1,721,012 research outputs found
Supporting Data for: Mineral precipitation and geometry alteration in porous structures: How to upscale variations in permeability-porosity relationship?
No full textPorous materials in natural and engineered environments are subject to morphological changes resulting from interacting chemical and physical processes. The complexity of coupled flow, transport, and chemical processes that occur on different temporal and spatial scales makes it difficult to predict the resulting porosity and permeability alterations. Delineating the controls of mineral precipitation reactions is particularly challenging because it requires the implementation of nucleation criteria and precipitation attributes. By conducting pore-scale simulations, we studied how the amount and stochastic distribution of crystallites, controlled by nucleation, affect the pore geometry and permeability in two-dimensional porous structures. The observed relationships between porosity and permeability show characteristics that differ from ones that are typically applicable in dissolving porous media because of the clogging effect. Additionally, we propose a stochastic framework that upscales the co-evolution of permeability and porosity across length scales. This framework enables precise communication of clogging behavior to continuum-scale simulations based on statistical probability distributions of permeability-porosity variations.
Keywords: Upscaling; Probabilistic nucleation; Mineral precipitation; Geometry evolution; Permeability-Porosity; Reactive transport; Porous medi
Pore-scale numerical investigation of solute transport variability in fractured rocks: from fracture surface topography to pore space heterogeneity
No full textThe heterogeneity of mineralogical and hydraulic properties in fractured crystalline rocks plays a critical role in accurately assessing the subsurface transport of radionuclides. This doctoral thesis combines both experimental and simulation approaches to examine the geometric and mineral heterogeneity affecting the primary reactive transport processes in fracture-matrix systems, including advection, diffusion, and surface reactions. A significant aspect of this study is the introduction of a cross-scale topographic analysis method, with a focus on the interactions between fluid transport and surface reactivity.
Fractures serve as the primary transport pathways for solutes in so-called crystalline rocks. Solute species migrate rapidly through fractures by advection, but this process is partly retarded by diffusion and adsorption. Accurately characterizing the geometric and topological structure of fractures is critical because it affects the assessment of solute dispersion within fractures, as well as the coupling between fluid flow and surface reactions. Surface topographic features can vary with different mineral composition, their grain size, and their grain surface. Therefore, we implemented a fracture model constructed from computed tomography (CT) images with modified surface detail in order to simulate solute transport under different surface roughness conditions. We introduced a cross-scale surface analysis tool (power spectra density, PSD) to evaluate the length scale sensitivity in surface modifications. We found that increasing the scale of surface topography does not always result in a linear or steady change in breakthrough curves (BTCs). This finding provides valuable guidance for using simplified or oversimplified geometries in reactive transport modeling.
In the rock volume surrounding fractures in crystalline rocks, solute species are primarily transported by diffusion through intergranular pores and microfractures that connected to the fractures, controlled by, varying effective diffusivities. Fracture mineralization, such as calcite, occurs quite frequently. The resulting channeling effect increases with the degree of mineralization, ultimately shifting transport from the advection regime to the diffusion regime and lowering migration. To quantitatively analyze the resulting transport in such complex systems, we conducted an in-situ diffusion experiment using positron emission tomography (PET) scanning. This experiment allowed us to quantify transport behavior and elucidate the role of heterogeneous, multigeneration fracture precipitates in influencing radionuclide migration. The reactivity of mineral surfaces determines the efficiency with which dissolved species are adsorbed or desorbed, and thus, the quantitative assessment of possible retention. The heterogeneity of nano- and microtopographic surfaces results in different reaction rates. Surfaces with a higher density of reactive sites, such as kink sites and edges, exhibit higher reaction rates than plateau surfaces. Concurrently, this micro-topographically induced variation in intrinsic surface reactivity continuously alters the surface topography. The coupling of surface evolution and reaction rate is crucial for the parameterization of pore-scale reactive transport models, which require the consideration of time-dependent variables. Linking the evolution of reactive surfaces across spatial (from nanometers to millimeters) and temporal scales with their reaction patterns answers a central question: Can the reaction rate be treated as a stable parameter across space and time? This question is becoming increasingly urgent given the demand for improved predictability of reactive transport models, particularly with regard to long-term radionuclide migration prediction. This work demonstrates for the first time that the quantitative variability of the reaction rate is stationary, establishing a stationary microtopography of the reactive surface. This dissertation provides a comprehensive analysis of how geometric and mineralogical heterogeneity in fractured rock types influences radionuclide transport through advection, diffusion, and reactivity. By integrating the aforementioned topics of reactive transport, it contributes to a better understanding of the controlling mechanisms of nuclear disposal. Based on this analysis, the dissertation proposes the use of specific geometry and reactivity parameters to enhance the predictive capabilities of existing reactive transport models in complex fractured rocks. Overall, this work contributes to the improved safety assessment of the geological disposal of high-level radioactive waste
Interactions between CO2, saline water and minerals during geological storage of CO2
Full text linkThis doctoral thesis is based on the following papers: Paper A: Hellevang, H., Aagaard, P., Oelkers, E.H., and Kvamme, B., 2005. Can dawsonite permanently trap CO2? Environmental Science and Technology, 39, 8281-8287. Paper B: Hellevang, H., Kvamme, B., and Aagaard, P., 2004. Long term interactions between minerals and reactive fluids - Stability of dawsonite. In Proceedings of the Third Annual Conference on Carbon Capture and Sequestration DOE/NETL, Alexandria, VA, May 3-6, 1-7 Paper C: Hellevang, H., and Kvamme, B. CO2-water-rock interactions - ACCRETE simulations of geological storage of CO2. Submitted to Applied Geochemistry. Paper D: Hellevang, H., Kumar, S., Fladmark, G., and Kvamme, B. CO2 storage in the Utsira Formation – ATHENA 3D reactive transport simulations. Submitted to Basin Research. Paper E: Hellevang, H., and Kvamme, B. 2006. ACCRETE – Geochemistry solver for CO2-water-rock interactions. Paper submitted to GHGT8, Trondheim, June 19-22, 2006. Paper F: Khattri, S.K., Hellevang, H., Fladmark, G.E., and Kvamme, B. 2006. Deposition of Green House Gases by Compositional Simulator: Long Term Reactive Transport of CO2 in the Sand of Utsira. Submitted to Transport in Porous Media
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Constraints on natural global atmospheric CO2 fluxes from 1860 to 2010 using a simplified explicit forward model
No full textLand-use changes until the beginning of the 20th century made the terrestrial biosphere a net source of atmospheric carbon. Later, burning of fossil fuel surpassed land use changes as the major anthropogenic source of carbon. The terrestrial biosphere is at present suggested to be a carbon sink, but the distribution of excess anthropogenic carbon to the ocean and biosphere sinks is highly uncertain. Our modeling suggest that land-use changes can be tracked quite well by the carbon isotopes until mid-20th century, whereas burning of fossil fuel dominates the present-day observed changes in the isotope signature. The modeling indicates that the global carbon isotope fractionation has not changed significantly during the last 150 years. Furthermore, increased uptake of carbon by the ocean and increasing temperatures does not yet appear to have resulted in increasing the global gross ocean-to-atmosphere carbon fluxes. This may however change in the future when the excess carbon will emerge in the ocean upwelling zones, possibly reducing the net-uptake of carbon compared to the present-day ocean
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