Direct Quantification of Coal Pore Dynamics during Methane Depletion via Low-Field Nuclear Magnetic Resonance

This study presents a novel low-field nuclear magnetic resonance (LF-NMR) framework to directly quantify sorption-induced pore strain and pore compressibility in coal reservoirs and thereby provides key parameters for predicting permeability during coalbed methane (CBM) production. Three coal samples of varying ranks (high-, middle-, and low-rank) were subjected to controlled methane adsorption/desorption and confining stress experiments under constant effective stress. By correlating transverse relaxation time (T2) spectra with methane phase dynamics, we resolved adsorbed gas (micropores) and free gas (mesopores, macropores, fractures) contributions, enabling real-time tracking of pore deformation. Analysis on the measurements reveals that the sorption-induced pore volumetric strain displays a linear relationship with adsorption gas content, ranging from 0.0108 to 0.0613 g·cm–3; the range of pore compressibility variation was calculated using an exponential relationship between transport pore volume and effective stress, and it ranges from 0.0509 to 0.0902 MPa–1. These two factors directly characterize the volumetric strain of the methane transport space within the coal reservoir, providing a direct, assumption-free approach to characterize pore-scale mechanics, particularly for heterogeneous coal reservoirs.