Prediction of Hydrogel Degradation Time Based on Central Composite Design

In this study, a self-degrading hydrogel was formed by free-radical-initiated copolymerization, which can be used for oil and gas well strip pressure operations. Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (¹H NMR), scanning electron microscopy (SEM), and thermogravimetry-mass spectrometry (TGA-MS) were used to study the reaction mechanism as well as the microstructure of the gels. Then, the effects of the four factors and their interactions on gel degradation time were determined by central composite design (CCD). Then, the effects of copolymer concentration, cross-linker, initiator, and reaction temperature and their interactions on gel degradation time were determined by central composite design (CCD), and the corresponding second-order polynomial models were generated. Finally, the gelation conditions were optimized by a response surface methodology and verified by degradation experiments. Both FTIR and ¹H NMR indicated that the gel was formed by a copolymerization reaction between the monomer and the cross-linker. SEM showed that the gel structure collapsed, which was caused by the poor mechanical properties of the gel, but it was also able to withstand some wellbore pressure and degraded more easily. TGA-MS showed that the gel possessed good degradation properties. In addition, analysis of variance (ANOVA) showed that the second-order polynomial model was highly significant. The results also showed that the expected values of the gelation conditions optimized by the response surface methodology did not differ significantly from the actual values. The degradation model can be used to predict the degradation time of the gel and optimization of gelation conditions. This study can help petroleum engineers in applying self-degrading gels to seal the wellbore pressure.