Abstract:

In shale gas reservoirs, CH₄ often coexists with heavy alkanes such as C₂H₆ and C₃H₈. To investigate the mechanism by which heavy alkanes affect the competitive adsorption of CH₄ and CO₂ in organic nanopores of shale, this study combines density functional theory and Grand Canonical Monte Carlo simulations. The interaction types and strengths between gas molecules and kerogen were systematically analyzed in terms of adsorption energy, structural characteristics, weak interaction analysis, and isothermal adsorption. The adsorption performance of pure-component gases was examined, followed by an evaluation of the influence of varying C₂H₆ and C₃H₈ concentrations on CH₄ adsorption performance. Furthermore, the influence of C₂H₆ and C₃H₈ on CO₂ and CH₄ competitive adsorption was analyzed. The results showed that: (1) With the elongation of alkane carbon chains, the interaction strength between gas molecules and kerogen progressively intensified. The adsorption energies of the four gas molecules on the kerogen surface followed the order: C₃H₈>C₂H₆>CO₂>CH₄. (2) For single-component gas adsorption, the total adsorption capacity decreased from CH₄ to C₃H₈ due to the increased adsorption sites occupied by individual alkanes. Meanwhile, CO₂ exhibited a higher adsorption capacity, which was attributed to its unique geometric structure. (3) With rising system temperature, the adsorption capacity of various gas molecules declined. As system pressure increased, the incremental rise in gas adsorption capacity gradually diminished. (4) In binary mixture adsorption, the existence of C₂H₆ and C3H8 significantly reduced the CH₄ adsorption capacity, with C₃H₈ exhibiting a more significant effect. Meanwhile, the addition of CO₂ greatly decreased CH₄ adsorption capacity. (5) In ternary mixture adsorption, the coexistence of C₂H₆, C₃H₈, and CO₂ demonstrated a synergistic effect on promoting CH₄ desorption. Specifically, the most effective promotion of CH4 production was achieved when C₂H₆ and C₃H₈ accounted for 4% and 8% of the total gas mixture by mass. In addition, compared with C₃H₈, C₂H₆ was more favorable to the storage of CO₂. These findings provide theoretical support for multicomponent competitive adsorption in shale gas and for CO₂-enhanced shale gas extraction.

Key words: kerogen, multicomponent shale gas, competitive adsorption, computational chemistry, molecular simulation