Investigations on groundwater dewatering by using vertical circulation wells: Numerical simulation method development and field validation

Die künstliche Grundwasserabsenkung stellt eine wichtige Maßnahme für die Entwässerung von Baugruben und bergbaulich genutzten Flächen dar. Eine erfolgreiche und zielgerichtete Absenkung des Grundwasserspiegels setzt ein zweckmäßiges Design und die richtige Auswahl der genutzten Absenkungstechniken voraus. Dabei sind insbesondere die Dimension des abzusenkenden Bereichs, die Untergrundbeschaffenheit sowie zu erfüllende umweltschutzrechtliche Regelungen zu berücksichtigen. Zur Grundwasserabsenkung kommen üblicherweise verschiedene Ausführungen und Anordnungen von Pumpbrunnen zum Einsatz. Konventionelle Pumpbrunnen, welche für Absenkungsmaßnahmen eingesetzt werden, entnehmen Grundwasser aus dem Aquifer. Durch das fortwährende Abpumpen von in der Regel erheblichen Wassermengen können jedoch Umweltprobleme entstehen, und es ist mit zusätzlichen Entsorgungskosten für die Ableitung des geförderten Wassers zu rechnen. Im Gegensatz hierzu stellen vertikale Zirkulationsbrunnen (VCW) einen innovativen Ansatz dar, der eine lokale Grundwasserabsenkung ohne Nettowasserentnahme aus dem Aquifer erlaubt. Ein VCW kann als ein Einbohrlochsystem aufgefasst werden, bei dem im oberen Bereich eines Brunnens Wasser entnommen und dieses in einem separaten, weiter unten installierten Brunnenbereich wieder injiziert wird. Die erfolgreiche Anwendung dieser neuen Grundwasserabsenkungstechnik erfordert die genaue Kenntnis der Faktoren, welche für die Grundwasserströmungsverhältnisse relevant sind und somit die Absenkung bestimmen. Traditionelle Berechnungsansätze vernachlässigen oft vertikale Grundwasserbewegungen und sind deshalb für die Beschreibung der komplexen Strömungsverhältnisse in unmittelbarer Nähe eines VCW nicht geeignet. Aus diesem Grund steht die systematische Untersuchung der Grundwasserströmung unter Berücksichtigung vertikaler Strömungskomponenten im Hauptfokus dieser Arbeit. Die Untersuchungen beschäftigen sich in erster Linie mit der Entwicklung einer geeigneten Simulationsmethode, mit der Evaluierung des Einflusses relevanter hydrogeologischer Parameter sowie mit der Durchführung und Auswertung von Pumpversuchen an einem Feldstandort. Die hier vorgestellte neue Simulationsmethode koppelt den sogenannten Arbitrary‐Lagrangian‐Eulerian‐(ALE)‐Algorithmus mit der Grundwasserströmungsgleichung. Die Simulationsergebnisse werden mit mehreren analytischen Lösungen verglichen und verifiziert. Das entwickelte numerische Modell berücksichtigt auch Vertikalströmungen und eignet sich somit zur Simulation der Grundwasserströmung in der Nähe von VCW. Folglich kann nun die Lage des Grundwasserspiegels, vor allem für ungespannte Grundwasserleiter, präzise berechnet werden. Nach erfolgter Kalibrierung des numerischen Modells anhand von Felddaten wurde eine Sensitivitätsanalyse relevanter Parameter im Hinblick auf die erzielte Absenkung und deren Auswirkungen auf die Grundwasserströmungssituation durchgeführt. Die dabei erhaltenen Ergebnisse zeigen, dass die Grundwasserabsenkung proportional zur Pumprate, indirekt proportional zur hydraulischen Leitfähigkeit und fast unabhängig von der Anisotropie des Grundwasserleiters um den VCW ist. Des Weiteren zeigte sich, dass die Lage des oberen Entnahmepunktes einen größeren Einfluss auf die Absenkung als die Lage des darunter liegenden Injektionspunktes hat. Die Größe des von der Grundwasserzirkulation beeinflussten Bereiches hängt dagegen neben dem Abstand dieser beiden Punkte hauptsächlich auch von der Anisotropie des Aquifermaterials ab. Um den Einfluss der Hydrostratigraphie auf die Grundwasserströmung zu untersuchen, wurden die Eigenschaften der einzelnen Schichten genau charakterisiert. Hierfür wurden Direct‐Push‐, Pump‐, Injektions‐ sowie Zirkulationsversuche an einem Feldstandort durchgeführt. Zudem wurden Bohrkerne entnommen und mithilfe von Siebanalysen vertikale Korngrößenverteilungsprofile im Labor bestimmt. Die eingesetzten experimentellen Methoden stellen in Kombination mit numerischen Simulationsrechnungen eine gute Basis dar, um die Rolle der Schichtstruktur im Aquifer besser beurteilen zu können. Die Untersuchungen leisten somit einen wichtigen Beitrag für das zukünftige Design und den Betrieb von VCW für Grundwasserabsenkungszwecke in ungespannten Grundwasserleitern. Zudem zeigt die hier vorliegende Arbeit das große Potential dieser neuen Grundwasserabsenkungstechnik als vielversprechende Alternative zu konventionellen Absenkungsverfahren auf

