Study on Ultrasonic Modification of CH₄/CO₂ Competitive Adsorption Behavior in Coal: A Pore Wall Molecular Structure Perspective.

Ultrasonic excitation can increase the coal seam permeability to improve the CO₂ injection efficiency during the CO₂-ECBM process, but it may also cause changes in the coal molecules. To investigate the effect of chemical change caused by ultrasonics on CH₄ and CO₂ competitive adsorption behavior, this paper constructs the pore wall molecular structure model based on the coal chemical structure test results. The molecular simulation results show that the CH₄ adsorption heat and adsorption density decreased with an increased pore size in single-component gas adsorption. The oxygen-containing structure slightly increased CH₄ adsorption heat by reducing the effective pore size. Compared with CH₄, CO₂ had more significant adsorption characteristics. The CH₄ and CO₂ adsorption densities were more obviously affected by the effective pore size and dominant adsorption sites, respectively. In the gas competitive adsorption behavior, the energy probability density distribution curves showed that with increased CO₂ injection, part of CH₄ transferred from the low-energy to the high-energy region. During the CO₂ injection, the gas density in the pores gradually increased, making the interaction energy between CH₄ and CO₂ more significant, especially for small pores. The injection of CO₂ into large pores more effectively weakened the interaction energy between coal and CH₄, weakening the CH₄ adsorption capacity. The oxygen-containing structure may inhibit the competitive adsorption behavior in small pores, and this effect gradually weakened or even reversed with the increase of pore size. Ultrasonic excitation reduced the oxygen-containing structure, which improved the competitive adsorption behavior in small pores.