Molecular-Level Insights into CO₂ Dissolution Trapping in Deep Saline Aquifers: Diffusion Behavior in NaCl Brines

Carbon capture, utilization, and storage (CCUS) is critical for carbon neutrality, and deep saline aquifers are promising reservoirs for CO₂ sequestration. CO₂ diffusion in brine directly affects dissolution trapping efficiency and is strongly influenced by salt ions. Molecular dynamics simulations were employed to investigate CO₂ diffusion in NaCl brines under varying concentrations (0.1-5.0 mol/L), temperatures (298-353 K), and pressures (3-40 MPa). Diffusion coefficients were derived from mean square displacement, and radial distribution functions combined with hydrogen bond analysis were used to elucidate microscopic mechanisms. Results show that as NaCl concentration increases from 0.1 to 5.0 mol/L, the diffusion coefficient decreases by ~50%, reflecting the kinetic consequence of the salting-out effect. Raising temperature from 298 to 353 K enhances diffusion by ~149%, following Arrhenius behavior, while pressure shows negligible influence below 30 MPa but causes a 15% drop at 40 MPa. RDF analysis reveals that higher salinity densifies the CO₂ hydration shell without changing its coordination number, and ions do not accumulate near CO₂. Hydrogen bond analysis indicates that slower diffusion arises primarily from increased viscosity and steric hindrance from hydrated ions rather than disruption of hydrogen bonds. These molecular-level insights can guide site selection and injection strategy optimization for CO₂ geological storage in saline aquifers.