Pore space architecture and water binding state in clay-rich rocks

Clays and clay rocks are relevant materials in many natural or engineered systems. Particles and pores in these materials are very small, which results in very low permeabilities. Accordingly, clays or clay rocks are considered as sealing materials or as host rocks for the safe disposal of hazardous waste in the underground.

The architecture of the pore space, i.e., the pore size distribution and the pore connectivity, are fundamental characteristics that define macroscopic properties of these materials, such as water retention function, hydraulic conductivity, diffusion coefficients, or the mechanical behavior. Unfortunately, the resolution of imaging techniques is often insufficient for a direct visualization of all pores in clays, and mostly indirect methods have to be used. Moreover, porewater close to charged clay surfaces may be partly bound, and this can also affect hydraulic conductivities.

We applied a range of different methods (Hg injection, N₂ and H₂O ad-/desorption, simultaneous thermal analysis coupled with evolved gas analysis STA-EGA) to characterize the pore space architecture and physical properties of porewater of a set of twelve very different rocks. We addressed the following questions: (1) Is porewater close to solid surfaces more strongly bound compared to porewater far from surfaces? (2) Are physical porewater properties related to basic properties of the clay rocks, such as clay-mineral content, cation exchange capacity, or the pore solution composition?

When comparing pore size distributions derived from the above methods and from NMR cryoporometry (Fleury et al., 2022), we see that mostly similar size ranges are obtained, but specific peaks should not be overinterpreted. Only NMR cryoporometry allows measurements at the original saturation state (except for high salinity solutions), which minimizes potential artefacts from drying. During STA, mainly water was released up to ~200°C (heating rate 10°C/min) in all samples. Vaporization enthalpy distributions derived from the STA data – indicators of water binding states – are unimodal in many cases, meaning that no clearly distinct water populations exist. However, the width of the distributions varied considerably among the samples. Comparably narrow distributions with a main peak in the region of bulk water vaporization enthalpies were seen for samples with relatively large pores, and wider or very wide distributions for samples with small pores, complex pore networks, higher surface charge concentration per volume of pore water, or high salinity pore solutions. The latter demonstrates that the derived vaporization enthalpies do not only reflect surface interactions, but also interactions with solutes. Finally, the partly large differences in the energetic state of the porewater should be considered as a relevant pore-scale feature when trying to derive macroscopic hydraulic parameters.

Fleury, M., T. Gimmi, M. Mazurek (2022). Porewater content, pore structure and water mobility in clays and shales from NMR methods. Clays Clay Miner. 70, 417–437, https://doi.org/10.1007/s42860-022-00195-4