# 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  
The gel of an in situ crosslinked gelation polymer refers to the injection of a polymer (or monomer), crosslinker, and initiator into the target formation, forming an aqueous solution that undergoes crosslinking in the formation environment and creating a viscoelastic gel with a 3D network. In situ crosslinked gels often have excellent gel strength and are adapted to plugging pores and fractures in the formation. The gel types of in situ crosslinked gel systems can be divided into rigid and weak gels, and each gel type has its applicable conditions due to its unique properties. Rigid gels have suitable strength and temperature resistance and are suitable for profile control, water shutoff, and lost circulation control near the wellbore. Weak gels of in situ crosslinked gel systems have suitable deformation and migration properties and are suitable for profile control, water shutoff, oil displacement, and fracturing in remote-well zones. Both types have achieved suitable results in field applications [23; 24; 25]. Acrylamide-based polymers or copolymers are the most commonly used polymers for in situ crosslinked gels and have molecular chains rich in amide and hydrolyzed carboxyl groups. Thus, the carboxyl groups can be crosslinked with metal ions (e.g., Cr\({}^{3+}\) and Al\({}^{3+}\)) to form ionic-bonded gels. The first in situ crosslinked gel system for profile control was prepared by Philips (now ConocoPhillips) in the 1970s using partial HPAM with aluminum citrate [26; 27]. [PERSON] et al. compounded halfway HPAM polymers with a chromium acetate derivative to prepare a chromium ion-crosslinked gel water shutoff system suitable for medium-temperature reservoirs [2]. However, it had a short crosslinking time at high temperatures; thus, it could not be used for in-depth water shutoff operations in formations. In addition, the metal ion-crosslinked gels were prone to syneresis under high-temperature conditions; therefore, this type of gel was only suitable for low and medium-temperature formations (<80 \({}^{\circ}\)C). In addition to crosslinking with metal ions, AM-based polymers can undergo crosslinking reactions with phenolic crosslinkers due to the amide groups on the molecular chain. Commonly used phenolic crosslinkers include phenol/formaldehyde, resorcinol/hexamethylenetetramine (HMTA), and water-soluble phenolic resins. Because phenol/formaldehyde crosslinked systems are highly toxic, less toxic organic hydroquinone (HQ)/HMTA crosslinked systems are more widely used. [PERSON] et al. studied the crosslinking mechanism as well as temperature and salt resistance of PAM/HQ/HMTA gels (Figure 1), and the results showed that the gel system had suitable salt resistance and maintained suitable stability below 140 \({}^{\circ}\)C [28]. In addition, covalently bonded gels produced by the reaction of organic crosslinkers with AM-based polymers will have better properties than metal ion crosslinkers and, subsequently, a wider pH and temperature adaptation range [29]. [PERSON] et al. prepared a gel using phenolic resin crosslinkers compounded with non-ionic PAM as an indepth profile control agent in formations, and the system was stable at a high temperature of 143 \({}^{\circ}\)C and achieved suitable application results in the Xinjiang oilfield, China [30]. The weak gel oil-displacement agent, which was prepared by compounding the PAM/water-soluble phenolic resin, was applied to the Pu-125 block of the Daqing oilfield, China. The cumulative production of the whole block increased by 1400 t, comprehensive water cut decreased by 7.16%, and crude oil recovery increased by 3.58%, proving the effectiveness of the system [13]. Figure 1: Schematic diagram showing the crosslinking mechanism of the resorcinol/HMTA gel. (**a–d**) Three-dimensional network structure of gels from different perspectives. (Reprinted/adapted with permission from Ref. [28], 2016, [PERSON], et al.).