X-ray computed tomography investigation of structures in Opalinus Clay fromlarge-scale to small-scale after mechanical testing

In the past years X-ray computed tomography (CT) has became more and more common for geoscientific applications and is used from the µm-scale (e.g. for investigations of microfossils or pore-scale structures) up to the dm-scale (full drill cores or soil columns). In this paper we present results from CT imaging and mineralogical investigations of an Opalinus Clay core on different scales and different regions of interest, emphasizing especially the 3-D evaluation and distribution of cracks and their impact on mechanical testing of such material. Enhanced knowledge of the testing behaviour of the Opalinus Clay is of great interest, especially since this material is considered for a long-term radioactive waste disposal and storage facility in Switzerland. Hence, results are compared regarding the mineral (i.e. phase) contrast resolution, the spatial resolution, and the overall scanning speed. With this extensive interdisciplinary scale-down approach it has been possible to characterize the general fracture propagation in comparison to mineralogical and textural features of the Opalinus Clay. Additionally, and as far as we know, a so-called mylonitic zone, located at an intersect of two main fractures, has been observed for the first time for an experimentally deformed Opalinus sample. The multi-scale results are in good accordance to data from naturally deformed Opalinus Clay samples, which enables us to perform systematical research under controlled laboratory conditions. Accompanying 3-D imaging greatly enhances the capability of data interpretation and assessment of such a material

Pore-scale tomography and imaging: applications, techniques and recommended practice

[No abstract available

Development of a numerical workflow based on μ-CT imaging for the determination of capillary pressure-saturation-specific interfacial area relationship in 2-phase flow pore-scale porous-media systems: A case study on Heletz sandstone

In this case study, we present the implementation of a finite element method (FEM)-based numerical pore-scale model that is able to track and quantify the propagating fluid–fluid interfacial area on highly complex micro-computed tomography (μ-CT)-obtained geometries. Special focus is drawn to the relationship between reservoir-specific capillary pressure (pc), wetting phase saturation (Sw) and interfacial area (awn). The basis of this approach is high-resolution μ-CT images representing the geometrical characteristics of a georeservoir sample. The successfully validated 2-phase flow model is based on the Navier–Stokes equations, including the surface tension force, in order to consider capillary effects for the computation of flow and the phase-field method for the emulation of a sharp fluid–fluid interface. In combination with specialized software packages, a complex high-resolution modelling domain can be obtained. A numerical workflow based on representative elementary volume (REV)-scale pore-size distributions is introduced. This workflow aims at the successive modification of model and model set-up for simulating, such as a type of 2-phase problem on asymmetric μ-CT-based model domains. The geometrical complexity is gradually increased, starting from idealized pore geometries until complex μ-CT-based pore network domains, whereas all domains represent geostatistics of the REV-scale core sample pore-size distribution. Finally, the model can be applied to a complex μ-CT-based model domain and the pc–Sw–awn relationship can be computed

Disaggregation bands as an indicator for slow creep activity on blind faults

Hidden, blind faults have a strong seismic hazard potential. Consequently, there is a great demand for a robust geological indicator of neotectonic activity on such faults. Here, we conduct field measurements of disaggregation bands above known underlying blind faults at several locations in Central Europe. We observe that the disaggregation bands have the same orientation as that of the faults, indicating their close connection. Disaggregation bands develop in unconsolidated, near-surface, sandy sediments. They form by shear-related reorganization of the sediment fabric, as a consequence of grain rolling and sliding processes, which can reduce the porosity. Using an analogue shearing experiment, we show that disaggregation bands can form at a velocity of 2 cm h−1, which is several orders of magnitude slower than seismogenic fault-slip velocities. Based on the field data and the experiments, we infer that disaggregation bands can form in the process zone of active blind faults and serve as an indicator of neotectonic activity, even if the fault creeps at very low slip velocity. Disaggregation bands could open a new path to detect hidden active faults undergoing aseismic movements. © 2022, The Author(s)

Disaggregation bands as an indicator for slow creep activity on blind faults

Hidden, blind faults have a strong seismic hazard potential. Consequently, there is a great demand for a robust geological indicator of neotectonic activity on such faults. Here, we conduct field measurements of disaggregation bands above known underlying blind faults at several locations in Central Europe. We observe that the disaggregation bands have the same orientation as that of the faults, indicating their close connection. Disaggregation bands develop in unconsolidated, near-surface, sandy sediments. They form by shear-related reorganization of the sediment fabric, as a consequence of grain rolling and sliding processes, which can reduce the porosity. Using an analogue shearing experiment, we show that disaggregation bands can form at a velocity of 2 cm h−1, which is several orders of magnitude slower than seismogenic fault-slip velocities. Based on the field data and the experiments, we infer that disaggregation bands can form in the process zone of active blind faults and serve as an indicator of neotectonic activity, even if the fault creeps at very low slip velocity. Disaggregation bands could open a new path to detect hidden active faults undergoing aseismic movements.publishedVersio

