Microrheological Evolution of C-S-H and M-S-H Gels During Gelation and the Regulatory Mechanism of Cations

Inorganic silicate gels, including calcium silicate hydrate (C-S-H) gel and magnesium silicate hydrate (M-S-H) gel, exhibit notable advantages in profile control and water shutoff applications in high-salinity oilfields both domestically and internationally. To elucidate the influence of ions on the gelation state and process of silicate gels in high-salinity reservoirs, this study employs diffusing wave spectroscopy (DWS) to investigate the effects of common reservoir cations on the microrheological behavior of C-S-H gel and M-S-H gel. The results indicate that the gelation process of C-S-H gel can be divided into four stages: an initial viscosity-dominated stage; a rapid cross-linking stage (1-2 h) with sharply decreased mean square displacement (MSD) and emerging elastic dominance; a stable equilibrium stage (2-12 h) with stabilized network structure; and a long-term relaxation stage (12-72 h) featuring increased MSD, declining modulus, and local network relaxation. In contrast to C-S-H gel, M-S-H gel consistently exhibits viscosity-dominated fluid behavior throughout the gelation process, with the slope of the MSD curve remaining close to unity, indicating that it does not form a dense elastic network. Cation type exerts a significant regulatory effect on the microrheological behavior of the two silicate gels. For C-S-H gel, Ca²⁺ promotes particle aggregation through strong charge bridging, resulting in the lowest MSD and the highest storage modulus. The monovalent cations Na⁺, K⁺, and H⁺ exhibit weak cross-linking effects, leading to higher MSD values, with the H⁺ system being the loosest due to the acidic environment inhibiting polycondensation. Although Mg²⁺ is divalent, it tends to form a loose M-S-H phase, which increases MSD and reduces the modulus. For M-S-H gel, the modulus values under different cation systems follow the order of MgCl₂ < HCl < NaCl < KCl < CaCl₂. Keywords: C-S-H gel; M-S-H gel; gelation process; cations; micro-rheology; high-salinity reservoirs.