Variable-Density Flow Processes in Porous Media On Small, Medium and Regional Scales

Nowadays society strongly depends on its available resources and the long term stability of the surrounding ecosystem. Numerical modelling has become a general standard for evaluating past, current or future system states for a large number of applications supporting decision makers in proper management. In order to ensure the correct representation of the investigated processes and results of a simulation, verification examples (benchmarks), that are based on observation data or analytical solutions, are utilized to evaluate the numerical modelling tool. In many parts of the world, groundwater is an important resource for freshwater. While it is not only limited in quantity, subsurface water bodies are often in danger of contamination from various natural or anthropogenic sources. Especially in arid regions, marine saltwater intrusion poses a major threat to groundwater aquifers which mostly are the exclusive source of freshwater in these dry climates. In contrast to common numerical groundwater modelling, density-driven flow and mass transport have to be considered as vital processes in the system and in scenario simulations for fresh-saltwater interactions. In the beginning of this thesis, the capabilities of the modelling tool OpenGeoSys are verified with selected benchmarks to represent the relevant non-linear process coupling. Afterwards, variable-density application and process studies on different scales are presented. Application studies comprehend regional groundwater modelling of a coastal aquifer system extensively used for agricultural irrigation, as well as hydro-geological model development and parametrization. In two process studies, firstly, a novel method to model gelation of a solute in porous media is developed and verified on small scale laboratory observation data, and secondly, investigations of thermohaline double-diffusive Rayleigh regimes on medium scale are carried out. With the growing world population and, thus, increasing pressure on non-renewable resources, intelligent management strategies intensify demand for potent simulation tools and development of novel methods. In that way, this thesis highlights not only OpenGeoSys’ potential of density-dependent process modelling, but the comprehensive importance of variable-density flow and transport processes connecting, both, avant-garde scientific research, and real-world application challenges.:Abstract Zusammenfassung Nomenclature List of Figures List of Tables I Background and Fundamentals 1 Introduction 1.1 Motivation 1.2 Structure of the Thesis 1.3 Variable-Density Flow in Literature 2 Theory and Methods 2.1 Governing Equations 2.2 Fluid Properties 2.3 Modelling and Visualization Tools 3 Benchmarks 3.1 Steady-state Unconfined Groundwater Table 3.2 Theis Transient Pumping Test 3.3 Transient Saltwater Intrusion 3.4 Development of a Freshwater Lens II Applications 4 Extended Inverse Distance Weighting Interpolation 4.1 Motivation 4.2 Extension of IDW Method 4.3 Artificial Test and Regional Scale Application 4.4 Summary and Conclusions 5 Modelling Transient Saltwater Intrusion 5.1 Background and Motivation 5.2 Methods and Model Setup 5.3 Simulation Results and Discussion 5.4 Summary, Conclusion and Outlook 6 Gelation of a Dense Fluid 6.1 Motivation 6.2 Methods and Model Setup 6.3 Results and Conclusions 7 Delineating Double-Diffusive Rayleigh Regimes 7.1 Background and Motivation 7.2 Methods and Model Setup 7.3 Results 7.4 Conclusions and Outlook III Summary and Conclusions 8 Important Achievements 9 Conclusions and Outlook Bibliography Publications Acknowledgements Appendi

Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry

The evolution of the planetary interior during plate tectonics is controlled by slow convection within the mantle. Global-scale geochemical differences across the upper mantle are known, but how they are preserved during convection has not been adequately explained. We demonstrate that the geographic patterns of chemical variations around the Earth’s mantle endure as a direct result of whole-mantle convection within largely isolated cells defined by subducting plates. New 3D spherical numerical models embedded with the latest geological paleo-tectonic reconstructions and ground-truthed with new Hf-Nd isotope data, suggest that uppermost mantle at one location (e.g. under Indian Ocean) circulates down to the core-mantle boundary (CMB), but returns within ≥100 Myrs via large-scale convection to its approximate starting location. Modelled tracers pool at the CMB but do not disperse ubiquitously around it. Similarly, mantle beneath the Pacific does not spread to surrounding regions of the planet. The models fit global patterns of isotope data and may explain features such as the DUPAL anomaly and long-standing differences between Indian and Pacific Ocean crust. Indeed, the geochemical data suggests this mode of convection could have influenced the evolution of mantle composition since 550 Ma and potentially since the onset of plate tectonics

