Nuclear Magnetic Resonance Study of the Transition from Bulk- to Surface-Dominated Relaxation of Hydrogen in Micron-Scale Pores

Understanding the proton relaxation mechanism of hydrogen gas in porous media is critical for underground hydrogen storage. This study investigates the proton relaxation mechanisms of hydrogen gas using variable-pressure NMR experiments on idealized glass bead pack models (6.8–65.9 μm). Results indicate: (1) The proton spin–spin relaxation time (T2) of bulk H2 gas is linearly proportional to pressure, confirming the dominance of the spin–rotation (SR) interaction. (2) In pores larger than 16.4 μm, bulk relaxation prevails, rendering the T2 distribution single-peaked and pore-size independent. (3) Conversely, in 6.8 μm pores, a distinct bimodal T2 distribution emerges, separating free-gas and surface-dominated components. A theoretical critical pore size (≈11.5 μm) was estimated based on a two-phase exchange model. This work elucidates the fundamental regime transition from bulk- to surface-dominated proton relaxation in micron-scale pores.