Numerical simulations of coupled processes in rock fractures

Reservoirmodelle sind unerlässlich, um das Potential und Risiken bei der Gewinnung geothermischer Energie zu beurteilen. Eine wesentliche Einschränkung zuverlässiger Reservoirmodelle ist jedoch die Verfügbarkeit benötigter Kluftparameter, die oftmals auf allzu einfachen oder auch fehlenden Methoden beruhen oder extrem aufwändige experimentelle Aufbauten benötigen. Um passende Klufteigenschaften zu definieren, ist es daher nötig, die zugrundeliegenden (oftmals gekoppelten) thermischen, hydraulischen, chemischen und mechanischen (THCM) Prozesse zu verstehen und zu quantifizieren, indem man wirkungsvolle, aber anwendungsorientierte Methoden zur Kluftanalyse entwickelt. Um diese Erfordernisse anzugehen, werden in dieser Arbeit drei voneinander unabhängige Herangehensweisen vorgestellt: In Studie 1 wird ein kontaktmechanischer Ansatz basierend auf einer neuartigen, frei zugänglichen Web-Anwendung, die sowohl elastische als auch elastisch-plastische Kontaktdeformation innerhalb der Kluft betrachtet, vorgestellt und anhand von Uniaxialtests an einer kreisrunden Granodiorit-Kluft validiert. Die Simulationsergebnisse zeigen, dass insbesondere der elastisch-plastische Ansatz eine gute Übereinstimmung mit den Ergebnissen der experimentellen Normalverschiebung aufweist. Im Gegensatz zu anderen schnellen und leicht anwendbaren Kontaktmodellen, berücksichtigen sowohl das elastische als auch das elastisch-plastische Model eine realitätsnahe Wiedergabe der Kontaktbereiche (< 2 % bei 10 MPa Auflast) und das Auftreten ungleichmäßiger lokaler Vertikalversätze. Obwohl ein genereller Widerstand gegen ausgeprägte nicht-elastische Deformation (sogar bei 10 MPa) zu beobachten ist, was auch durch die neu hergeleitete relative Härte von 0.14 bekräftigt wird, bestätigen die lokalen Kontaktbelastungen die höhere Validität des elastisch-plastischen Kontaktmodels. In Studie 2 wird eine neuartige Vorgehensweise, basierend auf zerstörungsfreien in situ Scans mittels medizinischer Computertomografie (CT), vorgestellt, die sich als geeignet erweist um den Fluidfluss in weniger rauen Klüften mit kleinen Öffnungsweiten (< 35 μm) annäherungsweise numerisch zu bestimmen. Die Ergebnisse werden anhand von Durchflussexperimenten an einer geklüfteten Sandsteinprobe unter Be- und Entlastungsbedingungen validiert, die zeitgleich zu der CT-Scans durchgeführt wurden. Abweichungen von den experimentellen Durchflussergebnissen sind bedingt durch unvermeidbare Abweichungen während der Kalibrierung der Öffnungsweiten, durch Einflussnahme der Gesteinsmatrix und durch die grobe Auflösung des CT Geräts. Trotz dieser Abweichungen, zeigen die Simulationen ein ausgeprägtes lastabhängiges Fließverhalten („Channeling“) und ein permanentes oder vorübergehendes Schließen einzelner Fließwege, wobei deutlichere Änderungen des Fließregimes unterhalb der Auflösung (0.5 × 0.5 × 1.0 mm³) der CT-Scans zu erwarten sind. In Studie 3 wird ein Phasenfeld-Model für hydrothermalbedingten Quarzwachstum angewandt, um den Einfluss der entstehenden Fällungs-Geometrien auf die hydraulischen Eigenschaften von Klüften zu untersuchen. Es wird gezeigt, dass der Fluidfluss in teilweise mineralisierten Klüften deutlich abhängig vom Kristallhabitus der ausgefällten Minerale ist. Für eher gestreckte Kristalle entwickeln sich typischerweise sogenannte „Kristallbrücken“, die ausgeprägte Fließbarrieren bilden und die Bildung von hydraulischen Klufteigenschaften begünstigen, die denen eines porösen Medium ähneln. Auf Basis der Strömungssimulationen kann eine neuartige semi-empirische Gleichung hergeleitet werden, um die hydraulischen Eigenschaften von teilweise mineralisierten Klüften abzuschätzen. In Rahmen dieser Thesis werden somit drei einfach anwendbare Methoden vorgestellt, welche die Grundlage für weitere Untersuchungen an Klüften darstellen und für verschiedenartige Kluft-Fragestellungen verwendet werden können. Die Web-Anwendung stellt eine schnelle und frei zugängliche Methode dar, die genutzt werden kann, um die druckabhängige Normalverschiebung von Klüften basierend auf herkömmlichen Oberflächenscans zu untersuchen. Der vorgeschlagene zerstörungsfreie Ansatz für medizinische CT-Scans ist sinnvoll, um beispielsweise die hydraulischen Eigenschaften von dicht verschlossenen, chemisch alterierten Klüften abzuschätzen. Die neu hergeleitete Gleichung der dritten Studie liefert eine effizientere Alternativlösung zu herkömmlichen Modellen, die zur Bestimmung der hydraulischen Kluftöffnungsweite entwickelt wurden, und kann genutzt werden, um die hydraulischen Eigenschaften von teilweise mineralisierten Klüften relativ unkompliziert abzuschätzen

