Microscopic pore structure and macroscopic fluid flow-chemical transport in host rocks and barrier materials

Wang, Q. M.*; Hu, Q.*; Zhao, C.*; Zhang, T.*; Fukatsu, Yuta  ; Tachi, Yukio  

Fluid flow and chemical transport in porous media are the macroscopic consequences of pore structure, which integrates geometry (e.g., pore size and surface area, pore-size distribution) and topology (e.g., pore connectivity). Low-permeability geological media whose pores are poorly interconnected will exhibit the characteristics of anomalous diffusion and sample size-dependent effective porosity, which will strongly impact long-term net diffusion and retention of radionuclides in geological repository settings. A suite of experimental approaches is utilized to study the microscopic pore structure and macroscopic fluid flow and chemical transport for a range of natural rocks (such as clay/shale, crystalline rock, salt), in addition to clay minerals. With a particular focus on quantifying the presence and magnitude of isolated pores for a reduced effective porosity in low-permeability geomedia, the integrated methodologies for basic properties and pore structure characterization include X-ray diffraction, thin section petrography, grain size distribution, water immersion porosimetry, mercury intrusion porosimetry, nitrogen physisorption, scanning electron microscopy, X-ray computed tomography, and (ultra-)small angle neutron (or X-ray) scattering. In addition, custom-designed gas diffusion, tracer recipe involving a range of anionic and cationic chemicals with subsequent analyses by laser ablation and inductively coupled plasma-mass spectrometry, along with batch sorption, column transport, and imbibition tests were conducted for coupled effects of pore structure and chemical retention/transport.