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

Abstract

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