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

Abstract

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 (T2) 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, T2 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 T2 spectra of the TDC contain three distinctive peaks; the first, P1 (T2 < 2 ms), represents adsorbed methane, and the second, P2 (2–40 ms), corresponds to free methane. As the pressure increases, both the T2 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 T2 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.