Molecular simulation study on CO₂ sequestration and shale gas displacement in the quartz-calcite composite system

Industrialization has raised atmospheric CO₂ levels and worsened global warming, driving the urgent demand for CO₂ emission reduction. CO₂ enhanced shale gas recovery (CO₂-ESGR) simultaneously boosts gas production and CO₂ storage. This study constructs a quartz calcite composite nanopore model and employs GCMC, MD, and DFT simulations to investigate CO₂ adsorption and shale gas displacement. The results show that CO₂ adsorption follows the Langmuir isotherm. CO₂ preferentially adsorbs on pore surfaces (especially calcite) with an asymmetric distribution. The isosteric heat for CO₂ declines with the elevation of pressure and water content. When the moisture content is 15 wt%, compared to the dry state, the absorption of CO₂ decreases by 45.9% because water can significantly inhibit adsorption. This indicates that compared to CO₂ molecules, H₂O molecules preferentially occupied the adsorption sites. DFT calculations indicate that the adsorption energy of CO₂ on both quartz and calcite surfaces is lower than that of CH₄ and C₂H₆. The electronic structure analysis reveals the microscopic essence of the preferential adsorption of CO₂. It provides a theoretical basis for a deeper understanding of CO₂ sequestration in shale reservoirs and the study of CO₂-ESGR.