# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 3 Improving Temperature Resistance of Polymer Gels
The hydroxyl groups (-OH) on the nanoparticle surfaces could form hydrogen bonds with the amide side chains (-CONH\({}_{2}\)) on the polymer molecular chains. However, the metal ions (Ca\({}^{2+}\) and Al\({}^{3+}\)) on the clay particle surfaces could be complex with carboxyl (-COOH) oxygen on the polymer molecular chains. Both reactions enhanced the 3D network structure of the gel, improving its temperature  
Figure 8: Crosslinking principles of the polymer gel with and without the addition of STPP crosslinker, respectively (Reprinted/adapted with permission from Ref. [59]. 2015, [PERSON], et al.). resistance. In addition, nano-montmorillonite could also assemble together in the gel structure, forming an armor that acted as an insulator against high temperatures and inhibited the high-temperature degradation of the gel. [PERSON] et al. prepared an in situ crosslinked gel system by reacting zirconia/graphene nanocomposites with low molecular weight PAM and showed that the addition of zirconia/graphene enabled the gel to form a honeycomb microscopic network structure with a smooth surface [61]. This allowed the gel to lock the internal water molecules at high temperatures. Nano-silica (SiO\({}_{2}\)) has also been commonly used as a solid additive for gels. SiO\({}_{2}\) can be used to fill voids in a 3D network gel structure, to increase the strength of the gel skeleton. However, SiO\({}_{2}\) can also bind to the hydrophilic groups on a gel skeleton through hydrogen bonding, increasing the compactness of the gel network structure (Figure 9). [PERSON] et al. introduced nano-silica into the composite gel agent for lost circulation control, and the thermal stability of the gel increased up to 38% [62]. [PERSON] et al. found that small-sized SiO\({}_{2}\) could increase the interfacial area of the gel and subsequently increase the surface free energy, which was conducive to enhancing the temperature resistance of the gel [63]. However, filling nanomaterials for polymer gels has been mainly based on the principle of physical toughening to improve the high-temperature stability of gels. Thus, solving the problem of high-temperature polymer gel failure is still a challenge. In general, to improve the high-temperature stability of polymer gel materials, current studies have focused on the preparation of high-temperature-resistant polymers and crosslinkers and the filling of high-temperature-resistant solid materials in the gel structure. It will be necessary to carry out in-depth research and development of new high-temperature-resistant polymers and crosslinkers from the perspective of their molecular design. Considering that solid filling materials can improve high-temperature gel stability to a certain extent, it will also be necessary to improve research on the interaction mechanisms between the gel components and the solid particles to provide a reference for the further selection of solid filling materials with excellent properties (Table 1). Figure 9: Microstructure of the polymer gel with the addition of different concentrations of SiO\({}_{2}\). (**a**) 0% nano-silica; (**b**) 1% nano-silica; (**c**) 3% nano-silica; (**d**) 5% nano-silica. (Reprinted/adapted with permission from Ref. [62]. 2020, [PERSON], et al. ).