Effects of CO₂-brine-rock Fracture Interaction on Reactive Solute Transport Properties During CO₂ Sequestration in Deep Saline Aquifers

The deep saline aquifer has a storage potential of CO₂ over 60%, making it one of the most recognized storage method. The complex reactive solute transport between saline water, CO₂ and surrounding rock minerals directly affects the CO₂ sequestration in deep saline aquifers and long-term environmental safety. Reactive solute transport experiment on a sandstone fracture was conducted to determine the main mineral reactions. The governing equations for the evolution of fracture aperture considering chemical reaction processes were given, while coupled with advection–diffusion equation and Navier–Stokes equation to calculate the fracture aperture, specie concentration and fluid pressure, respectively. The species concentration distribution, fluid flow and fracture aperture evolution characteristics at short and long cycles were obtained under four combinations of Pe number and Da number, considering the dynamic variation of the grid and cyclic seepage boundary under chemical reaction. The experimental results indicated that the primary mineral reaction in the rock fracture was CaAl₂Si₂O₈ + CO₂ + 2H₂O =  > Ca²⁺ +  \({\text{CO}}_{3}^{{2}-}\) ${\text{CO}}_3^{2-}$ +Al₂Si₂O₅(OH)₄. The results of the dimensionless parameter analysis revealed that the Pe number was the governing factor of the solute transport process in the short cycle, and the time to reach the peak solute concentration decreased with increasing Pe number. Under the long cycle, the solute transport process was governed by the Da number. The reaction rate is relatively faster under high Da number conditions during the early stage, as are the product concentration variation rate and fracture aperture variation rate. The fracture closed under a high Da number, resulting in a progressive decrease in the variation rate of Ca²⁺ concentration during the late stage. Additionally, the bypassing flow phenomenon of streamlines and species transport pathways was also observed. The research results can provide a foundation for the long-term potential assessment, transport pathways prediction, and safety evaluation of CO₂ sequestration in saline aquifers.