Visual Techniques for Geological Fieldwork Using Mobile Devices

Visual techniques in general and 3D visualisation in particular have seen considerable adoption within the last 30 years in the geosciences and geology. Techniques such as volume visualisation, for analysing subsurface processes, and photo-coloured LiDAR point-based rendering, to digitally explore rock exposures at the earth&rsquo;s surface, were applied within geology as one of the first adopting branches of science. A large amount of digital, geological surface- and volume data is nowadays available to desktop-based workflows for geological applications such as hydrocarbon reservoir exploration, groundwater modelling, CO2 sequestration and, in the future, geothermal energy planning. On the other hand, the analysis and data collection during fieldwork has yet to embrace this &rdquo;digital revolution&rdquo;: sedimentary logs, geological maps and stratigraphic sketches are still captured in each geologist&rsquo;s individual fieldbook, and physical rocks samples are still transported to the lab for subsequent analysis. Is this still necessary, or are there extended digital means of data collection and exploration in the field ? Are modern digital interpretation techniques accurate and intuitive enough to relevantly support fieldwork in geology and other geoscience disciplines ? This dissertation aims to address these questions and, by doing so, close the technological gap between geological fieldwork and office workflows in geology. The emergence of mobile devices and their vast array of physical sensors, combined with touch-based user interfaces, high-resolution screens and digital cameras provide a possible digital platform that can be used by field geologists. Their ubiquitous availability increases the chances to adopt digital workflows in the field without additional, expensive equipment. The use of 3D data on mobile devices in the field is furthered by the availability of 3D digital outcrop models and the increasing ease of their acquisition. This dissertation assesses the prospects of adopting 3D visual techniques and mobile devices within field geology. The research of this dissertation uses previously acquired and processed digital outcrop models in the form of textured surfaces from optical remote sensing and photogrammetry. The scientific papers in this thesis present visual techniques and algorithms to map outcrop photographs in the field directly onto the surface models. Automatic mapping allows the projection of photo interpretations of stratigraphy and sedimentary facies on the 3D textured surface while providing the domain expert with simple-touse, intuitive tools for the photo interpretation itself. The developed visual approach, combining insight from all across the computer sciences dealing with visual information, merits into the mobile device Geological Registration and Interpretation Toolset (GRIT) app, which is assessed on an outcrop analogue study of the Saltwick Formation exposed at Whitby, North Yorkshire, UK. Although being applicable to a diversity of study scenarios within petroleum geology and the geosciences, the particular target application of the visual techniques is to easily provide field-based outcrop interpretations for subsequent construction of training images for multiple point statistics reservoir modelling, as envisaged within the VOM2MPS project. Despite the success and applicability of the visual approach, numerous drawbacks and probable future extensions are discussed in the thesis based on the conducted studies. Apart from elaborating on more obvious limitations originating from the use of mobile devices and their limited computing capabilities and sensor accuracies, a major contribution of this thesis is the careful analysis of conceptual drawbacks of established procedures in modelling, representing, constructing and disseminating the available surface geometry. A more mathematically-accurate geometric description of the underlying algebraic surfaces yields improvements and future applications unaddressed within the literature of geology and the computational geosciences to this date. Also, future extensions to the visual techniques proposed in this thesis allow for expanded analysis, 3D exploration and improved geological subsurface modelling in general.publishedVersio