Numerical modeling of groundwater flow based on explicit and fully implicit schemes of finite volume method

This paper documents a novel numerical model for calculating the behavior of unsteady, one-dimensional groundwater flow by using the finite volume method. The developed model determined water table fluctuations for different scenarios as follows: Drainage and recession from an unconfined aquifer, and water table fluctuations above an inclined leaky layer due to ditch recharge with a constant and variable upper boundary condition. The Boussinesq equation, which is the governing equation in this domain, is linearized and solved numerically in both of the explicit and fully implicit conditions. Meanwhile, the upwind scheme is employed to discretize the governing equation. The computed outcomes of both the explicit and implicit approaches agreed well with the results of analytical solution and laboratory experiments. The main reason is that in the first half of simulation process explicit scheme obtains slightly better results and in the second half of the simulation process fully implicit scheme predicts more reliable outcomes that are hidden in the neighbor node points. As a final point, the numerical outcomes confirm that the developed model is capable of calculating satisfactory outcomes in engineering and science applications

INTEGRATED APPROACHES FOR SCALABLE AQUIFER NUMERICAL MODELING, INVERSION, AND UPSCALING

Ph.D

Modeling the drug release from hydrogel-based matrices

In this work the behavior of hydrogel-based matrices, the most widespread systems for oral controlled release of pharmaceuticals, has been mathematically described. In addition, the calculations of the model have been validated against a rich set of experimental data obtained working with tablets made of hydroxypropyl methylcellulose (a hydrogel) and theophylline (a model drug). The model takes into account water uptake, hydrogel swelling, drug release, and polymer erosion. The model was obtained as an improvement of a previous code, describing the diffusion in concentrated systems, and obtaining the erosion front (which is a moving boundary) from the polymer mass balance (in this way, the number of fitting parameters was also reduced by one). The proposed model was found able to describe all the observed phenomena, and then it can be considered a tool with predictive capabilities, useful in design and testing of new dosage systems based on hydrogels

