How Does Fluid Exchange Between Pores in Unsaturated Porous Media?

The inter-pore fluid exchange, driven by molecular diffusion, is referred to as pore coupling. The pore coupling affects subsurface pore-scale physicochemical dynamics. Monitoring pore coupling is essential for understanding numerous hydrological and ecological processes. Nuclear magnetic resonance (NMR) has effectively tracked pore coupling in saturated porous media. However, monitoring unsaturated pore coupling with NMR is challenging due to the complex pore network architecture and dynamic water-pathway connectivity. Based on X-ray computed tomography imaging, we develop a framework to simulate the NMR (Formula presented.) -distribution and (Formula presented.) -store- (Formula presented.) map of samples with distinct-sized pore environments at different water saturations, validated against measurements. NMR results, combined with visualized pore networks and phase distributions, demonstrate that reduced saturation induces increasingly tortuous and eventually disconnected water-pathways, thus restricting and ultimately blocking pore coupling. Our work enhances NMR applications in unsaturated media, providing accurate insights into the complex architecture and hydrodynamics in the pore network.