# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 3 Improving Temperature Resistance of Polymer Gels  
With the continuous depletion of medium and shallow oil-gas resources, deep and ultra-deep oil-gas resources are gaining increasing attention. As a result, high temperatures are one of the most critical challenges faced during deep and ultra-deep oil-gas drilling and production [52; 53]. Because polymers are susceptible to chain scission degradation  
Figure 7: Preparation mechanism, microscopic image, and strength variations of modified GO polymer microspheres; (**1**) synthetic routes of FRGO; (**2**) synthetic routes of RGO-PMSs; (**a**) synthetic routes of GO polymer microspheres; (**b**) morphology of modified GO polymer microspheres; (**c**) strength of polymeric microspheres versus aging time [50]. (Reprinted/adapted with permission from Ref. [50]. 2020, [PERSON], et al.). at high temperatures, the gel strength of the polymer gel material will decrease, which affects its efficiency in these applications. This is mainly due to the dissolved oxygen in the polymer gel or the residual initiators at high temperatures, which considerably influence gel performance. Due to the presence of oxygen/residual initiators, the main chain of the gel will be prone to chain oxidation reactions at high temperatures, likely resulting in scission at weak bond points where the peroxides are generated, resulting in gel instability. In addition, the methylene groups of the crosslinked polymer gel will be subjected to nucleophilic attack at high temperatures by the protonated solvent (water), breaking and forming a hydroxyl group on one side and an amide on the other side. When the protonated solvent attacks the amide carbonyl carbon, the amide will break and generate a primary amine and carboxylic acid. Nucleophilic attack on the carbonyl carbon of the side chain amide by the protonated solvent (water) will generate carboxylic acid and ammonia. Both cleavage and hydrolysis of the above groups will destroy the 3D network structure of the gel. Numerous studies have been carried out to address the challenging problem of the high-temperature resistance of polymer gels. Currently, the main research avenues include optimizing the polymer macromolecular chain structure, optimizing the crosslinker type, and adding high-temperature-resistant additives to the gel components [54]. Commonly used polymers in oilfields, such as PAM, starch, guar gum, and xanthan gum, are prone to chain scission and are challenging to degrade at high temperatures. Introducing cyclic structures (e.g., benzene ring) or temperature-resistant groups (e.g., sulfonic acid groups) into their molecular chains by grafting and copolymerization can significantly improve the temperature resistance and tolerance of the polymer molecular chains, and, subsequently, the high-temperature stability of the gels [23]. [PERSON] et al. prepared a high-temperature-resistant pre-crosslinked gel system by polymerizing and crosslinking AM, N-vinylpyrrolidone (NVP) with a cyclic structure, and divinylbenzene (DVB); the gel strength remained stable after aging for 90 days at 130 \({}^{\circ}\)C [55]. AMPS is a commonly used temperature-resistant monomer for the preparation of monomer gels and/or polymers due to its long-branched chains and sulfonic acid groups. AM/AMPS copolymers have suitable thermal stability due to the steric hindrance of many methyl propane and sulfonic acid groups on the polymer chain. [PERSON] et al. crosslinked an AM/AMPS copolymer with a phenolic crosslinker and used ethylenediamine as a crosslinking retard to prepare a gel with a dense 3D network structure [56]. The gel maintained suitable strength and stability after 100 days of aging at a high temperature of 130 \({}^{\circ}\)C and was successfully used for reservoir water shutoff operations in oilfields in Northwest China. [PERSON] et al. prepared an in situ crosslinked gel system with suitable thermal stability and an adjustable gel strength by reacting AM/AA/AMPS terpolymers with OC-3, a novel crosslinker with hydroxyl and phenolic rings [57].