# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 1 Introduction  
Polymer gels are elastomers with a three-dimensional (3D) network structure composed of polymers and crosslinkers as the primary agents and other additives. The 3D network structure of polymer gel can binds a large number of solvents and make it resistant to high formation pressure. As a result, polymer gels are polymer materials that have adjustable strength and suitable deformability and pumpability [1; 2; 3], which makes them widely used in oil-gas engineering, such as formation plugging, lost circulation control, profile control, water shutoff, fracturing during production, and wellbore strengthening during oil-gas drilling. Polymer gels can achieve a greater operating depth than conventional packers during profile control and water shutoff operations [4; 5; 6] and can overcome the size-matching difficulties of conventional rigid materials in lost circulation control operations during drilling. Crosslinked polymer acids have the advantages of both conventional acids and guar gum fracturing fluids in fracturing operations, resulting in smaller friction and higher sand-carrying capacities [7; 8]. Therefore, polymer gels have been very effective in various field applications of the oil-gas drilling process [9; 10; 11]. The polymer gels in oil-gas drilling and production engineering can be categorized into ground crosslinked polymer gel systems and in situ crosslinked polymer gel systems. The gelation process of ground crosslinked polymer gels occurs before injection, while the crosslinking of in situ crosslinked polymer gels occurs in the formation. The main advantage of ground crosslinked polymer gels is their ability to migrate at depth, while in situ crosslinked polymer gels have advantages of suitable mechanical strength and temperature resistance. Because both have advantages and disadvantages, the appropriate crosslinking system should be selected according to the specific field conditions [12; 13]. Polyacrylamide (PAM) is the most commonly used polymer for preparing gels, including in situ and ground crosslinked polymer gels. PAM is inexpensive, has numerous carboxyl and amide groups, and can be hydrolyzed under high-temperature or alkaline conditions to form stable partially hydrolyzed PAM (HPAM). The presence of carboxyl and amide groups ensures that they can crosslink well with various organic/inorganic crosslinkers, forming a gel system with suitable mechanical properties [14]. The most common inorganic crosslinkers are high-valent ion crosslinkers such as Cr\({}^{3+}\) and Al\({}^{3+}\). Metal ion crosslinkers are inexpensive and have achieved suitable results in field applications. However, gel systems that use metal ions as crosslinkers are often only suitable for reservoirs below 80 \({}^{\circ}\)C, and ionic crosslinkers have nonnegligible toxicity; thus, their applications in high-temperature reservoirs are limited. Commonly used organic crosslinkers include phenolic and polyethyleneimine (PEI), which have better temperature resistance and mechanical properties than inorganic crosslinkers due to their stronger covalent bonds, and as a result, have a wide range of applications and prospects [15; 16]. Polymer gels in oil-gas reservoirs are often subjected to high temperatures and salinity, and excessive temperatures and salinity can destroy the structural integrity of the polymer chains, resulting in substantial instability. Therefore, maintaining the suitable mechanical properties of polymer gels under high-temperature and high-salinity conditions is extremely difficult [17; 18; 19]. Researchers have optimized polymer gel properties from three aspects: polymer structure, crosslinker type, and the addition of other additives. Conventional PAM is prone to excessive hydrolysis at high temperatures, affecting the gels' stability in the formation.