Petrophysical Analyses for Supporting the Search for a Shale-Hosted Nuclear Repository
Authors:   BGE, Strobel Joachim 
Published: September 2024

The Bundesgesellschaft für Endlagerung mbH (BGE) is responsible for identifying a deep geological repository site in Germany, which should allow the disposal of high-level radioactive waste and ensure the best possible safety for at least one million years. The three-phase site selection process is currently in the second half of its first phase, during which around 90 potentially suitable sub-areas are evaluated via individual representative preliminary safety assessments. These subzones cover all types of potential containment rock: allochthonous and autochthonous salt, shale, and basement. Such a comprehensive assessment is especially challenging for a host rock that covers vast areas like shale or autochthonous salt. This paper focuses on the estimation of some relevant petrophysical parameters of the host-rock shale. One of the steps toward estimating the mentioned parameters 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 (GR) logs have been the petrophysicist’s workhorse for decades, they allow no quantification of shale properties. Radioactivity itself is no specific property of shale or its physical parameters. Computing a “min-max” curve may appeal as a good shale indicator, but it has little significance for evaluating the quality of the shale in the context of a nuclear repository. Density and acoustic logs are indirect measurements only, requiring a matrix parameter and, in the case of the acoustic log, a mixing law. Hydrogen-sensitive logs like neutron or nuclear magnetic resonance (NMR) show the best potential for evaluations. However, thermal neutron logs suffer from a magnitude of corrections and the presence of thermal absorbers. On the other hand, the NMR logs require a minimum relaxation time below 400 ms and consideration for perturbations caused by iron. Decades of research gave good algorithms for defining wet clay porosity from resistivity logs, but these methods are not validated in pure shale layers and need knowledge of the penetrating water salinity. Starting from critical wells with core control, a solver-based model using NMR, epithermal neutron, and resistivity data estimates robust shale porosities and clay volumes with tortuosity as a by-product. Transposing the model allows its application to wells with less comprehensive data. A second option is based on the fact that shale compaction acts on clay water and intergranular porosity. Hence, compiling porosity results from the log analyses allows the derivation of compaction-depth trends that guide parameter selection for wells with fewer data. Another log-derived parameter is the homogeneity of a rock formation as seen by statistical data quantifying the jaggedness of a logging curve, such as vertical variograms, curve differentiation, or variable filter techniques. The paper looks at the definition of shale and its porosity within the context of containment. It shows how a GR log can be deceiving, with a higher reading meaning worse shale properties. It offers a vague depth-compaction trend based on NMR and epithermal neutron data. That trend can be compared to early log-based porosity computations, potentially eliminating areas with a strong surplus porosity.