Long-term supercritical CO2 exposure dependence of physicochemical properties and CH4 adsorbability of moisture-equilibrated coal matrices.

Further understanding into the impacts of supercritical CO2 fluid exposure toward CO2 storage and coalbed methane (CH4) recovery is significant to evaluate validity, stability, and safety of CO2-ECBM. For purpose of addressing the potential impacts, the long-term static interactions between supercritical CO2 fluid and moisture-equilibrated coal matrices with various degrees of coalification were conducted at 318.15 K and 12.00 MPa for 240 days in this study. Furthermore, the changes of the physicochemical properties dominating adsorbability of moisture-equilibrated coals due to the exposure were revealed. Ultimately, the impacts of the exposure toward CH4 adsorbability of moisture-equilibrated coals were addressed. Results indicate that the long-term supercritical CO2 exposure induces complex effects typically including organic matter extraction, mineral dissolution and formation, and matrix swelling to the coals, thereby further reducing the micropores within pore diameter ranges of 0.38–0.71 nm and 0.74–0.90 nm of the low-rank moisture-equilibrated coals, and slightly increasing those of 0.40–0.70 nm and 0.75–0.90 nm of the high-rank moisture-equilibrated coal. Additionally, the complex effects reduce the mesopores with pore diameter less than 8.00 nm for all the coal samples. Moreover, the exposure alters the surface chemistry of the coals. Particularly, the exposure increases the oxygen-containing functional groups typically comprising C-O, C=O, and -COOH of the low-rank moisture-equilibrated coals by 25.32–27.16%, and weakens the graphitization of the high-rank coal by 34.00%. Moreover, the induced synergistic effects decrease the number of pores with diameter less than approximately 8.00 nm of all the moisture-equilibrated coals. Furthermore, the CO2 exposure significantly reduces CH4 adsorption capacity of the moisture-equilibrated coal matrices indicated by the decreasing amplitude in absolute adsorption amount of 6.20–18.00%. Overall, the long-term supercritical CO2 exposure could favor CH4 production during CO2-ECBM. Future study should focus on in-situ characterization and accurate quantitative analysis on extraction, mineral dissolution and formation, and matrix swelling due to CO2 exposure, and distinguishment in the contribution of these effects to pores and functional groups of coals. [ABSTRACT FROM AUTHOR]