# 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
prepared a novel elastic cement designed with sufficient strength and integrity using an additive containing carbon nanotubes (CNT) in a methyl methacrylate polymer matrix. The new additive provides the appropriate rheology, dramatically reduces fluid loss, and improves cement's mechanical properties and bonding strengths [45]. Figure 4: Physical photograph and microstructure of super-swelling PPG before and after swelling. (**a**) Physical photograph of super-swelling PPG before and after swelling; (**b**) microstructure of super-swelling PPG before and after swelling at room temperature. (Reprinted/adapted with permission from Ref. [40]. 2018, [PERSON], et al.). Figure 5: Mechanism of high-temperature stable/unstable crosslinker composite PFGs. (**a**) PPG Swells. (**b**) PPG degrades into viscous polymer solution. (Adapted from Ref. [41]. 2013, [PERSON], et al.). Unlike PPG preparation, which involves first preparing the bulk gel and then mechanical dispersion granulation, [PERSON] et al. proposed a method for preparing the dispersed particle gel (DPG) by direct mechanical shearing, where the particle gel was prepared by high-speed shearing using the polymer and crosslinker, and the particle size was controlled by the shear rate [37]. The DPG prepared using this method had a facile preparation process, had a wide adjustable particle size range, and was suitable for injection during preparation in the oilfield, which was beneficial for its promotion and application [43; 44]. To optimize the performance of the DPG under high-salinity conditions, [PERSON] et al. prepared a strengthened reinforced DPG (SDPG) with nano-silica as the reinforcing agent, resulting in better mechanical properties and viscoelasticity than conventional DPG (Figure 6) [46]. The particles of PPGs generally range from a few hundred microns to a few millimeters and can only enter formations that have large-sized high-permeability pore channels or fractures, where PPGs aim to form a high-strength plugging zone. However, due to their large size, PPG particles have a limited migration distance in formations and fail to meet the requirements of in-depth profile control or oil displacement in oil-gas reservoirs. To overcome this problem, some polymer microspheres with smaller sizes, a smooth surface, regular morphology, and suitable dispersion and suspension have been prepared. These microspheres can migrate and adsorb in the large pore channels with the dominant flow, achieve temporary plugging through the multi-particle bridge, and displace crude oil in small pores where the oil flow accumulates, thus realizing in-depth profile control and oil displacement in formations [47; 48; 49]. Inverse suspension polymerization and inverse emulsion polymerization are the two standard methods for polymer microsphere preparation. [PERSON] et al. prepared micron-sized PAM elastic microspheres (MPEMs) as profile control agents via inverse suspension polymerization [47]. The microspheres showed stable performance at 100 \({}^{\circ}\)C, a salinity of 30,000 mg/L, and different pH levels. The field application results also showed that the microspheres could effectively improve water flooding sweep and displacement efficiencies. The properties of microspheres can be optimized by introducing different materials into the polymer molecular chain. Graphene oxide (GO) has high strength and a large specific surface area. Therefore, by introducing modified GO into the structural chains of microspheres through inverse emulsion polymerization, the microspheres exhibited a significantly improved strength value of 115.8 kPa even after 30 days of aging (Figure 7), suitable retention in small pore channels, and maximum additional flow resistance of 7.12 kPa. The microspheres exhibited suitable performance for in-depth profile control and displacement in reservoirs [50]. Nanoparticles can also contribute considerably to the optimization of microsphere performance. [PERSON] et al.