Underground carbon dioxide storage in confined systems becomes a viable alternative to diminish atmospheric concentrations of this gas. Shale reservoirs exhibit mineralogical and pore size heterogeneities that are not deeply analyzed to evaluate the transport and adsorption capacities of carbon dioxide inside their matrix. Functionalized carbon nanotubes and inorganic nanochannels composed of calcite or silicon dioxide are excellent approximations to model the poral throats of the organic and inorganic matrices of shale reservoirs, respectively. In this work, through an extensive molecular dynamics study, we assess the impact on adsorption and transport properties of carboxylic functionalization of the nanochannel surfaces and oxidized inorganic nanochannels, considering only silicon dioxide on pure carbon dioxide and water and carbon dioxide mixtures. We find that the presence of a relevant concentration of carboxylic groups and silicon dioxide on both types of nanochannels significantly reduces the axial velocity of carbon dioxide, owing mainly to their geometrical contributions. Regarding carbon dioxide and water mixtures at different molar fractions, simulations show that there is a relevant increase in water adsorption for both organic and inorganic nanochannels due to strong Coulombic interactions, which partially occlude the available space where carbon dioxide molecules could be adsorbed and displaced. In Figure 1a, we observe how the water molecules nucleate, self-owing to their own Coulombic interactions. On the other hand, in Figure 1b, we observe how this fluid interacts with SiO2, owing to its chemical affinity with the hydrophilic surface. Additionally, based on our findings, the mineralogical composition, the O/C relationship of kerogen, and residual water saturation confined in the nanopores all play a relevant role in defining the storage capacity of carbon dioxide.