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling

Modeling coupled thermohaline flow and reactive solute transport in discretely-fractured porous media

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2005-2006Un modèle numérique tridimensionnel a été développé pour la simulation du système chimique quartz-eau couplé avec l’écoulement à densité et viscosité variable dans les milieux poreux discrètement fracturés. Le nouveau modèle simule aussi le transfert de chaleur dans les milieux poreux fracturés en supposant que l’expansion thermique du milieu est négligeable. Les propriétés du fluide, densité et viscosité, ainsi que les constantes chimiques (constant de taux de dissolution, constant d’équilibre, coefficient d’activité) sont calculées en fonction de la concentration des ions majeurs et de la température. Des paramètres de réaction et d’écoulement, comme la surface spécifique du minéral et la perméabilité sont mis jour à la fin de chaque pas de temps avec des taux de réaction explicitement calculés. Le modèle suppose que des changements de la porosite et des ouvertures de fractures n’ont pas d’impact sur l’emmagasinement spécifique. Des pas de temps adaptatifs sont utilisés pour accélérer et ralentir la simulation afin d’empêcher des résultats non physiques. Les nouveaux incréments de temps dépendent des changements maximum de la porosité et/ou de l’ouverture de fracture. Des taux de réaction au niveau temporel L+1 (schéma de pondération temporelle implicite) sont utilisés pour renouveler tous les paramètres du modèle afin de garantir la stabilité numérique. Le modèle a été vérifié avec des problèmes analytiques, numériques et physiques de l’écoulement à densité variable, transport réactif et transfert de chaleur dans les milieux poreux fracturés. La complexité de la formulation du modèle permet d’étudier des réactions chimiques et l’écoulement à densité variable d’une façon plus réaliste qu’auparavant possible. En premier lieu, cette étude adresse le phénomène de l’écoulement et du transport à densité variable dans les milieux poreux fracturés avec une seule fracture à inclinaison arbitraire. Une formulation mathématique générale du terme de flottabilité est dérivée qui tient compte de l’écoulement et du transport à densité variable dans des fractures de toute orientation. Des simulations de l’écoulement et du transport à densité variable dans une seule fracture implanté dans une matrice poreuse ont été effectuées. Les simulations montrent que l’écoulement à densité variable dans une fracture cause la convection dans la matrice poreuse et que la fracture à perméabilité élevée agit comme barrière pour la convection. Le nouveau modèle a été appliqué afin de simuler des exemples, comme le mouvement horizontal d’un panache de fluide chaud dans un milieu fracturé chimiquement réactif. Le transport thermohalin (double-diffusif) influence non seulement l’écoulement à densité variable mais aussi les réactions chimiques. L’écoulement à convection libre dépend du contraste de densité entre le fluide (panache chaud ou de l’eau salée froide) et le fluide de référence. Dans l’exemple, des contrastes de densité sont généralement faibles et des fractures n’agissent pas comme des chemins préférés mais contribuent à la dispersion transverse du panache. Des zones chaudes correspondent aux régions de dissolution de quartz tandis que dans les zones froides, la silice mobile précipite. La concentration de silice est inversement proportionnelle à la salinité dans les régions à salinité élevée et directement proportionnelle à la température dans les régions à salinité faible. Le système est le plus sensible aux inexactitudes de température. Ceci est parce que la température influence non seulement la cinétique de dissolution (équation d’Arrhenius), mais aussi la solubilité de quartz.A three-dimensional numerical model is developed that couples the quartz-water chemical system with variable-density, variable-viscosity flow in fractured porous media. The new model also solves for heat transfer in fractured porous media, under the assumption of negligible thermal expansion of the rock. The fluid properties density and viscosity as well as chemistry constants (dissolution rate constant, equilibrium constant and activity coefficient) are calculated as a function of the concentrations of major ions and of temperature. Reaction and flow parameters, such as mineral surface area and permeability, are updated at the end of each time step with explicitly calculated reaction rates. The impact of porosity and aperture changes on specific storage is neglected. Adaptive time stepping is used to accelerate and slow down the simulation process in order to prevent physically unrealistic results. New time increments depend on maximum changes in matrix porosity and/or fracture aperture. Reaction rates at time level L+1 (implicit time weighting scheme) are used to renew all model parameters to ensure numerical stability. The model is verified against existing analytical, numerical and physical benchmark problems of variable-density flow, reactive solute transport and heat transfer in fractured porous media. The complexity of the model formulation allows chemical reactions and variable-density flow to be studied in a more realistic way than previously possible. The present study first addresses the phenomenon of variable-density flow and transport in fractured porous media, where a single fracture of an arbitrary incline can occur. A general mathematical formulation of the body force vector is derived, which accounts for variable-density flow and transport in fractures of any orientation. Simulations of variable-density flow and solute transport are conducted for a single fracture, embedded in a porous matrix. The simulations show that density-driven flow in the fracture causes convective flow within the porous matrix and that the highpermeability fracture acts as a barrier for convection. The new model was applied to simulate illustrative examples, such as the horizontal movement of a hot plume in a chemically reactive fractured medium. Thermohaline (double-diffusive) transport impacts both buoyancy-driven flow and chemical reactions. Free convective flow depends on the density contrast between the fluid (hot brine or cool saltwater) and the reference fluid. In the example, density contrasts are generally small and fractures do not act like preferential pathways but contribute to transverse dispersion of the plume. Hot zones correspond to areas of quartz dissolution while in cooler zones, precipitation of imported silica prevails. The silica concentration is inversely proportional to salinity in high-salinity regions and directly proportional to temperature in low-salinity regions. The system is the most sensitive to temperature inaccuracy. This is because temperature impacts both the dissolution kinetics (Arrhenius equation) and the quartz solubility

