The pore characteristics and fluid transport behavior from the nano-to micrometer scale of clays at field conditions provide fundamental insights into their performance efficiency as barrier material and their role in regulating radionuclide transport in nuclear waste repositories. Small angle scattering is the canonical technique to be used when probing the inner structure of disordered systems in the 1 to 1000 nm range. In this study, we have characterized the pore features of synthetic and naturally occurring clays including illite, palygorskite, smectite, and WY bentonite using small-angle neutron scattering (SANS), and their evolution over the relevant ranges of humidity, temperature, pressure, composition, and length scale. Our results demonstrate that each clay shows unique pore features. The effects that impact on pore features include sample form and sample environment including humidity, temperature, and pressure. Analysis of the SANS data indicates significant alteration of clays on water absorption. These results suggest that SANS is particularly suited for in-situ studies of pore-scale characteristics of geological materials and the factors that impact their pore features. As emergent property, nano-to micro-scale structural characterization is crucial in providing insights into pore-scale processes, which are pertinent to upscale continuum model development for field applications.