Pore scale study of multiphase multicomponent reactive transport during CO₂ dissolution trapping

Solubility trapping is crucial for permanent CO₂ sequestration in deep saline aquifers. For the first time, a pore-scale numerical method is developed to investigate coupled scCO₂-water two-phase flow, multicomponent (CO₂(aq), H⁺, HCO₃ ⁻, CO₃ ²⁻ and OH⁻) mass transport, heterogeneous interfacial dissolution reaction, and homogeneous dissociation reactions. Pore-scale details of evolutions of multiphase distributions and concentration fields are presented and discussed. Time evolutions of several variables including averaged CO₂(aq) concentration, scCO₂ saturation, and pH value are analyzed. Specific interfacial length, an important variable which cannot be determined but is required by continuum models, is investigated in detail. Mass transport coefficient or efficient dissolution rate is also evaluated. The pore-scale results show strong non-equilibrium characteristics during solubility trapping due to non-uniform distributions of multiphase as well as slow mass transport process. Complicated coupling mechanisms between multiphase flow, mass transport and chemical reactions are also revealed. Finally, effects of wettability are also studied. The pore-scale studies provide deep understanding of non-linear non-equilibrium multiple physicochemical processes during CO₂ solubility trapping processes, and also allow to quantitatively predict some important empirical relationships, such as saturation-interfacial surface area, for continuum models.