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Author: Simon Kaspar Schnyder
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Languages : en
Pages : 0
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Author: Simon Kaspar Schnyder
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Category :
Languages : en
Pages : 0
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Author: Simon Schnyder
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Languages : en
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Author: Ralf Metzler
Publisher: Frontiers Media SA
ISBN: 2889663655
Category : Science
Languages : en
Pages : 221
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Author: Alessandro Comolli
Publisher:
ISBN:
Category :
Languages : en
Pages : 215
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Solute transport in heterogeneous porous media in general exhibits anomalous behaviors, in the sense that it is characterized by features that cannot be explained in terms of traditional models based on the advection-dispersion equation with constant effective coefficients. Signatures of anomalous transport are the non-linear temporal growth of the variance of solute concentration, non- Gaussian density profiles and heavy-tailed breakthrough curves. Understanding and predicting transport behavior in groundwater systems is crucial for several environmental and industrial applications, including groundwater management and risk assessment for nuclear waste repositories. The complexity of this task lies in the intrinsic multi-scale heterogeneity of geological formations and in the large amount of degrees of freedom. Hence, the predictive description of transport requires a process of upscaling that is based on measurable medium and flow attributes. The time domain random walk (TDRW) and continuous time random walk (CTRW) approaches provide suitable frameworks for transport upscaling. In this thesis, we identify different mechanisms that induce anomalous transport and we quantify their impact on transport attributes. We propose average transport models that can be parameterized in terms of flow and medium properties. Among the mechanisms that induce non-Fickian behaviors, a pivotal role is played by the heterogeneity of the flow field, which is directly linked to medium disorder. Due to its importance, the impact of advective heterogeneity is studied throughout the thesis, alongside with other mechanisms. First, we consider solute trapping due to physical or chemical heterogeneity, which we parameterize in terms of a constant trapping rate and a distribution of return times. We observe three distinct transport regimes that are linked to characteristic trapping time scales. At early times, transport is advection- controlled until particles start to get trapped. Then, the increasing distance between mobile and immobile particles gives rise to a superdiffusive regime which finally evolves towards a trapping-controlled regime. Second, we study transport in correlated porous media. We show that particle motion describes a coupled CTRW that is parameterized in terms of the distribution of flow velocity and length scales. We show that disorder and correlation may lead to similar behaviors in terms of displacement moments, but the difference between these mechanisms is manifest in the distributions of particle positions and arrival times. Next, we study the relationship between flow and transport properties and the impact of different injection conditions on transport. To this end, the relationship between Eulerian and Lagrangian velocities is investigated. Lagrangian statistics evolves to a steady-state that depends on the injection conditions. We study the velocity organization in Darcy flows and we develop a CTRW model for transport that is parameterized in terms of flow and medium attributes only. This CTRW accounts for non-stationarity through Markovian velocity models. We study the impact of advective heterogeneity by considering different disorder scenarios. Finally, we quantify the impact of diffusion in layered and fibrous heterogeneous media by considering two disorder scenarios characterized by quenched random velocities and quenched retardation properties, respectively. These mechanisms lead to different, dimension-dependent disorder samplings that give rise to dual transport processes in space and time. Specifically, transport describes correlated Lévy flights in the random velocity model and correlated CTRWs in the random retardation model.
Author: Peter Kyungchul Kang
Publisher:
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Category :
Languages : en
Pages : 144
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Anomalous transport, understood as the nonlinear scaling with time of the mean square displacement of transported particles, is observed in many physical processes, including contaminant transport through porous and fractured geologic media, animal and human foraging patterns, tracer diffusion in biological systems, and transport in complex networks. Understanding the origin of anomalous transport is essential, because it determines the likelihood of high-impact, low-probability events and therefore exerts a dominant control over the predictability of a system. The origin of anomalous transport, however, remains a matter of debate. In this thesis, we first investigate the pore-scale origin of anomalous transport through sandstone. From high-resolution (micron-scale) 3D numerical flow and transport simulation, we find that transport at the pore scale is markedly anomalous. We demonstrate that this anomalous behavior originates from the intermittent structure of the velocity field at the pore scale, which in turn emanates from the interplay between velocity heterogeneity and velocity correlation. Finally, we propose a continuous time random walk (CTRW) model that honors this intermittent structure at the pore scale and captures the anomalous 3D transport behavior at the macroscale. To show the generality of our finding, we study transport through lattice networks with quenched disorder. We again observe anomalous transport originating from the interplay between velocity heterogeneity and velocity correlation. We extend the developed CTRW model to capture the full multidimensional particle transport dynamics for a broad range of network heterogeneities and for both advection- and diffusion-dominated flow regimes. We then study anomalous transport through fractured rock at the field-scale. We show that the interplay between heterogeneity and correlation in controlling anomalous transport can be quantified by combining convergent and push-pull tracer tests because flow reversibility is strongly dependent on correlation, whereas late-time scaling of breakthrough curves is mainly controlled by velocity heterogeneity. Our transport model captures the anomalous behavior in the breakthrough curves for both push-pull and convergent flow geometries, with the same set of parameters. Moreover, the inferred flow correlation length shows qualitative agreement with geophysical measurements. Thus, the proposed correlated CTRW modeling approach furnishes a simple yet powerful framework for characterizing the impact of flow correlation and heterogeneity on transport in porous and fractured media. Finally, we propose a joint flow-seismic inversion methodology for characterizing fractured reservoirs. Traditionally, seismic interpretation of subsurface structures is performed without any account of flow behavior. With the proposed methodology, we reduce the uncertainty by integrating dynamic flow measurements into the seismic interpretation, and improve the predictability of reservoir models by this joint use of seismic and flow data. This work opens up many possibilities of combining geophysical and flow information for improving subsurface characterization.
