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

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.

KEYWORDS:

• CO₂ sequestration
• long-term supercritical CO₂ exposure
• coalbed methane recovery
• moisture-equilibrated coals
• pores
• functional groups

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study is financially supported by the National Natural Science Foundation of China (Grant Nos. 52264001 and 42272202), the Yunnan Fundamental Research Projects (Grant No. 202201AT070144), and the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project (Grant No. YNWRQNBJ-2019-164).

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.