# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 2 Types of Polymer Gels Used for Oil-Gas Drilling and Production
### In Situ Polymer Gels Crosslinked
Conventional PAM is prone to degradation via chain scission at high temperatures, considerably reducing gel stability, which can be optimized through modification. PEI is an efficient high-temperature crosslinker because it can undergo the transamidation reaction with AM-based polymers, forming high-strength covalent bonds [31]. [PERSON] et al. prepared a hydrophobically modified PAM polymer, HMP, by micellar copolymerization using AM, octadecyl acrylate (OA), and AMPS as the raw materials, where OA increased the rigidity of the polymer molecular chains and AMPS enhanced the temperature resistance of the molecular chains, crosslinking it with PEI to prepare a temperature-resistant gel as a lost circulation control agent HMP-Gel (Figure 2) [32]. This gel exhibited a controllable gelation time in the temperature range of 100-150 \({}^{\circ}\)C and showed an excellent plugging effect on carbonate formations [32]. In addition to the AM-based polymers, polysaccharide polymers (e.g., starch, guar gum, and xanthan gum) and their modified derivatives have been used to prepare many types of gels, and these gels are environmentally friendly but have lower temperature resistance and gel strength than AM-based gels [5]. In addition to those mentioned above, in situ chemically crosslinked polymer gels, which require crosslinkers, and in situ physically crosslinked polymer gels have emerged in recent years that do not require the use of crosslinkers. The latter gel type is formed through the entanglement or association of polymer chains with special functional groups. Physically crosslinked gels differ from chemically crosslinked polymer gels mainly because they have unique shear thixotropy and use different crosslinking methods, giving them higher flushing resistance and better retention in fractures, making them more easily plug  
Figure 2: Mechanism and effect of the HMP-Gel as a lost circulation control agent. (**a**) The gelation mechanism of HMP-Gel; (**b**)the microscopic morphology of the HMP-Gel; (**c**) the performance of the HMP-Gel for different size chips bed; (**d**) comparison of plugging performance of different gel. (Reprinted/adapted with permission from Ref. [32]. 2021, [PERSON], et al.). large fractures. To address the vicious lost circulation of drilling fluid during drilling, [PERSON] et al. [33] developed a physically crosslinked gel plugging agent (GP-A), which mainly consisted of a spider web-type star-shaped structure, unlike the linear gel structure generated by HPAM. The GP-A gel exhibited high mechanical strength and had a strong adhesive surface; thus, it could attach to the fracture wall to increase its retention capacity (Figure 3). Based on the principle of supramolecular chemistry, [PERSON] et al. [34] introduced hydrophobic functional groups on PAM macromolecular chains to synthesize a highly hydrophobic-associated polymer, which could spontaneously assemble through intermolecular interactions, forming a supramolecular gel with a dynamic physically crosslinked network. This gel has been used successfully for lost circulation control in more than 30 wells with a severe loss in China. Based on a similar principle, [PERSON] et al. [35] synthesized a gel agent for lost circulation control using a supramolecular polymer thickener (SMPT) and viscoelastic surfactant (VES) worm-like micelles. This gel had very efficient physical crosslinkers and high viscosity, strong shear thinning, and self-healing abilities at low shear rates.