring of OC-3, forming a dense 3D network structure. Scanning electron microscope (SEM) images revealed that the microscopic network gel network was approximately 10 \(\upmu\)m in size and had a large grid thickness; thus, the system showed only minor syneresis after it was heated at 150 \({}^{\circ}\)C for five months. So far, the optimization of the polymer molecular structure has been studied systematically and in-depth; however, because polymer gels are composed of multiple components, studies on enhancing the high-temperature stability of gels by simply improving the temperature resistance of polymer molecules have hit a ceiling limit. Crosslinkers are necessary components of crosslinked polymer gels. Commonly used crosslinkers for polymer gels include organic phenolics and metal ions. Metal ion-crosslinked gels are prone to hydrolysis at high temperatures, resulting in gel syneresis; however, stable covalent bonds can be formed between the amide group of the polymer and the organic crosslinker at high temperatures. Thus, gels prepared by organic crosslinkers generally have suitable temperature resistance. For monomer-crosslinked polymer gels, N,N'-methylenebisacrylamide and N-methylolacrylamide are the most commonly used crosslinkers. [PERSON] et al. prepared a PAM gel based on resorcinol crosslinked with formaldehyde. The gel had a temperature resistance of up to 120 \({}^{\circ}\)C, but it was too sensitive to salt and pH. [PERSON] et al. sulfonated the resorcinol used in the above gel to improve the salt and pH resistance of the gel. Unlike toxic phenolic crosslinkers, organic polymer crosslinkers such as PEI are non-toxic and environmentally friendly. The crosslinking reaction between PEI and PAM involves nucleophilic mine nitrogen on PEI replacing the amide group on PAM, forming a stable covalent bond. Therefore, PEI-crosslinked gels have better high-temperature stability than metal ion-crosslinked gels [31]. To solve the issue of metal ion-crosslinked gels being prone to hydrolysis at high temperatures, resulting in gel syneresis, organic-inorganic composite crosslinkers are commonly used to enhance the compactness of the 3D network structure, thereby improving the high-temperature stability of the gel. [PERSON] et al. prepared a composite-crosslinked PAM gel system using both Cr3+ and phenolic resin as the crosslinkers [58]. Because the system had both weak bonds generated by the metal crosslinkers and strong bonds generated by the organic crosslinkers, the presence of the double-bond network effectively inhibited the high-temperature syneresis of the metal-crosslinked structure, which significantly improved the temperature resistance of the gel, allowing it to remain stable when aged at 140 \({}^{\circ}\)C for 120 days (Figure 8). Similarly, an organic-inorganic composite crosslinker composed of a phenolic resin and sodium tripolphosphate (STPP) was crosslinked with an acryloxyethyl trimethyl ammonium chloride/AM (DAC/AM) copolymer to prepare a double-group crosslinked hydrogel that was capable of operating at 130 \({}^{\circ}\)C [59]. The filling of high-temperature-resistant solid materials (e.g., clay, nanoparticles, fibers, and fruit shells) during polymer gel synthesis can also improve the high-temperature stability of the polymer gel to a certain extent. Nanoparticles have advantages of small particle size, large specific surface area, and high surface activity, and they have been widely used in oil-gas drilling and production engineering in recent years. Nanomaterials can be added as property modifiers to gel components to increase the degree of crosslinking between the polymer molecules and densify the network structure, thus optimizing the temperature resistance of the gel. [PERSON] et al. found that nano-montmorillonite was a suitable temperature-resistant additive [60].