How Does Pore Structure Affect the NMR Relaxation in Unsaturated Porous Media: A Simulation Study

Monitoring groundwater dynamics within the vadose zone is important for the investigation of many hydrological and ecological processes. Nuclear magnetic resonance (NMR) technology can be utilized to reveal these dynamics due to its unique sensitivity to water. The correlation between water content and distribution with NMR signals (relaxation times and amplitude) aids in discerning water retention patterns in porous media. However, interpreting NMR data to understand unsaturated pore-water dynamics is challenging due to complex pore environments and fluid-rock-air interactions. Especially, previous studies often misinterpret the increased amplitude of shorter relaxation T2 components of unsaturated T2 to erroneously imply that the small pore exceeds their maximum saturation capacity. We develop a simulation framework to accurately track pore-water dynamics and NMR responses (T2 and T1-T2) in unsaturated multi-pore systems. Dual-spherical and dual-triangular pore systems with different pore size distributions modeled the imbibition process. Simulations clarified the decrease in shorter relaxation T2 components in unsaturated states, revealing that unsaturated macropores and saturated micropores can exhibit the same short relaxation times when their fluid volume to fluid occupied surface area ratio is identical. Our simulation also demonstrates that different pore shapes and pore size distributions lead to distinct unsaturated NMR signals. Furthermore, we propose a new method evaluating water distribution in various natural porous media at various saturations by interpreting experimental unsaturated NMR data. Our work enhances unsaturated NMR data interpretation, providing accurate insights into water distribution and pore structure in unsaturated porous media.