AIMES: advanced computation and I/O methods for earth-system simulations

Dealing with extreme scale Earth-system models is challenging from the computer science perspective, as the required computing power and storage capacity are steadily increasing. Scientists perform runs with growing resolution or aggregate results from many similar smaller-scale runs with slightly different initial conditions (the so-called ensemble runs). In the fifth Coupled Model Intercomparison Project (CMIP5), the produced datasets require more than three Petabytes of storage and the compute and storage requirements are increasing significantly for CMIP6. Climate scientists across the globe are developing next-generation models based on improved numerical formulation leading to grids that are discretized in alternative forms such as an icosahedral (geodesic) grid. The developers of these models face similar problems in scaling, maintaining and optimizing code. Performance portability and the maintainability of code are key concerns of scientists as, compared to industry projects, model code is continuously revised and extended to incorporate further levels of detail. This leads to a rapidly growing code base that is rarely refactored. However, code modernization is important to maintain productivity of the scientist working with the code and for utilizing performance provided by modern and future architectures. The need for performance optimization is motivated by the evolution of the parallel architecture landscape from homogeneous flat machines to heterogeneous combinations of processors with deep memory hierarchy. Notably, the rise of many-core, throughput-oriented accelerators, such as GPUs, requires non-trivial code changes at minimum and, even worse, may necessitate a substantial rewrite of the existing codebase. At the same time, the code complexity increases the difficulty for computer scientists and vendors to understand and optimize the code for a given system. Storing the products of climate predictions requires a large storage and archival system which is expensive. Often, scientists restrict the number of scientific variables and write interval to keep the costs balanced. Compression algorithms can reduce the costs significantly but can also increase the scientific yield of simulation runs. In the AIMES project, we addressed the key issues of programmability, computational efficiency and I/O limitations that are common in next-generation icosahedral earth-system models. The project focused on the separation of concerns between domain scientist, computational scientists, and computer scientists

Spatio-temporal information system for the geosciences: concepts, data models, software, and applications

The development of spatio–temporal geoscience information systems (TGSIS) as the next generation of geographic information systems (GIS) and geoscience information systems (GSIS) was investigated with respect to the following four aspects: concepts, data models, software, and applications. These systems are capable of capturing, storing, managing, and querying data of geo–objects subject to dynamic processes, thereby causing the evolution of their geometry, topology and geoscience properties. In this study, five data models were proposed. The first data model represents static geo–objects whose geometries are in the 3–dimensional space. The second and third data models represent geological surfaces evolving in a discrete and continuous manner, respectively. The fourth data model is a general model that represents geo–objects whose geometries are n–dimensional embedding in the m–dimensional space R^m, m >= 3. The topology and the properties of these geo–objects are also represented in the data model. In this model, time is represented as one dimension (valid time). Moreover, the valid time is an independent variable, whereas geometry, topology, and the properties are dependent (on time) variables. The fifth data model represents multiple indexed geoscience data in which time and other non–spatial dimensions are interpreted as larger spatial dimensions. To capture data in space and time, morphological interpolation methods were reviewed, and a new morphological interpolation method was proposed to model geological surfaces evolving continuously in a time interval. This algorithm is based on parameterisation techniques to locate the cross–reference and then compute the trajectories complying with geometrical constraints. In addition, the long transaction feature was studied, and the data schema, functions, triggers, and views were proposed to implement the long transaction feature and the database versioning in PostgreSQL. To implement database versioning tailored to geoscience applications, an algorithm comparing two triangulated meshes was also proposed. Therefore, TGSIS enable geologists to manage different versions of geoscience data for different geological paradigms, data, and authors. Finally, a prototype software system was built. This system uses the client/server architecture in which the server side uses the PostgreSQL database management system and the client side uses the gOcad geomodeling system. The system was also applied to certain sample applications

