Multiscale insights into CO2/CH4 competitive adsorption and diffusion: Molecular dynamics and pore-scale modeling for enhanced shale gas recovery and carbon storage optimization

CO₂ injection in shale reservoirs is highly promising and feasible in enhanced shale gas recovery and carbon capture and storage (CCS). It is essential to fully understand the complex competitive adsorption and diffusive behaviors of CO₂ and CH₄ in shale reservoirs at multiscale levels. This study integrates molecular dynamics (MD) and pore-scale simulations on nano-CT-reconstructed 3D shale matrices to unravel CO₂/CH₄ competitive adsorption and diffusion across multiple scales. MD results demonstrate the preferential adsorption of CO₂ over CH₄, with binding affinities ranked: organic matter > SiO₂ > kaolinite. The confinement effects in small nanopores (4 nm) amplify the density of CO₂ adsorption by 60–70 % compared to larger pores (10 and 20 nm), while the adsorption/desorption rates derived from MD simulation and the diffusion coefficients calculated from MSD govern the transport dynamics. The uniform porous model (UPM) achieves 43–64 % CO₂ recovery at optimized CO₂ concentrations (300–600 mol/m³), while the fractured porous model (FPM) exhibits lower recovery due to preferential flow bypassing adsorbed CO₂. The diminishing returns beyond 600 mol/m³ highlight a critical balance between methane production and injection of CO₂.