Quantitative characterization of methane adsorption on coal using a low-field NMR relaxation method

• An NMR-transparent setup for measuring sorption isotherms has been constructed.
• Methane corresponds to the T2 signal of < 7 ms.
• Methane confined in coal pores corresponds to the T2 spectrum of 7–240 ms.
• Adsorption isotherms measured by NMR and the volumetric method agree within ± 2 m3/t.
• NMR can be applied to qualitatively characterize the methane adsorption capacity of coals.

To develop a nuclear magnetic resonance (NMR) method for characterizing the methane adsorption capacity of coals, we built an NMR transparent isotherm adsorption experimental setup. Proton (1H) NMR measurements were first performed with bulk methane to obtain a hydrogen amplitude index reflecting the methane volume concentration. Then, dry coals were pressurized with methane to assess the quantity of adsorbed methane at pressures up to 6.1 MPa. The adsorption isotherms obtained by this procedure were compared with the corresponding sorption isotherms determined by the traditional volumetric method under the same experimental conditions. The methane transverse relaxation time (T2) spectra have three distinct peaks at T2 < 7 ms, T2 = 7–240 ms, and T2 = 240–2000 ms. The peaks at T2 < 7 ms and T2 = 7–240 ms both correspond to the surface relaxation mechanism, and individually, they are interpreted as “coal-adsorbed methane” and “porous medium-confined methane”, respectively. The peak at T2 = 240–2000 ms represents the relaxation of bulk methane. The integrated amplitude of the “porous medium-confined methane” peak has a positive linear relationship with pressure, whereas the integrated amplitude of the “coal-adsorbed methane” peak with increasing pressure follows the Langmuir equation. The adsorption isotherms from the NMR and volumetric methods are nearly identical. The absolute deviations of the experimental data points from the two methods fall within ± 2 m3/t, and the calculated Langmuir volumes have absolute deviations of < 0.38 m3/t and relative deviations of < 1.24%. We suggest that the low-field NMR may be applied as a noninvasive analytical technique to characterize the methane adsorption capacity of coals.

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