ABSTRACT
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.
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Notes on contributors
Haitao Wang
Haitao Wang is currently a Researcher mainly focused on unconventional oil and gas exploration and production.
Ran Yang
Rang Yang is currently a graduate student majoring in chemical engineering. His research mainly involves geologic sequestration of greenhouse gas.
Xuewei Cai
Xuewei Cai is currently a graduate student majoring in chemical engineering. Her research mainly involves unconventional natural gas exploration and production.
Yi Xu
Yi Xu is currently a PhD student majoring in chemical engineering. Her research mainly involves shale gas exploration and production.
Dongyang Li
Dongyang Li is currently a graduate student majoring in chemical engineering. His research mainly involves geologic sequestration of greenhouse gas.
Xuexiang Fu
Xuexiang Fu is currently a PhD student majoring in chemical engineering. His research mainly involves coalbed methane exploration and production.
Dengfeng Zhang
Dengfeng Zhang is currently a Professor. His research mainly involves carbon dioxide capture and sequestration, unconventional oil and gas exploration and production.