# Types and Performances of Polymer Gels for Oil-Gas Drilling and Production: A Review
## 5 Improving Rheological and Mechanical Properties of Polymer Gels  
In oil-gas drilling and production engineering, one of the main uses of polymer gels is to plug pores and fractures in formations and resolve problems such as drilling fluid loss during drilling and water channeling during production. After gel plugging of the target formation, the gel structure will be quickly destroyed under the actions of formation pressure and water scouring. Therefore, the mechanical properties of the gel will play a decisive role in their plugging effectiveness in the formation, and the rheological and mechanical properties are the two most commonly used parameters for evaluating the mechanical performance of a gel. The mechanical properties of a gel depend mainly on the polymer, crosslinker, filler material type, and concentration. They are susceptible to the influence of the formation environment, such as temperature and pressure [69; 71]. Figure 11: **The** Schematic diagram of the preparation of salt-insensitive microgels by pre-embedding of Cr\({}^{3+}\) ligands. **(a)** The schematic representation of the Cr\({}^{3+}\)-embedded microgels; **(b,c)** schematic representations of the poly (acrylic acid)-based microgels and the Cr\({}^{3+}\)-embedded microgels influenced by the sodium ions. (Reprinted/adapted with permission from Ref. [70]. 2019, [PERSON], et al.). Gel component type and concentration will most intuitively influence the mechanical properties of the gel. The polymer and crosslinker type and concentration will determine the crosslinking sites that can react, thereby affecting the formation of the 3D network structure of the gel. [PERSON] et al. prepared a gel system for water shutoff based on an AMPS-NVP-PAM copolymer, PEI, and bentonite and evaluated the influence of each factor on the rheological properties of the gel [72]. Increasing the AMPS and AM concentrations also increased the storage modulus and strength of the gel, while increasing the NVP components or clay content increased the compactness of the gel grid, which increased the gel strength, and the post-gelation composite modulus reached 20-40 Pa. [PERSON] et al. prepared a polymer gel with nano-silica as the reinforcing agent (Figure 12). [73]. The study showed that enhancing gel strength is the main reason for the excellent water plugging effect and oil recovery of the C-SiO\({}_{2}\)/PAM/PEI gel. Multiple crosslinked chemical bonds can also be used to construct a gel network to improve the mechanical properties of the gel. Multiple crosslinks can form an interpenetrating composite network, densifying the gel network. [PERSON] et al. developed a multi-crosslinked gel system based on a high-temperature-resistant AM-based polymer (sulfonated polyacrylamide or SPAM), chromium acetate, and PEI [74]. Because the crosslinking rate of SPAM with chromium acetate was much higher than with PEI, SPAM was initially crosslinked with Cr\({}^{3+}\) to form a weakly crosslinked gel with suitable injectability and viscoelasticity. This was conducive to injecting it into the formation, draining the formation water from the pores and fractures, and maintaining the gel component stability. After entering the target formation, the gel was crosslinked with PEI again to form a strong covalent bond network, and the presence of the double network ensured the excellent injectability and suitable gel strength of the gel system. In addition to using PAM-based polymers, modified starch has also been commonly used as a raw material for gel preparation. Gels based on modified starch are characterized by low initial and high gel viscosities, which are beneficial for achieving a suitable plugging performance [75; 76]. On this basis, [PERSON] et al. prepared a modified starch gel system with suitable mechanical strength, suitable injectability, and an initial viscosity below 60 mPas [77].