# An Ultra-Stable Polysaccharide Gel Plugging Agent for Water Shutoff in Mature Oil Reservoirs
## 3 Results
### Cts/MBA Gelation Mechanism  
The microstructure of the CTS/MBA gel system before and after gelation is presented in Figure 11. Before gelation, the CTS/MBA system exhibited an irregular network structure, while after gelation, it displayed a distinct three-dimensional honeycomb structure, indicating that cross-linking occurred between CTS and MBA, which subsequently altered the microstructure of the gel.  
A comparison of the infrared spectra in Figure 12 reveals that the characteristic peaks of the CTS/MBA gel system largely overlap with those of CTS and MBA. Notably, significant peaks at 1675 cm\({}^{-1}\) and 1621 cm\({}^{-1}\) are absent in the CTS spectrum but are present in both the MBA and CTS/MBA spectra. In the MBA spectrum, transmittance at 1621 cm\({}^{-1}\) is higher than at 1675 cm\({}^{-1}\), whereas in the CTS/MBA spectrum, this trend is reversed, with transmittance at 1675 cm\({}^{-1}\) exceeding that at 1621 cm\({}^{-1}\). Given that the peak at 1675 cm\({}^{-1}\) is attributed to C=O stretching vibrations and the peak at 1621 cm\({}^{-1}\) is attributed to C=C stretching vibrations, it can be inferred that the number of C=C bonds decreased following the cross-linking reaction.  
The TG-DTG curves of CTS and the CTS/MBA gels, presented in Figure 13, reveal that CTS undergoes weight loss in two main stages. The first stage, involving approximately 12% weight loss, occurs as the temperature increases from 30 \({}^{\circ}\)C to 100 \({}^{\circ}\)C, likely due to the evaporation of crystal water. No significant weight loss is observed during the subsequent temperature increase from 100 \({}^{\circ}\)C to 250 \({}^{\circ}\)C; however, weight loss resumes as the temperature exceeds 250 \({}^{\circ}\)C, continuing until 630 \({}^{\circ}\)C, with the fastest rate of weight loss at 302.8 \({}^{\circ}\)C. The CTS/MBA gel also exhibits weight loss in two stages--below 200 \({}^{\circ}\)C, weight loss is likely related to the evaporation of crystal water and acetic acid, whereas above 200 \({}^{\circ}\)C, weight loss may be attributed to the thermal decomposition of the molecular structure, with the fastest rate of weight loss occurring at 272.8 \({}^{\circ}\)C. These findings indicate that the thermal stability of CTS/MBA is lower compared to CTS, likely due to the lower structural stability of MBA relative to the rigid cyclic structure of CTS. Although the presence of MBA reduces the decomposition temperature of the gel, it also demonstrates that cross-linking occurred between CTS and MBA, thereby altering the thermal properties of the molecular network.  
Figure 11: Microstructure of the CTS/MBA gel system (2.0 wt% CTS, 0.6 wt% MBA). (**a**) Before gelation; (**b**) after gelation at 120 \({}^{\circ}\)C.  
Based on the above analysis, the proposed gelation mechanism is illustrated in Figure 14. The NH\({}_{2}\) groups of CTS undergo a Michael addition reaction with the \(\alpha\),\(\beta\)-unsaturated carbonyl groups of MBA, resulting in the covalent cross-linking of the CTS molecular chains by MBA, ultimately forming a three-dimensional gel network structure.  
Figure 14: (**a**) Chemical reaction during the gelling process. (**b**) Gelation mechanism of CTS/MBA.  
Figure 12: FTIR spectrum of all initial reactants and CTS/MBA.  
Figure 13: TGA curves of CTS (**a**) and CTS/MBA (**b**).