Re-evaluating Adsorbed and Free Methane Content in Coal and Its Ad- and Desorption Processes Analysis

Discriminating methane mass stored between free and adsorbed states in coal and shale is crucial in devising optimal gas recovery strategies and greenhouse control. Existing methods of estimating free and adsorbed gas contents in core plugs cannot fully discriminate between methane phases due to high levels of compaction and the resulting complex architecture of micropores. We propose a method using nuclear magnetic resonance (NMR) spectroscopy to define adsorbed/free methane ratios in powdered coal at pressures up to 15 MPa and to verify its fidelity against standard isothermal adsorption measurements. The methane T2 spectra exhibit four distinct peaks in the intervals 0.01-1, 1-20, 20-100, and - 1000 ms, respectively. The three peaks located < 100 ms all correspond to surface relaxation indicating the presence of adsorbed (0.01-1 ms) and free (1-100 ms) methane. The free methane can be interpreted as occupying both small (1-20 ms) and large (20-100 ms) pores as indicated by relaxation times and their pressure-dependency. The adsorption capacity generally reaches a maximum at - 10 MPa with free methane content only lower than adsorbed content below - 5 MPa. Adsorbed/ free methane ratios recovered by NMR compare favorably with isothermal adsorption results with NMR also capable of calibrating absolute adsorbed methane contents.