Integration of 3D geological and numerical models based on tetrahedral meshes for hydrogeological simulations in fractured porous media

Une nouvelle approche de modélisation des milieux géologiques fracturés représentés par un modèle conceptuel de fractures discrètes et déterministes est présentée dans cette thèse. L'objectif principal de l'étude est de reproduire l'hétérogénéité et la complexité des milieux poreux fracturés dans un modèle géométrique tridimensionnel afin d'effectuer des simulations numériques dans le but d'améliorer les capacités de modélisation en hydrogéologie. Ceci est réalisé à travers le couplage entre une plateforme de modélisation géologique (GOCAD) et un code numérique (HydroGeoSphere). Les principaux défis à relever sont: la représentation géométrique du réseau de fractures, la sélection d'un maillage approprié pour la discrétisation spatiale du domaine de simulation et l'adaptation du code numérique à ce maillage. La nouvelle approche est basée sur une première phase de modélisation géologique 3D, suivie par la génération d'un maillage tétraédrique 3D et par la simulation numérique de l'écoulement souterrain en conditions saturées et du transport de solutés. En général, le maillage tétraédrique s'avère plus adéquat que les maillages de blocs ou de prismes pour discrétiser les geometries complexes telles que les milieux fracturés. De plus, une définition alternative du maillage "dual", qui est essentiel pour appliquer la méthode numérique élément finis - volume de contrôle utilisée par HydroGeoSphere, est analysée et intégrée dans le code numérique. Le code numérique proposé est d'abord vérifié par l'intermédiaire de simples scénarios de simulation dont les solutions, analytiques et numériques, sont déjà connues. La complexité des simulations est augmentée de façon graduelle. L'approche de modélisation est finalement appliquée au site Olkiluoto (Finlande) où un laboratoire de recherche souterrain est en construction afin d'évaluer la faisabilité du stockage géologique profond de déchets nucléaires à haute activité. Les techniques de modélisation géologique mises au point permettent de modéliser facilement la géométrie des fractures identifiées à travers la caractérisation géologique in situ. De plus, le modèle numérique s'avère adéquat pour la simulation de l'écoulement et du transport de solutés dans ce site complexe. Ce travail de recherche présente une contribution au développement des techniques de modélisation hydrogéologique des milieux fracturés

Viticulture in the Laetanian Region (Spain) during the Roman Period: Predictive Modelling and Geomatic Analysis

Geographic information system (GIS)-based predictive modelling is widely used in archaeology to identify suitable zones for ancient settlement locations and determine underlying factors of their distribution. In this study, we developed predictive models on Roman viticulture in the Laetanian Region (Hispania Citerior-Tarraconensis), using the location of 82 ancient wine-pressing facilities or torcularia as response variables and 15 topographical and 6 socio-economic cost distance datasets as predictor variables. Several predictor variable subsets were selected either by expert knowledge of similar studies or by using a semi-automatization algorithm based on statistical distribution metrics of the input data. The latter aims at simplifying modelling and minimizing the necessity of a priori knowledge. Both approaches predicted the distribution of archeological sites sufficiently well. However, the best prediction performance was obtained by an expert knowledge model utilizing a predictor variable combination based on recommendations on viticulture by Lucius Junius Moderatus Columella, the prominent ancient Roman agronomist. The results indicate that the accessibility of a location and its connectivity to trade routes and distribution centres, determined by terrain steepness, was decisive for the settlement of viticultural facilities. With the knowledge gained, the ancient cultivated area and number of wine-pressing facilities needed for processing the vineyard yields were extrapolated for the entire study region