4.7 Article

Influence of pore structure and surface free energy on the contents of adsorbed and free methane in tectonically deformed coal

Journal

FUEL
Volume 285, Issue -, Pages -

Publisher

ELSEVIER SCI LTD
DOI: 10.1016/j.fuel.2020.119087

Keywords

Tectonically deformed coal; Adsorbed and free methane; Nuclear magnetic resonance; Pore structure; Adsorption potential

Funding

  1. Major National Science and Technology Projects [2016ZX05044002-003]
  2. China Scholarship Council [201906420039]
  3. Postgraduate Research & Practice Innovation Program of Jiangsu Province [KYCX19_2126]
  4. Postgraduate Research & Practice Innovation Program of China University of Mining and Technology [KYCX19_2126]
  5. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of the Ministry of Education (China University of Mining and Technology) [2019-0011]

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Differences in pore and surface free energy in tectonically deformed coal affect coalbed methane extraction and mine safety. Strongly deformed coal has larger pore volume and specific surface area, leading to higher contents of adsorbed and free methane.
Differences in pore and surface free energy affect the occurrence of adsorbed and free methane in tectonically deformed coal (TDC), which influences coalbed methane extraction and coal mine safety. Therefore, eight middle-rank TDC samples from the Yueliangtian coal mine (Guizhou Province, Southwest China) were screened. Pore volume and specific surface area were explored via mercury intrusion, nitrogen adsorption/desorption, and carbon dioxide adsorption approaches. Based on the low-field nuclear magnetic resonance analysis for powdered TDC, the relaxation time (T-2) amplitudes and the contents of adsorbed and free methane, the surface free energy reduction (SFER), and adsorption potential were measured. 1) The pore specific surface area, pore volume, T-2 amplitudes and contents of adsorbed and free methane, SFER, and adsorption potential of the strongly deformed coal are obviously larger than those of the weakly deformed coal. Adsorbed methane content is dominantly affected by micropore specific surface area, and free methane content is primarily influenced by macropore volume. 2) The methane T-2 spectra of the TDC contain three distinctive peaks; the first, P-1 (T-2 < 2 ms), represents adsorbed methane, and the second, P-2 (2-40 ms), corresponds to free methane. As the pressure increases, both the T-2 amplitude rate and content rate of adsorbed methane decrease, while those of free methane rise. 3) With the enhancement of tectonic deformation, the specific surface area and volume of pores increase (especially those of micropores and macropores), more adsorption positions and storage space for adsorbed and free methane become available, and the SFER and adsorption potential of the TDC are enhanced, leading to increases in the T-2 amplitudes and contents of adsorbed and free methane. The present research will help improve the accuracy of coalbed methane evaluation and the prevention of coal and gas outbursts.

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