Classification and quantification of pore shapes in sandstone reservoir rocks with 3-D X-ray micro-computed tomography

Recent years have seen a growing interest in the characterization of the pore morphologies of reservoir rocks and how the spatial organization of pore traits affects the macro behavior of rock–fluid systems. With the availability of 3-D high-resolution imaging, such as x-ray micro-computed tomography (µ-CT), the detailed quantification of particle shapes has been facilitated by progress in computer science. Here, we show how the shapes of irregular rock particles (pores) can be classified and quantified based on binary 3-D images. The methodology requires the measurement of basic 3-D particle descriptors (length, width, and thickness) and a shape classification that involves the similarity of artificial objects, which is based on main pore network detachments and 3-D sample sizes. Two main pore components were identified from the analyzed volumes: pore networks and residual pore ganglia. A watershed algorithm was applied to preserve the pore morphology after separating the main pore networks, which is essential for the pore shape characterization. The results were validated for three sandstones (S1, S2, and S3) from distinct reservoirs, and most of the pore shapes were found to be plate- and cube-like, ranging from 39.49 to 50.94 % and from 58.80 to 45.18 % when the Feret caliper descriptor was investigated in a 10003 voxel volume. Furthermore, this study generalizes a practical way to correlate specific particle shapes, such as rods, blades, cuboids, plates, and cubes to characterize asymmetric particles of any material type with 3-D image analysis

Porosity and distribution of water in perlite from the island of Milos, Greece

A perlite sample representative of an operating mine in Milos was investigated with respect to the type and spatial distribution of water. A set of different methods was used which finally provided a consistent view on the water at least in this perlite. Infrared spectroscopy showed the presence of different water species (molecular water and hydroxyl groups / strongly bound water). The presence of more than 0.5 mass% smectite, however, could be excluded considering the cation exchange capacity results. The dehydration measured by thermal analysis occurred over a wide range of temperatures hence confirming the infrared spectroscopical results. Both methods point to the existence of a continuous spectrum of water binding energies. The spatial distribution of water and/or pores was investigated using different methods (CT: computer tomography, FIB: scanning electron microscopy including focused ion beam technology, IRM: infrared microscopy). Computer tomography (CT) showed large macropores (20 – 100 μm) and additionally revealed a mottled microstructure of the silicate matrix with low density areas up to a few μm in diameter. Scanning electron microscopy (FIB) confirmed the presence of μm sized pores and IRM showed the filling of these pores with water. In summary, two types of pores were found. Airfilled 20 – 100 μm pores and μm-sized pores disseminated in the glass matrix containing at least some water. Porosity measurements indicate a total porosity of 26 Vol%, 11 Vol% corresponding to the μm-sized pores. It remains unsolved wether the water in the μm-sized pores entered after or throughout perlite formation. However, the pores are sealed and no indications of cracks were found which indicated a primary source of the water, i.e. water was probably entrapped by quenching of the lava. The water in these pores may be the main reason for the thermal expandability which results in the extraordinarily porous expanded perlite building materials

Anwendung und Bewertung der Gitter Boltzmann Methode für die Modellierung des Fluid Transports in porösen Gesteinen

