Twenty years of claystone transport research: Methods, challenges and lessons learned

Claystones are important in a wide range of geological applications, functioning both as effective caprocks for subsurface fluid storage and as potential host formations for radioactive waste disposal. For over 20 years, the Petrophysics Group - led by Dr. Bernhard Krooss and Prof. Dr. Ralf Littke at the Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University - has investigated different flow regimes, including advection, slip flow and diffusion, using various fluids such as water, hydrogen, methane and carbon dioxide. The primary focus has been the precise characterization of transport properties under varying boundary conditions, including temperature, lithostatic and pore pressures, fluid composition and degree of water saturation. To achieve this, the group has applied many different measurement techniques, ranging from steady-state methods to nonsteady-state pulse-decay tests, classic and “unconventional”, such as radial and axial uptake techniques. In this contribution, we discuss the sensitivities and potential pitfalls encountered when using these methods, illustrating our findings with representative datasets collected over the past 20 years. We also present recent findings from the ongoing MATURITY project (co-funded by the Federal Ministry of Education and Research and the Federal Company for Radioactive Waste Disposal), in which we systematically compare laboratory-based permeability coefficients, obtained through various measurement techniques, with field-scale observations to identify potential discrepancies and evaluate their implications. In addition, we examine how different core storage conditions affect transport properties by analyzing samples from the same locations, where one set has been stored under ambient conditions since the 1980s, and another set was vacuum-sealed in aluminum-laminate foil immediately after retrieval. Finally, we emphasize the need for establishing standardized testing protocols and adopting benchmark samples to ensure both reproducibility and comparability of permeability data across different laboratories and research teams, complementing the discussion with our own datasets. This overview underscores the importance of robust experimental design and careful data interpretation in advancing our understanding of claystone transport properties.