Evolution of Seepage in Sandstone Cracks Repaired with Mineralization of Low-Viscosity Chemical Solution

Long-term geological CO2 storage has a leakage risk, and traditional high-viscosity plugging materials fail to penetrate deep into microcracks for leakage sealing. Chemical solutions, as a novel low-viscosity sealing material, can react with escaping CO2 to form precipitates, penetrating microfractures to achieve repair and sealing. However, the long-term seepage behavior of gases within fractures during this repair process remains unclear. This study screened low-viscosity systems (Na2O·3SiO2, Ca(OH)2, Mg(OH)2, NaAlO2) through rheological experiments. Results indicate that Mg(OH)2 exhibits low solubility, NaAlO2 readily undergoes hydrolysis, and Ca(OH)2 demonstrates periodic fluctuations in fracture flow rate during prolonged interaction with sandstone, rendering it unsuitable for long-term sealing. The reaction between Na2O·3SiO2 and CO2 produces a SiO2 that adheres to the fracture, causing the flow rate to steadily decrease over time. In summary, Na2O·3SiO2 is the most promising low-viscosity system. Through soaking and seepage experiments, the seepage evolution mechanism was investigated. After 60 days under 14 MPa, the fracture flow decreased by 97.67%, and the fracture surface area increased by 46.48%. The flow reduction during repair exhibited a nonlinear relationship with effective stress changes, where the nonlinear coefficient B’ increased by 2–3 orders of magnitude with soaking time. The equivalent hydraulic aperture of the fracture decreased 6 times, and seepage entered the nonlinear stage earlier. SiO2 precipitation filled the pores, forming a dense structure that complexified flow channels and reduced fracture flow capacity. The study clarifies the optimal treatment time for Na2O·3SiO2 to block the fracture seepage in sandstone and reveals the mechanism of mineralization repair fracture.