ONLINE WORKSHOP ON EARTH AND SPACE SCIENCES - ATMOSPHERIC PHYSICS AND CLIMATE Proceedings Book 2020

This book was produced in the scope of the Curricular Unit Seminar I (Physics of Atmosphere and Climate) of the PhD program in Earth and Space Sciences, which included the organization of the 2020 edition of the Online Workshop on Earth and Space Sciences - Atmospheric Physics and Climate of the University of Évora. This volume brings together the research articles produced by students who attended the course. Since its first edition in 2013, the Workshop has been a space for sharing knowledge and training in science communication. In this sense, it seeks to provide students who attend Seminar Units with an effective experience not only in the preparation and presentation of oral communications and research articles, but also in the organization of the event itself. In the 2020 edition, which took place on June 4, 11 abstracts were submitted and presented, covering the various themes of the PhD program, in addition to two invited lectures

GPlates – Building a Virtual Earth Through Deep Time

GPlates is an open‐source, cross‐platform plate tectonic geographic information system, enabling the interactive manipulation of plate‐tectonic reconstructions and the visualization of geodata through geological time. GPlates allows the building of topological plate models representing the mosaic of evolving plate boundary networks through time, useful for computing plate velocity fields as surface boundary conditions for mantle convection models and for investigating physical and chemical exchanges of material between the surface and the deep Earth along tectonic plate boundaries. The ability of GPlates to visualize subsurface 3‐D scalar fields together with traditional geological surface data enables researchers to analyze their relationships through geological time in a common plate tectonic reference frame. To achieve this, a hierarchical cube map framework is used for rendering reconstructed surface raster data to support the rendering of subsurface 3‐D scalar fields using graphics‐hardware‐accelerated ray‐tracing techniques. GPlates enables the construction of plate deformation zones—regions combining extension, compression, and shearing that accommodate the relative motion between rigid blocks. Users can explore how strain rates, stretching/shortening factors, and crustal thickness evolve through space and time and interactively update the kinematics associated with deformation. Where data sets described by geometries (points, lines, or polygons) fall within deformation regions, the deformation can be applied to these geometries. Together, these tools allow users to build virtual Earth models that quantitatively describe continental assembly, fragmentation and dispersal and are interoperable with many other mapping and modeling tools, enabling applications in tectonics, geodynamics, basin evolution, orogenesis, deep Earth resource exploration, paleobiology, paleoceanography, and paleoclimate

