First-Principles Calculation of Adsorption of Shale Gas on CaCO ₃ (100) Surfaces

Background To demonstrate the adsorption strength of shale gas to calcium carbonate in shale matrix, the adsorption of shale gas on CaCO ₃ (100) surfaces was studied using the first-principles method, which is based on the density functional theory (DFT). Methods The structures and electronic properties of CH ₄ , C ₂ H ₆ , CO ₂ and N ₂ molecules were calculated by the generalized gradient approximation (GGA), for a coverage of 1 monolayer (ML). Under the same conditions, the density of states (DOS) of CaCO ₃ (100) surfaces before and after the adsorption of shale gas molecules at high-symmetry adsorption sites were compared. Results The results showed that the adsorption energies of CH ₄ , C ₂ H ₆ , CO ₂ and N ₂ on CaCO ₃ (100) surfaces were between 0.2683 eV and -0.7388 eV. When a CH ₄ molecule was adsorbed at a hollow site and its 2 hydrogen atoms were parallel to the long diagonal (H3) on the CaCO ₃ (100) surface, it had the most stable adsorption, and the adsorption energy was only -0.4160 eV. The change of adsorption energy of CH ₄ was no more than 0.0535 eV. Compared with the DOS distribution of CH ₄ before adsorption, it shifted to the left overall after adsorption. At the same time, the partial density of states (PDOS) curves of CaCO ₃ (100) surfaces before and after adsorption basically overlapped. Conclusions This work showed that the adsorption effect of shale gas on calcium carbonate is very weak, and the adsorption is physisorption at the molecular level.