Modelling of hydraulic fracturing and its engineering application

The Hydraulic Fracturing process and its engineering applications have been studied and reported in this thesis. The Distinct Element Method (DEM) was adopted as the main and preferred numerical technique because of its distinctive features and advantages. This method allows the phenomenon to be modelled and viewed microscopically at the inter-particle level by conceptualising the rock mass as an assembly of discrete particles interacting with each other via contacts. This method allows for a more detailed and dynamic monitoring of the hydraulic fracturing process. Sequel to a detailed review on the study of the hydraulic fracturing phenomenon, the research was extended to investigate specific cases of applications of hydraulic fracturing in geo-mechanical and environmental problems. Examples of such cases include carbon dioxide injection and storage in a reservoir system, and the behaviour of naturally occurring faults subjected to hydrostatic fluid pressures. The key factors governing the geo-mechanical responses of porous media (rocks), including reservoir formations were identified and further examined to ascertain the following: the role and inter-relationship between operating and material/fluid variables such as injection flow rate, fluid pressure, and interstitial velocity; type and pattern of fracture propagation; influence of environmental conditions as well as the configuration of the well-reservoir system, amongst others. Because of broad similarities in enabling conditions, analyses and applications of the phenomenon were also extended to study the sand production process. However, since the emphasis of the study was on identifying and examining the controlling variables as well as establishing patterns of sanding production rates rather than the study of the cavitation process, investigations were conducted using a finite element procedure; moreover, the limit of computational capacity has prevented a large scale DEM model for such problems. Modelling results show that fracturing mode, pattern and intensity are highly dependent on operating and environmental conditions; the reservoir erosion processes also indicate likewise tendencies. The numerical modelling techniques adopted and results obtained facilitate an improved understanding of geo-mechanical mechanisms at sub-surface systems, and could be further improved for industrial applications, such as site evaluation and assessment of the efficiency of stimulation techniques

Analysis and modeling of the behavior of hydrogels-based systems for biomedical and agro-food applications

