Influence of natural gas composition on adsorption in calcite Nanopores: A DFT study

Highlights: • Cylindrical carbonaceous nanopore of tight gas reservoir is studied using DFT. • CH₄ and CO₂ adsorption is studied with higher affinity to CO₂ and both physisorbed. • E_ads of both gases increased ~5 exponentially with decreasing nanopore diameter. • The capacity test revealed an adsorption of 28/24 of CH₄/CO₂ on the nanopore. • After that, the surface still has affinity to absorb more molecules energetically. Density functional theory is used to study the adsorption of natural gas components in calcite (10.4) cylindrical nanopores with 1–4 nm diameters. The change of adsorption energy with the diameter of the nanopores is studied for CH₄ and CO₂ gases. The results of the simulation calculations showed that as the nanopore diameter decreases, the adsorption energy increases exponentially due to the geometry of the smallest pore that increases the affinity of the molecules to the surface. Compared to the flat surface, for both molecules, CH₄ and CO₂, the interaction energy of the molecule with the nanopore could increase to more than five times depending on pore radius and molecule type. Additionally, in all cases, CO₂ has a greater affinity to the surface than CH₄; thus, it is more affected by the surface curvature and energy. For methane, adsorption energy on the flat surface is $-$0.0025 eV/Ų, while on the smallest nanopore, it increases to $-$0.0139 eV/Ų. On the other hand, the adsorption energy of carbon dioxide has increased from $-$0.0046 eV/Ų on the flat surface to $-$0.0263 eV/Ų on the smallest nanopore. To estimate the nanopore saturation of the gas, the capacity of the gases' adsorption was calculated. The nanopores absorbed up to 28 and 24 molecules of CH₄ and CO_2, respectively, and the adsorption energy decreased to −0.0062 and −0.0075 eV/Ų for each. Although the nanopore was filled spatially by the molecules, its surface still has an affinity to absorb more gas molecules energetically. These findings could be useful for estimating the adsorbed gas on carbonate rocks.