Die Modellierung physikalischer Eigenschaften und Phänomene von natürlichen Festgesteinen stellt inzwischen einen wichtigen Arbeitsbereich innerhalb der modernen Gesteinsphysik dar. In den letzten Jahren hat dabei die Röntgen-Computertomografie als schnelles, zerstörungsfreies sowie bildgebendes Verfahren die Grenzen des Verständnisses der Fluid Dynamik in komplexen Systemen signifikant vorangetrieben. Diese Promotionsschrift reiht sich konsequent in die komplexe Thematik mit ein und gibt zunächst einen umfassenden Einblick in den Stand der Technik von Strömungssimulationen in porösen Medien, bevor neue Ansätze für hochgenaue Modellrechnungen in echten Porennetzwerken aufgezeigt werden. Zielführend werden dabei zunächst die theroretischen Grundlagen bezüglich der Fluid Dynamik, dem bildgebenden Verfahren sowie der Modellierungstechnik gegeben. Diesem theoretischen Teil folgt eine ausführliche, interdisziplinäre Gesteinscharakterisierung, bevor die Ergebnisse der Strömungssimulationen an synthetischen und realen Porennetzwerken dargestellt werden. Besonderes Augenmerk dieser Arbeit liegt dabei auf zwei thematischen Bereichen. Zum Einen, wird der essentielle Arbeitsablauf zur Gewinnung hochaufgelöster 3D Datensätze für die Extraktion der in-situ Porennetzwerke analysiert. Aufgrund der hohen erreichbaren Auflösung moderner CT-Systeme (wenige Mikrometer, teils einige hundert Nanometer) können erstmalig auch sehr kleine Porenstrukturen für die Modellierung aufbereitet werden. Dazu wird ein verbesserter Extraktions-Algorithmus getestet, der in der Lage ist, die segmentierten Strukturen mit hoher Genauigkeit und Zuverlässigkeit in numerische Gitter zu transferieren. Zum Anderen wird ein neues Verfahren zur Überprüfung der Repräsentativität von Transportmodellierungen eingeführt. Die so ermittelten repräsentativen Volumina werden im Anschluss daran mittels der Gitter Boltzmann Methode auf ihre Transporteigenschaften hin untersucht. Aufgrund dieser numerisch sehr einfachen, aber dennoch eleganten Methode können beliebige Strukturen (z.B. grobe und feine Porennetzwerke, sekundäre Porositäten, Tonporositäten, etc.) auf beliebigen Skalen mit hoher Auflösung modelliert und gleichfalls charakterisiert werden. Ergänzend dazu werden vergleichende Simulationen mit gebräuchlichen Softwarepaketen evaluiert und mit den Ergebnissen der Boltzmann Modellierung auf verschiedenen (Mikro-) Skalen gegenübergestellt. Abschließend werden die Ergebnisse aller Teilbereiche zusammengefasst, beurteilt und diskutiert.Modeling physical properties and phenomena of natural rocks forms an important field of work for modern petrophysics. In recent years, the fast and non-destructive X-ray computed micro tomography has pushed the knowledge and understanding of fluid dynamics in complex pore systems extensively forward. This PhD thesis is in accordance to this wide and complex field of research and gives a comprehensive overview about state of the art transport modeling in porous media. New approaches for high precisely modeling aspects in real pore geometries are shown. Conducive to this thesis, theoretical background on fluid dynamics, modern CT imaging and modeling techniques is introduced first. Afterwards, a detailed and interdisciplinary rock characterization is performed, before results of transport modeling on a synthetic and on real pore networks are given. Special attention is given onto two main topics. On the one hand, the essential workflow to obtain high resolution 3D data sets for the advanced pore network extraction is introduced in detail. Since modern CT devices are able to scan samples with micron to submicron resolution, even very small pore structures can be taken into account for modeling purposes. For this, a special network extraction algorithm, which is able to preserve these small sample features with high accuracy during grid generation, is tested. On the other hand, an advanced method to evaluate the representiveness of transport modeling phenomena, depending on the voxel size of the scanned data, is introduced. The so derived representative elementary volumes (REV) are then used to apply the Lattice Boltzmann Method (LBM) for the estimation of fluid transport properties of these structures. Due to this numerically easy but elegant approach, even structures of arbitrary complexity (e.g. coarse and fine pore networks, secondary porosities, clay related porosities, etc.) can be investigated and taken into account on different scales as well. Additionally, comparative simulations with a commonly used modeling technique are performed and evaluated in close combination with the results of the Boltzmann transport modeling on different scales. Finally, results of each partition of this thesis are combined, assessed, discussed and summarized

Impedance Spectroscopy on Carbonates

ABSTRACT With impedance spectroscopy (IS), or spectral induced polarization (SIP) as it is also termed, the complex electrical conductivity of rocks is measured over a wide frequency range, typically ranging from 1 mHz to 10 kHz. Originally developed for the exploration of ore deposits, today's SIP measurements are used for a wide range of applications within geophysics. On the field scale, this technique is (e.g.) used to characterize the hydraulics of shallow aquifers, to assess pollution for environmental protection purposes, or even to support archaeological investigations. On the laboratory scale, SIP is commonly used for the investigation of electric rock properties for clastic reservoir rocks in general, and sandstones in particular. The frequency dependent complex electrical conductivity gives access to an advanced rock characterization, even beyond a more qualitative and in most cases general pore space characterization. Parameters particularly relevant for hydraulics -as for example specific internal surface, (dominant) pore size and permeability -can be estimated. Some robust empirical relations between induced polarization (IP) parameters and petrophysical parameters (BET surface, surface conductivity, cation exchange capacity) have been identified for sandstones and sandy material. However, for carbonates these relations do not seem to work and some of the resulting parameters show a quite different behavior. More systematic studies are needed to evaluate the potential of SIP for characterization of carbonates. The authors would like to show the results of a first case study on a limestone sample set originating from shallow wells of the Tushka area, south Western Desert, Egypt. Besides basic principles of SIP in general, adaptation of theory and evaluation technique will be highlighted in particular