Petrophysical Analyses for Supporting the Search for a Claystone-Hosted Nuclear Repository

The Bundesgesellschaft für Endlagerung mbH (BGE; “Federal Company for Radioactive Waste Disposal”) is responsible for identifying a repository site in Germany for the storage of high-level radioactive waste, ensuring the best possible safety for at least 1 million years (StandAG, 2017). The three-phase site selection process is currently in Step 2 of its Phase I. Around 90 potentially suitable sub-areas are evaluated via individual representative preliminary safety assessments. These sub-areas cover all types of potential containment rock: allochthonous and autochthonous rock salt, claystone, and crystalline rock. Such a comprehensive assessment is challenging for a host rock that covers vast areas like claystone or autochthonous salt. One of the steps towards estimating parameters for a site search process requires the analysis of numerous logs from the areas of interest and their vicinity. Unfortunately, computing these parameters is a much more complex problem than it seems. While gamma ray logs have been the petrophysicist’s workhorse for decades, they allow no quantification of clay properties. Nuclear magnetic resonance and neutron activation logs yield critical information but are scarce in old wells. Decades of research have given good algorithms for defining wet clay porosity from resistivity logs, but inverting those for porosity and tortuosity remains challenging. Another log-derived parameter is a formation’s homogeneity, which quantifies the jaggedness of a logging curve. A vertical variogram or variable filter technique can accomplish this.