This dissertation extensively examines the brittle structural elements of the Boda Claystone Formation (BCF), which could serve as a host formation for high-level radioactive waste in Hungary. Despite the formation’s low porosity and permeability, which give its retentive features, it is crucial to assess the tectonic setting of the rock body to comprehend the timing and magnitude of faults and folds.
The idea behind the research is that each successive phase of the study examines the problem at an increasing scale, from microstructural observations to models of the fracture network based on more than one borehole. The main goals of this study were to identify the geometry and hydrogeological properties of the fracture network of the Boda Claystone Formation as well as the investigation of the spatial extensibility of the properties of the fracture network.
The BAF–2 well was regarded as a base well in this study because it explores the maximum thickness of the formation (917 m) with almost 100% core recovery and offers unique opportunity to evaluate its brittle structural elements. In addition, the analysis of the BAF–4 well was also done to investigate the extensibility of the properties of the fracture system.
The petrographic description of the sigmoidal veins in the shear zones was carried out on thin sections. The volume and density changes of the shear zones were analysed based on the geometry of the sigmoidal veins, using the isocon method and micro-computed tomography. The geometry of the fracture network was investigated using the discrete fracture network modelling method, while its hydrological properties were characterised using the flow zone indicator. This indicator designates hydraulic flow units based on porosity and permeability. The relationship between the fracture network and well log data was investigated using multiple linear regression analysis.
Based on petrographic observations, the sigmoidal veins in the BCF can be interpreted as “pennant” veins whose formation was caused by conjugate Riedel fractures. Despite the formation of sigmoidal veins, a 5-8% volume loss occurred in the shear zones. Based on fracture network modelling, five fracture network geometries can be distinguished in the BAF-2 well. In some models, the fracture network forms a communicating system along the entire length of the borehole, while in other cases it breaks down into subsystems that do not communicate with each other at depths of 100, 400, or 700 meters. The fracture system can be divided into seven hydraulic units, the boundaries of which coincide with the boundaries designated by fracture network modelling. In the upper 100 meters of the borehole, the hydrological properties of the rock were affected by weathering. Based on the properties of the fracture network, the rock column of the well can be divided into two very distinct blocks with a border at a depth of 400 m. The hydrological properties of the formation at 700 meters were determined by the by fine-grained sandstone layers.
Based on multiple linear regression analysis, the fracture density of the BCF can be estimated based on density and resistance well log data. Fracture density can also be estimated in boreholes without acoustic borehole televiewer data using the regression equation.