Impacts of CO₂/CH₄/N₂ adsorption/desorption on the macromolecular structure of anthracite: Insights from in situ Raman spectroscopy

Objective and Methods  The adsorption/desorption of coalbed methane (CBM) not only induces the macroscopic deformations of coals but also significantly alters their macromolecular structures, with different gases exhibiting greatly varying mechanisms and influence degrees. Utilizing an independently developed in situ laser Raman spectroscopy system, this study revealed the differential structural responses and mechanisms of anthracite under CO₂, CH₄, and N₂ injection.

Results and Conclusion  Experimental results indicate that gas injection led to notable blue shifts in the D₄, D, D₃, and G peaks characterizing the microcrystalline structures of coals. In contrast, red shifts were observed during desorption, with the structures failing to recover completely. With an increase in the adsorption pressure, the A-to-G area and intensity ratios (A_D/A_G and I_D/I_G, respectively), as well as the full width at half maximum (FWHM) of the D peak, increased significantly. These results indicate reduced microcrystalline sizes, decreased degrees of structural order, and increased defects. Among the studied gases, CO₂ induced the most substantial structural variations, with the influence degrees of the three gases decreasing in the order of CO₂, CH₄, and N₂. These microstructural variation patterns provide a molecular-scale theoretical basis for enhanced CBM recovery in deep coal seams via CO₂/N₂ injection and the geological sequestration of CO₂.