Geophysical and geological characterization of fault-controlled geothermal systems: The Vallès Basin case of study

[eng] Geothermal energy is a renewable source of energy that harnesses heat from the Earth's interior. Temperature increases with depth, defining the geothermal gradient, which can be variable depending on the geological context. The geological setting of western Europe favors a relatively high geothermal gradient that could be exploited to generate electricity or for its direct use, for example, for its application in industry, greenhouses, or heating systems. In each of these cases, geothermal could favor the community's energy independence and reduce the use of polluting energy sources. To appropriately exploit areas with a significant geothermal gradient, it is essential to know the origin of the temperature anomaly and the system's functioning. In this context, developing appropriate exploration methodologies and techniques is essential for its adequate and efficient use. This thesis develops a methodology focused on a geothermal system type characterized by being located in highly fractured zones. These fractures connect the surface with great depths, allowing the rapid ascent of deep fluids at high temperatures without giving them time to cool down. Specifically, this thesis applies this methodology to a study case located in the Vallès Basin, close to Barcelona city (NE Iberian Peninsula), where some localities, such as La Garriga and Caldes de Montbui towns, have thermal hot springs (60ºC and 70ºC, respectively). In particular, the methodology applied to study the Vallès Basin geothermal fractured system, is focused on two main cores, geophysical and geological techniques. Geophysical methods allow the characterization of the subsurface physical properties, reaching great depths without having to drill. For example, if the physical characteristics of the subsurface have enough contrast, they could allow distinguishing between different types of rocks, fractured zones, or if there is any fluid circulation. However, the geophysical results have to be complemented with other geoscientific studies in order to make a proper interpretation. In this case, this thesis includes a characterization of the area's geology, fracturing, and hydrology. Finally, the integration of the applied techniques has allowed the understanding of the origin and system's functioning, which is presented in the form of a 3D conceptual model, geological model, and temperature model. This innovative methodology, which integrates different geoscientific techniques at different scales, combining traditional techniques with novel digital tools, has facilitated the characterization of a geothermal system controlled by geological structures. Therefore, it is established as a methodical option to characterize systems of similar origin.[cat] La Geotèrmia és una font renovable d'energia que aprofita la temperatura de l'interior de la Terra. El grau en què aquesta temperatura augmenta en profunditat, ve definint pel gradient geotèrmic, el qual pot ser variable segons el context geològic. La geologia de la regió oest del continent europeu afavoreix un gradient geotèrmic relativament alt que podria ser aprofitat per generar electricitat o per a ús directe, com és el cas d'aplicacions en indústria, hivernacles o sistemes de calefacció. En qualsevol cas, la geotèrmia podria afavorir la independència energètica i una disminució en l’ús de fonts d’energia contaminants. Per a un aprofitament d'aquestes zones amb un gradient geotèrmic significatiu, és essencial conèixer-ne l'origen i el funcionament. En aquest context, és basic desenvolupar metodologies d'exploració que siguin adequades i eficients. Aquesta tesis desenvolupa una metodologia aplicada a un exemple de sistema geotèrmic caracteritzat per estar ubicat en una zona molt fracturada. Aquestes fractures connecten la superfície amb grans profunditats, permetent l'ascens ràpid de fluids profunds que es troben a temperatures altes, sense que els doni temps a refredar-se. Concretament, aquesta zona d'estudi es situa a la Conca del Vallès (NE Península Ibèrica), on