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Languages : en
Pages :
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Transport phenomena in highly heterogeneous media can be dramatically different from those in homogeneous media and therefore are of great fundamental and practical interest. Anomalous transport occurs in semiconductor physics, plasma physics, astrophysics, biology, and other areas. It plays an especially important role in hydrogeology because it may govern the rate of migration and degree of dispersion of groundwater contaminants from hazardous waste sites. The series of four articles in this special section of Vadose Zone Journal is devoted to transport phenomena in heterogeneous media in the context of geologic disposal of radioactive waste. It contains the results of joint investigations performed at the Nuclear Safety Institute of the Russian Academy of Sciences and Lawrence Berkeley National Laboratory in California. The work was supported by the U.S. DOE (under Contract No. DEAC02-05CH11231). The problems addressed in this research involve a broad range of space and time scales and were approached using modern methods of theoretical and computational physics, such as scaling analysis and diagrammatic techniques used before in critical phenomena theory. Special attention is paid to the asymptotics of concentration behavior (concentration tails). This issue is exceptionally important for the reliability assessments of radioactive waste disposal because, depending on the structure of the tails, concentrations at large distances from the source can differ by many orders of magnitude. In the first paper of this special section, Bolshov et al. (2008b) present an overview of field and laboratory observations that demonstrate nonclassical flow and transport behavior in geologic media. It is recognized that natural fracture networks as a rule have fractal geometry and can be classified as percolation systems. This is one of the main factors giving rise to anomalous transport in geologic media. Another important factor is the presence of contaminant traps provided by low-permeable rock matrix and dead-ends of fracture percolation clusters. Physical concepts to describe transport phenomena in fractured rocks are discussed. The second paper (Dykhne et al., 2008) is devoted to the analysis of diffusion in heterogeneous media with sharply contrasting properties. The authors show that as time progresses, three different transport regimes can be realized in the model. Here, an intermediate regime corresponds to subdiffusion. The change of regimes results in a complex structure of concentration tails, with the shapes of the more-distant tail segments determined by earlier-time transport behavior. In the third paper (Bolshov et al., 2008a), new elements are developed to generalize the dual-porosity model for moisture infiltration and solute transport in unsaturated rocks, taking into account fractal aspects of the percolation process. It is shown that the solute transport regime is determined by a competition of two mechanisms: random advection through a fracture network and trapping caused by sharply contrasting properties of the medium. As a result, superdiffusive, subdiffusive, or classical diffusive regimes may occur. The complex structure of concentration tails and effects due to medium characteristic fluctuations is also discussed. In the fourth paper, Goloviznin et al. (2008) develop a stochastic random walk numerical model of anomalous diffusion to simulate solute transport in highly heterogeneous media. Solutions of the one- and symmetric two-dimensional stochastic problem are compared with computations performed on the basis of fractional advection--diffusion equation models. The new model is in reasonable agreement with experimental data on solute transport in highly heterogeneous media.
Author: Brian Berkowitz
Publisher: Springer Science & Business Media
ISBN: 9401712786
Category : Science
Languages : en
Pages : 261
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In spite of many years of intensive study, our current abilities to quantify and predict contaminant migration in natural geological formations remain severely limited. The heterogeneity of these formations over a wide range of scales necessitates consideration of sophisticated transport theories. The evolution of such theories has escalated to the point that a review of the subject seems timely. While conceptual and mathematical developments were crucial to the introduction of these new approaches, there are now too many publications that contain theoretical abstractions without regard to real systems, or incremental improvements to existing theories which are known not to be applicable. This volume brings together articles representing a broad spectrum of state-of-the-art approaches for characterization and quantification of contaminant dispersion in heterogeneous porous media. Audience: The contributions are intended to be as accessible as possible to a wide readership of academics and professionals with diverse backgrounds such as earth sciences, subsurface hydrology, petroleum engineering, and soil physics.
