# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 5 Improving Rheological and Mechanical Properties of Polymer Gels
The post-gelation modulus and viscosity of the gel increased with increasing modified starch content, and the pressure-bearing strength was as high as 24.9 MPa for fractures that were 0.42 mm wide. Figure 12: The schematic diagram of mechanism of C-SiO\({}_{2}\)/PAM/PEI withstands long-term stability. (Reprinted/adapted with permission from Ref. [73]. 2022, [PERSON], et al.). Adding inorganic materials such as nano-silica and clay (e.g., bentonite and Li-bentonite) to the gel system can considerably improve the mechanical properties of the gel through physical filling, chemical reaction, or the synergistic effect of nano-silica and clay. [PERSON] et al. prepared a Li-bentonite-enhanced nanocomposite hydrogel based on anionic partially hydrolyzed AM [78]. The main principle was as follows. First, the negatively charged polymer molecular chains would bind to the edges of the positively charged Li-bentonite sheets, and second, the hydroxyl groups (-OH) on the surface of Li-bentonite would interact with the amide groups (-CONH\({}_{2}\)) on the amide side chains of the polymer, forming hydrogen bonds. In addition, the cations on the Li-bentonite surface would form complexes with the carboxyl groups of the polymer (Figure 13). The experimental results showed that the elastic modulus of the gel increased by approximately 30,000 Pa as the Li-bentonite concentration in the gel components increased. [PERSON] et al. prepared a hybrid nanocomposite of poly(styrene-methyl methacrylate acrylic acid)/clay as a novel rheology-improvement additive for drilling fluids. The new drilling fluids maintained fluid-loss control properties to temperatures up to about 200 \({}^{\circ}\)F and salinities of up to about 40 g/L NaCl concentration and reduced fluid loss by about 77% for the terpolymer-treated drilling fluid [79; 80]. [PERSON] et al. prepared PAM/PEI and PAM/PEI-CFA gels using PEI as the crosslinker and coal fly ash (CFA) as the inorganic filler as filling materials [81]. The CFA was mainly composed of alumina and calcium oxide. The experimental results showed that the gel with CFA addition exhibited a larger elastic modulus and a denser network structure when observed under an SEM (Figure 14). Conventional inorganic nanomaterials will improve the mechanical properties of gels mainly through physical embedment. The gel properties can be significantly improved by combining the active nanomaterials with the gel structure through chemical methods, and intercalation polymerization is a commonly used method. First, the modified layered silicate inorganic minerals are dispersed in the monomers by intercalation polymerization, allowing some monomers to enter the interlayers. Then, the monomers will start to polymerize and crosslink, thereby forming a gel with suitable mechanical properties. Using the above principle, [PERSON] et al. [82] developed a composite gel material with a storage modulus of 15 kPa and compressive strength of 8.5 MPa at 160 \({}^{\circ}\)C (Figure 15). The above intercalation polymerization method could serve as an inspiration for the development of highly elastic, high-strength gels. Figure 13: Mechanism of interaction between the anionic polymer and Li-bentonite surface cations [78]. (Adapted from Ref. [78]. 2012, [PERSON], et al. ).