Pore Structure Affecting the NMR Relaxation in Unsaturated porous media

Nuclear magnetic resonance (NMR) uniquely reveals pore water properties due to the magnetization and relaxation dynamics of water molecule hydrogen atoms. Importantly, the correlation between NMR signals (amplitude and relaxation times) and water content and distribution aids in discerning water retention patterns in porous media. While this relationship is well understood in saturated media, comprehending water dynamics under unsaturated conditions using NMR datasets is a considerable challenge, owing to the complexities of the pore environment (e.g. pore structure and interactions between different phases and components). In many previous studies, an increased amplitude of shorter relaxation T₂ time distribution components has often been associated with enhanced water bound in micropores in unsaturated versus saturated media. The interpretation is counterintuitive as smaller pores cannot exceed their saturation water capacity, implying a potential misinterpretation of water distribution dynamics and pore structure within unsaturated media. To address this misinterpretation, our study develops a model to simulate the T₂ peak shift from unsaturated to saturated pore states. The simulations successfully reconcile these anomalies, indicating that unsaturated macropores can display short relaxation times akin to saturated micropores and demystifying the decrease in shorter relaxation time components in T₂ distributions of non-expansive, multi-pore-sized media from saturated to unsaturated states. By establishing different models to idealize pore structure characteristics (e.g. size and shape), the simulated NMR relaxation can clarify how pore structure affects the NMR relaxation, and how information about water distribution and pore structure are interpreted from NMR outcomes in unsaturated porous media.

Keywords: Nuclear magnetic resonance; porous media; water distribution; pore structure