algunes localitats com La Garriga i Caldes de Montbui, tenen surgències d'aigua termal (60ºC i 70ºC, respectivament). Concretament, la metodologia aplicada es basa en dues parts principals: l'exploració geofísica i la geològica. Els mètodes geofísics ens permeten conèixer les propietats físiques del subsol arribant a grans profunditats sense haver de fer perforacions. Si les característiques físiques del terreny presenten un contrast suficient, poden permetre, per exemple, distingir entre tipus de roques, zones fracturades, o si hi ha circulació d'algun fluid. Tot i així, els resultats geofísics s'han de complementar amb altres estudis geocientífics per una correcta interpretació dels resultats. En aquest cas, aquesta tesis inclou una caracterització de la geologia, la fracturació i la hidrologia de la zona. La integració final de totes les dades ha permès entendre l'origen i el funcionament d'aquest sistema, resultat del qual es presenta en forma d'un model 3D conceptual, geològic i de temperatures. Aquesta metodologia innovadora, que integra diferents tècniques geocientífiques a escala diferent, ha combinat tècniques tradicionals amb eines digitals noves, facilitant la caracterització d'un sistema geotèrmic controlat per estructures geològiques. Per tant, s’estableix com una opció metòdica a seguir per a la caracterització de sistemes d’origen similar.[spa] La Geotermia es una fuente renovable de energía que aprovecha el calor del interior de la Tierra. La temperatura del interior de la Tierra aumenta con la profundidad, y este aumento, definido como gradiente geotérmico, puede ser variable según el contexto geológico. El contexto geológico del oeste del continente europeo favorece un gradiente geotérmico relativamente alto que podría ser aprovechado para generar electricidad o para su uso directo, como es el caso de aplicaciones en industria, invernaderos o sistemas de calefacción. En cualquier caso, la geotermia podría favorecer la independencia energética y una disminución del uso de fuentes de energía contaminantes. Para un apropiado aprovechamiento de estas zonas con un gradiente geotérmico significativo, es esencial conocer su origen y funcionamiento. En este contexto, es necesario un avance en el desarrollo de metodologías de exploración que sean adecuadas y eficientes. Esta tesis desarrolla una metodología aplicada a un tipo de sistema geotérmico caracterizado por estar ubicado en zonas muy fracturadas. Estas fracturas conectan la superficie con grandes profundidades, permitiendo el ascenso rápido de fluidos profundos que se encuentran a altas temperaturas sin que les dé tiempo a enfriarse. Geográficamente, esta zona de estudio se encuentra en la Cuenca del Vallès, cerca de Barcelona (NE Península Ibérica), donde algunas localidades como La Garriga y Caldes de Montbui, tienen surgencias de agua termal (60ºC y 70ºC, respectivamente). Concretamente, esta metodología se puede separar en dos partes principales, la exploración geofísica y la geológica. Los métodos geofísicos nos permiten conocer las propiedades físicas del subsuelo, llegando a grandes profundidades, sin tener que hacer perforaciones. Si las características físicas del terreno presentan un contraste suficiente, nos pueden permitir, por ejemplo, distinguir entre tipos de rocas, zonas fracturadas, o si hay circulación de algún fluido. Aun así, los resultados geofísicos tienen que complementarse con otros estudios geocientíficos para poder hacer una apropiada interpretación. Esta tesis incluye una caracterización de la geología, la fracturación y la hidrología de la zona, cuya integración final ha permitido entender el origen y funcionamiento de este sistema. Los resultados finales se presentan en forma de un modelo 3D conceptual, geológico y de temperaturas. Esta metodología innovadora integra distintas técnicas geocientíficas a distinta escala, combinando técnicas tradicionales con herramientas digitales novedosas, facilitando la caracterización de un sistema geotérmico controlado por estructuras geológicas. Por lo tanto, se establece como una opción metódica a seguir para la caracterización de sistemas de origen similar