CO₂ storage in organic nanopores with varying widths: Molecular simulation and simplified local density model

Accurately characterizing CO₂ adsorption behavior in organic nanopores is a prerequisite for estimating CO₂ storage capacity in shale reservoirs with complex pore structures. This study employs grand canonical Monte Carlo (GCMC) molecular simulations to investigate the CO₂ adsorption characteristics in organic nanopores with varying widths over a wide range of temperatures and pressures. Based on GCMC simulations, the pore width in the simplified Local-Density (SLD) adsorption model is related to key adsorption parameters, enabling the SLD model to evaluate CO₂ storage density rapidly and accurately in organic nanopores under complex reservoir conditions. The research findings suggest that the features of CO₂ excess adsorption curves are mainly influenced by temperature and the number of adsorption layers. Higher temperatures and increased adsorption layers result in higher pressure requirements to achieve the CO₂ excess adsorption peak. Under high pressures, the number of adsorption layers plays a critical role in enhancing the adsorption capacity. Due to the constraints of pore size, there are fewer adsorption layers in micropores, leading to higher CO₂ excess adsorption in mesopores than in micropores under high pressures. In the SLD model, the key parameter Λ_b decays exponentially with the increase of pore width. The average density of CO₂ in the nanopores calculated by the modified SLD model is in good agreement with the simulation results. Finally, the research reveals that smaller organic nanopore widths result in higher CO₂ densities, with limited sensitivity to temperature and pressure variations. Low pressure and high temperature reservoir conditions are unfavorable for CO₂ storage, but adsorption significantly enhances CO₂ storage density in nanopores, particularly in micropores.