Author: Anna Russian
Publisher:
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Languages : en
Pages : 178
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This thesis studies diffusion phenomena in heterogeneous media, which includes Darcy flow and diffusive solute transport in geological media. Natural media are heterogeneous at different scales, which induces complexity in diffusion phenomena. The work is centered on the integration of the effects of heterogeneity on Darcy flow and solute diffusion into large scale models. The quantification of the effects of heterogeneity in diffusion phenomena is highly important for a large number of problems such as diffusion and reaction of chemicals and radionuclides in low permeability media, which is essential in subsurface hazardous waste storage problems, CO2 sequestration performance and groundwater management. In a stochastic framework we quantify the effects of heterogeneity in large scale models considering two interrelated strategies that can be called Ôcoefficient approachÕ, which deals with the derivation of effective coefficients to insert in equivalent homogeneous models, and Ôdynamic approachÕ, which deals with the upscaling of the local scale equations and the derivation of large scale formulations which can differ from their local counterparts. Whenever a diffusion process cannot be described in terms of effective coefficient, that behaviour is named anomalous or non-Fickian. Anomalous diffusion behaviours observed experimentally are frequently modelled by effective theories such as fractional diffusion equations, continuous time random walks. One limitation of these models is that often they are rather phenomenological and the relation to the local scale heterogeneity and dynamics may not be clear. In the dynamic approach we derive large scale descriptions that can explain anomalous behaviour and link it with a description of the local scale medium heterogeneity. To this end, we upscale the local scale governing equations using different methods depending on the type of medium heterogeneity. For moderately heterogeneous media we upscale flow equation by stochastic averaging. Starting from the classical flow equation at local scale determined by Darcy's law, we derive an upscaled non-local effective formulation. The non-local effective formulation is compared with its local counterpart by considering the head response for a pulse injection. Numerically, we solve flow and diffusion in heterogeneous media using particle tracking methods. While classical random walk particle tracking is an efficient numerical tool to solve for diffusion problems in moderately heterogeneous media, strong medium contrasts, as encountered in fractured media, render this method inefficient. For highly heterogeneous media efficiency of classical random walk can be increased by the use of the time domain random walk (TDRW) method. We rigorously derive the equivalence of the TDRW algorithm and the diffusion equation and we extend the classical TDRW method to solve diffusion problem in a heterogeneous medium with multi-rate mass transfer properties. Moreover we use the TDRW method in connection with a stochastic model for the heterogeneity in order to upscale heterogeneous diffusion processes. For a certain class of heterogeneity, the upscaled dynamics obey a CTRW. Analytically we upscale diffusion in highly heterogeneous media by using a multicontinuum representation of the media. Using volume and ensemble averaging we derive a multicontinuum model that can explain anomalous diffusion behavior and link it with a suitable local scale description of the medium heterogeneity. Finally, we integrate the multicontinuum model derived in the context of aquifer modelling. We derive a multicontinuum catchment model that can explain anomalous behavior observed in the aquifer dynamics at basin scale. We identify the physical mechanisms that induce anomalous behaviour and we determine the time scales that control its temporal evolution.
Author: Rulan Gong
Publisher:
ISBN:
Category : Reactivity (Chemistry)
Languages : en
Pages :
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Reactive transport models are essential tools for predicting contaminant fate and transport in the subsurface and for designing effective remediation strategies. Sound understanding of subsurface mixing in heterogeneous porous media is the key for the realistic modeling of reactive transport. This dissertation aims to investigate the extent of mixing and improve upscaling effective macroscopic models for mixing-controlled reactive transport in connected heterogeneous formations, which usually exhibit strongly anomalous transport behavior. In this research, a novel approach is developed for an accurate geostatistical characterization of connected heterogeneous formations transformed from Gaussian random fields. Numerical experiments are conducted in such heterogeneous fields with different connectivity to investigate the performance of macroscopic mean transport models for simulating mixing-controlled reactive transport. Results show that good characterization of anomalous transport of a conservative tracer does not necessarily mean that the models may characterize mixing well and that, consequently, it is questionable that the models capable of characterizing anomalous transport behavior of a conservative tracer are appropriate for simulating mixing-controlled reactive transport. In connected heterogeneous fields with large hydraulic conductivity variances, macroscopic mean models ignoring concentration variations yield good prediction, while in fields with intermediate conductivity variances, the models must consider both the mean concentration and concentration variations, which are very difficult to evaluate both theoretically and experimentally. An innovative and practical approach is developed by combining mean conservative and reactive breakthrough curves for estimating concentration variations, which can be subsequently used by variance transport models for prediction. Furthermore, a new macroscopic framework based on the dual-permeability conceptualization is developed for describing both mean and concentration variation for mixing-controlled reactive transport. The developed approach and models are validated by numerical and laboratory visualization experiments. In particular, the new dual-permeability model demonstrates significant improvement for simulating mixing-controlled reactive transport in heterogeneous media with intermediate conductivity variances. Overall, results, approaches and models from this dissertation advance the understanding of subsurface mixing in anomalous transport and significantly improve the predictive ability for modeling mixing-controlled reactive transport in connected heterogeneous media.
Author: Leonid Bolshov
Publisher: LAP Lambert Academic Publishing
ISBN: 9783659519055
Category :
Languages : en
Pages : 88
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This book contains results of contaminant transport studies in heterogeneous sharply contrasting media with taking into account possible sorption both in weakly permeable domain and on mobile colloidal particles as well. It is shown that in such media anomalous transport regimes (when contaminant distribution is not described by classical Gaussian law) are rather the rule than the exception. Moreover, transport regime itself for a given type of medium usually changes with time. The results allow estimating the role of different mechanisms and heterogeneity types in the formation of ultimate transport regime of contaminant near the source and at far distances.
