Methane – Natural Clay Interfacial Interactions as Revealed by High-Pressure Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) Spectroscopy

The present study aims to provide fundamental, molecular- to microscopic-level descriptions of methane gas within natural source clay minerals. Texas montmorillonite (STx-1), Georgia kao-linite (KGa-2), and Ca2+ saturated Texas montmorillonite (Ca-STx-1, Ca-bentonite) were used as subsurface model systems for elucidating nano-confinement behaviors of 13C labeled methane gas. High-pressure Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) was utilized to characterize the interactions between methane and the clays by varying temperature and pressure. In the pure state, no significant thermal effect on the behavior of methane was observed. However, there was a perceptible change in the chemical shift position of confined methane in the mixtures with the clays up to 346 K. Conversely, the 13C-NMR chemical shift of methane changed as a func-tion of pressure in a pure state, and the mixtures with clays, attributed to the interaction of me-thane with the clay surfaces or the nanopore network of the clay-silica mixed phase. There was only one 13C-NMR peak of methane in the mixture with either kaolinite (KGa-2) or Ca-bentonite with line-broadening compared to that of pure methane, but two peaks were observed in the mix-ture with STx-1, explained by the imbibition and mobility of methane in the pore network.