2015 - 2016Hydrogels are three-dimensional, hydrophilic, polymeric networks capable of imbibing large amounts of water or biological fluids. Depending on the type of polymer, number of cross-links, presence of ionic species the swelling/shrinking behavior can be greatly modified. This peculiar behavior, which has led to define this soft matter as “smart materials”, makes hydrogels and hydrogel-based systems very attractive by several frontier fields, such as biomedical applications, as well as for sectors that are less demanding technology, i.e. agro-food applications. The general aim of this Ph.D. thesis is to analyze, with ad hoc experiments, and to describe/simulate, through mathematical modeling, the behavior of hydrogels and hydrogel-based systems. A first question to answer when approaching hydrogels is: “are they multiphasic or monophasic systems”? The answer cannot be taken for granted. Despite in most experimental cases the response is simply avoided, it become fundamental when the aim is to develop a mechanistic mathematical model of the system. The most natural approach is to consider hydrogels as single-phase matter, in which several components can coexist, like it would be indisputably done for polymeric solutions (hydrosols). Another vision is to consider hydrogels as made of different phases, i.e. the water phase is separated from the polymeric phase, and these can exchange momentum. During this work a general modeling framework has been proposed to which several models from literature, multiphasic or monophasic, can be traced back or, vice versa, depending on the chosen approach the framework can be particularized to give the multiphasic or the monophasic balance equations. In this thesis, in light of its thermodynamic and numerical robustness, the monophasic approach, which is more consistent, has been chosen. Another important question is related to the need of modeling/analyze the full behavior, mass transport plus mechanics, or just one aspect, mass transport only. The difficulties related to the solution/analysis of the full hydrogels behavior have led many researchers to describe hydrogel-based systems with a “mass transport only” approach. This is, in example, common in drug delivery applications. During this PhD a mechanistic model based on a “mass transport only” approach for drug release from hydrogel-based system has been developed and validated against experimental data. HPMC-based tablets, loaded with Theophylline have been studied. Differently to what is normally done in dissolution tests, in this work besides the evaluation of the drug release via spectrophotometric analysis, the water and polymer residue have been determined by gravimetric analysis. This has been done on the entire tablets, as well as on portion of them, obtaining internal profiles of the components. The partially swollen tablets have been also subjected to indentation tests, which after an opportune calibration have allowed obtaining information on the water distribution inside the system. A 2Daxisymmetric model has been built on the water and drug mass transport equations; the polymer has been obtained from the mass fraction constraint. The deformations have been described with an ALE moving mesh method, whose boundaries move in relation to the amount of water and drug entering or leaving the system. The comparison between the detailed experimental results and the modeling results has shown a good agreement, in terms of masses, shape and components distribution, demonstrating that the main features had been correctly described. Such a formulated model has been applied to describe commercial-like tablets (in which excipients were present), with two type of HPMC with different substitution pattern (i.e. different degree of cross-links) and tested in non-standard apparatus (NMR cell). Despite after a proper tuning the model has been able to describe the drug and polymer release, the shape and the water distribution inside the system (experimentally taken from MRI technique) have not been correctly described. This application demonstrated the limits of a “mass transport only” approach. In the analyzed case the forces acting on the swelling tablet (shear, centrifugal, gravitational) could have a relevant impact, but most of all the different degree of cross-links of the HPMC played the major role. In order to consider the hydrogel mechanics, the pure hydrogel behavior has been studied. Hydrogels normally couple solvent mass transport to system deformation and vice versa. This phenomenon is generally called poroelasticity and it is characteristic also of other materials (i.e. biological tissues, soils etc.). Another peculiarity of hydrogels is that the constituent polymeric network can have viscoelastic characteristics (i.e. like polymeric melts), which eventually translate in an overall hydrogel viscoelastic behavior. Depending on the time interval of interest and on the characteristic times of relaxation and diffusion, hydrogels can behave viscoelastically, poroelastically or poroviscoelastically (when the diffusion time is comparable with the relaxation time). A 3D model describing the poroviscoelastic behavior of hydrogels, still scarcely implemented in literature, has been developed within the field of non-equilibrium thermodynamics and non-linear solid mechanics (large deformations) and implemented in a commercial FEM-based software. The results of such kind of model permit to discriminate between and to study the poroelastic and viscoelastic regime as well as Abstract Diego Caccavo it permits to study the poroviscoelastic behavior. Experimental unconfined stress-relaxation tests have been performed on agarose-gels at different concentrations with radius and height of 1 cm, and imposing a deformation of 10%. In the time range analyzed (1200 s) the agarose-gel has shown a predominant viscoelastic behavior, releasing only little amount of water. The model, after an initial tuning of the parameters, has been able to fairly predict the experimental data. Characteristic of the developed approach is that, once the model parameters are derived, it is possible to describe the hydrogel subjected to different stimuli (mechanicals or chemicals). The proposed poroviscoelastic model is extendable to multicomponent diffusion systems, which could be, in example, controlled release systems based on hydrogels. For the first time, to the author’s knowledge, in the hydrogel-based systems modeling literature, in this thesis it has been shown how to extend the poroviscoelastic model to consider the presence of another diffusing species. The transport and constitutive model equations, opportunely modified, have been implemented in a commercial FEM-based software and, as an example, the drug release from a swelling system has been reported. [edited by author]XV n.

Integrated Environmental Modelling Framework for Cumulative Effects Assessment

Global warming and population growth have resulted in an increase in the intensity of natural and anthropogenic stressors. Investigating the complex nature of environmental problems requires the integration of different environmental processes across major components of the environment, including water, climate, ecology, air, and land. Cumulative effects assessment (CEA) not only includes analyzing and modeling environmental changes, but also supports planning alternatives that promote environmental monitoring and management. Disjointed and narrowly focused environmental management approaches have proved dissatisfactory. The adoption of integrated modelling approaches has sparked interests in the development of frameworks which may be used to investigate the processes of individual environmental component and the ways they interact with each other. Integrated modelling systems and frameworks are often the only way to take into account the important environmental processes and interactions, relevant spatial and temporal scales, and feedback mechanisms of complex systems for CEA. This book examines the ways in which interactions and relationships between environmental components are understood, paying special attention to climate, land, water quantity and quality, and both anthropogenic and natural stressors. It reviews modelling approaches for each component and reviews existing integrated modelling systems for CEA. Finally, it proposes an integrated modelling framework and provides perspectives on future research avenues for cumulative effects assessment