Influence of Natural Gas Composition on Reserve Estimation in Carbonates: Theoretical and Experimental Investigations

First-principles density functional theory (DFT) is used to study the adsorption and transport properties of the components of natural gas; including CH₄ and CO₂ molecules, inside a cylindrical nanopore created from CaCO₃ calcite. Three different diameters of calcite nanopores were considered; 1.16, 2.05, and 3.65 nm. The results show that the adsorption energy increases (i.e., the interaction becomes stronger) exponentially with decreased radius of the nanopore. Depending on the molecule type and pore radius, the interaction energy of the molecule with the nanopore can be more than five times stronger compared to the one obtained for the flat surface. The adsorption capacity of the nanopore for CH₄ molecule was calculated to estimate the saturation of the nanopores with gas molecules. Additionally, molecular dynamic calculations and capillary pressure measurements are conducted to provide information about the characteristics of carbonate porous media and the transport of the gases inside it. These findings can be useful for the ultimate recovery of natural gas (EUR) from tight gas reservoirs.