Re-evaluating adsorbed and free methane content in coal and its ad- and desorption processes analysis

Abstract 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