# 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
### Pre-Crosslinked Polymer Gels  
Ground pre-crosslinked gels were developed to address the problems associated with in situ crosslinked polymer gels. The pre-crosslinked gels are formed by mixing the polymer and crosslinker in the ground equipment. Then, after crosslinking is complete, it is injected in situ, often in the form of particles, to create a plugging zone through the gel particles' expansion, accumulation, and filling effects. Ground pre-crosslinked gels have certain advantages. Namely, they are insensitive to pH, temperature, shear, and salinity, giving them a more comprehensive range of applicable conditions. Commonly used ground pre-crosslinked gels mainly include pre-crosslinked particle gels (PPGs), polymer microspheres, and colloidal dispersion gels [19; 36; 37]. The concept of PPGs was first put forward by PetroChina in 1996. PPGs are usually made by producing a bulk gel with AM, crosslinkers, initiators, and other additives, followed by drying and crushing into particles ranging from microns to centimeters. PPG particles often contain hydrophilic groups such as amide and carboxyl groups. After dispersion in the aqueous phase, the particles absorb water and swell up to 200 times, thus achieving the high-strength plugging of the fractures. [PERSON] et al. [38] first prepared bulk gels with AM monomers, N,N-methylenebisacrylamide (MBAA), sodium peroxodisulfate, and bentonite as the raw materials. Then, they dried, crushed, and sieved the bulk gel to obtain PPG particles, which remained stable at 120 \({}^{\circ}\)C for more than a year and exhibited suitable salt resistance, with practical applications for profile control in water injection wells with a salinity of 150,000 mg/L and a temperature of 107 \({}^{\circ}\)C. Different polymers, crosslinkers, and additives led to PPGs with different properties. In general, particle gels with a higher swelling capacity have a lower mechanical strength, while conversely, particle gels with a lower swelling capacity have a higher mechanical strength. The introduction of hydrophilic groups can enhance the swelling capacity of the PPGs. [PERSON] et al. used AM, acrylic acid (AA), and MBAA as the raw materials to synthesize a PPG profile control agent via the free-radical method, and their study showed that the addition of AA improved the swelling capacity of the particle gel [39]. Some particular types of crosslinkers could also increase the water-absorbing capacity of the PPG. [PERSON] et al. prepared a super-swelling PPG water shutoff agent using sulfonated PAM powder and an aluminum nitrate nanohydrate crosslinker (Figure 4) [40]. Its swelling ratio was temperature and pH-responsive, and it could absorb water at a rate equivalent to 1000-2000 times its weight within a pH range of 5-9 while maintaining an intact 3D network structure. Researchers have also studied various PPG materials with special functionality to meet the plugging needs of different formations. [PERSON] et al. prepared a composite pre-crosslinked gel material using both high-temperature stable and high-temperature unstable crosslinkers [41]. Unlike conventional pre-crosslinked gels, the composite PPG swelled and plugged the high-permeability zone when injected into the reservoir. After a while, the unstable crosslinks of the particle gel were destroyed under the influence of reservoir temperature, and the PPG was degraded into a viscous polymer, which migrated deeper into the reservoir with the subsequent water flow, an action similar to polymer flooding; thus, it further improved the sweep efficiency and displacement efficiency (Figure 5). [PERSON] et al. prepared a re-crosslinkable PPG (RPPG) based on an AA-AM copolymer [42; 43; 44]. The RPPG could swell 38 times its initial volume and re-crosslink into a high-strength bulk gel while swelling, which enhanced the ability of the particle gel layer to plug pores and fractures in the formation. [PERSON] et al.