On the application of silica gel for mitigating CO₂ leakage in CCS projects: rheological properties and chemical stability

Silica sol gels have the potential to act as sealing agents to reduce leakage risks associated with long-term CO₂ storage. This study considers the effects of brines of varying chemical composition on the formation of sol gels, their viscosity, and their long term stability. The gelation times of sol–gel solutions were measured for different concentrations of SiO₂, Na⁺, K⁺, Ca^2＋, and Mg²⁺ as well as pH levels. Individually, increased concentrations of SiO₂, Na⁺, K⁺, Ca^2＋, and Mg²⁺ reduced gelation time. However, the combined effects of Na⁺, HCl, and Ca^2＋ or Mg²⁺ were found to delay gelation, compared to when only Ca^2＋ or Mg²⁺ is added. Gelation times were similarly found to be a complex function of the pH of the system. Empirical fits were obtained describing the gelation times and the precursor sol viscosities from the start of activation until gelation. Expressions are presented that relate the changes in the fitting parameters in response to variations in gel composition. There is good agreement between the experimental measurements and the models, which could be used to predict gelation rates in field-scale applications. The durability of the gel was also investigated through experiments in which the gels were exposed to different solutions of varying salinity and pH. These results showed that silica gels were stable after 45 days of brine exposure, with the most significant change being a slight expansion of the gel. Additional experiments revealed that the gels remained thermally stable for expended periods at a temperature of 60 °C.