Critical Parameters In Magmatic Degassing

Thesis (Ph.D.) University of Alaska Fairbanks, 2008Decompression experiments conducted at pressures up to 200MPa and temperatures of 825�C-880�C on hydrated K-phonolite and rhyolite melts were used to explore the critical parameters controlling nucleation, exsolution and degassing behavior. Experiments on the low viscosity/surface tension K-Phonolite melt highlighted the role of melt properties. Although the sample porosities deviated below equilibrium values for pressures less than ~40MPa, the melt exsolved water in equilibrium over all the pressures and decompression rates studied. Melt shearing is proposed to have caused bubble deformation and alignment, lowering the porosity at which extensive permeability develops and significant degassing occurs compared to rhyolite. Experiments on a rhyolite melt decompressed slowly from 100 MPa and then held at 10 MPa for up to 900 s highlighted the critical parameters controlling the formation and stability of a highly vesicular magma: bubble number density, bubble size distribution and porosity. The porosity of the interconnected, highly vesicular network decreased during "Stage I" degassing and the bubble size distribution evolved from a unimodal population to include a population of much larger bubbles. During Stage II degassing, the network collapsed. Pre-collapse and collapse degassing rates were obtained and a coalescence-induced coalescence model proposed to explain the rapid destabilization. The ability of a melt to efficiently exsolve volatiles and the ease of bubble coalescence are both a function of the initial distribution of nucleated bubbles. The development of a new method for quantifying this distribution using spatial statistics will allow future researchers to explore the underlying controls on nucleation such as melt structure and the occurrence of a prior nucleation event. To investigate the critical parameters controlling shallow dike intrusion and therefore magmatic ascent rate, the fracture mechanics of intrusion into homogeneous and layered (weak sandstone/strong granite) particle models under lithostatic, compressive and extensional regimes were examined. Although the scale of the model intrusions were an order of magnitude greater than field observations, extensive microfracturing across the weaker layers, parallel dike jointing in the stronger layers and a length scale dependence to fracture toughness were observed suggesting that the use of a particle code is a promising approach to intrusion modeling

The Importance of Sintering with Crystals in Volcanic Regimes

Sintering, or welding, in silicic volcanic conduits limits outgassing, and outgassing from these conduits has a first order effect on determining the explosivity of an eruption (Cassidy et al., 2018). Crystals in volcanic systems can act as rigid inclusions; particles that are not involved in the sintering process but can have a significant effect on porosity and permeability. However, the role of crystals in sintering in a volcanological context has rarely been investigated previously. Here, analogue glass and crystal mixtures were sintered in situ using 4D x-ray tomography data collected in real time at the Diamond Light Source JEEP (i12) beamline synchrotron. This non-destructive technique captures high resolution 3D images at multiple points allowing the visualisation and quantification of interior and exterior changes. The data were processed using a 3D visualisation software to quantify porosity and connectivity. When crystals are included in the starting mixture, the total porosity decreases, the disconnected porosity increases through the experiment, and the final porosity is higher with greater crystal content. The pore network preferentially disconnects through the z-axis first before pores are disconnected through other axes. A high connectivity is maintained to lower porosities (i.e. for a longer period of time) compared to crystal-free systems, suggesting that degassing is more efficient which may reduce the frequency of explosive events or quicken the transition to effusive behaviour. The results demonstrate that crystals do have an impact on sintering behaviour and further work is required to establish the effects of even greater crystal content and of different pressure conditions

Techniques de bas niveau en traitement d'images pour la télédétection des milieux non homogènes

La télédétection vise à acquérir l'information sur des cibles en étudiant leur réponse aux ondes électromagnétiques. Et partout nous rencontrons des milieux non homogènes et des composites. Connaître comment ces milieux non homogènes répondent a la sonde de télédétection est de la plus grande importance pour la praticabilité même de la télédétection. Le comportement macroscopique d'un composite peut séxprimer en fonction des caractéristiques macroscopiques de ses constituants, mais d'une manière complexe incluant la géométrie de leur arrangement. Si nous pouvons obtenir le tenseur diélectrique efficace d'un composé, nous pouvons modéliser sa réponse au champ électromagnétique, et donc sa réponse comme cible de télédétection. La nécessité pour inclure la géométrie détaillée du système d'une façon efficace dans des méthodes numériques, ainsi qu'une équivalence entre les images numériques et les modèles de treillis des composites, suggère le recours aux techniques de bas niveau de traitement d'images numériques. Le cadre de cette thèse est le traitement numérique d'un problème général de télédétection fondée sur le problème électromagnétique d'homogénéisation dans des microstructures. Dans ce contexte, deux techniques de traitement d'images de bas niveau sont présentées, à savoir, une nouvelle méthode pour l'étiquetage des composantes connexes, présentant des améliorations significatives par rapport aux méthodes existantes, et une méthode de codage des configurations locales avec plusieurs caractéristiques la rendant appropriée pour des applications variées. Leurs avantages sont discutés, et des exemples d'application sont fournis au-delà du domaine spécifique étant à leur origine, comme la vision artificielle, le codage d'image, ou encore la synthèse d'image.The aim of remote sensing is obtaining information about targets by studying their response to electromagnetic waves. And everywhere we found non homogeneous media. Knowing how these non homogeneous media respond to the remote sensing probe is of great importance for the very feasibility of remote sensing. The macroscopic behaviour of a composite can be expressed as a function of the macroscopic characteristics of its constituents, but usually in a complex way which includes the geometry of their arrangement. If we are able to obtain the effective permittivity tensor of any given composite, we can model its macroscale response to the electromagnetic field, and therefore its response as a remote sensing target. The necessity of including the detailed geometry of the system in an efficient way in the numerical methods, together with an equivalence between grid models and digital images, suggest the recourse to low level image processing techniques. The framework of this thesis is the numerical treatment of a general problem in remote sensing based on the electromagnetic problem of homogenization of microstructures. In this context, two low level image processing techniques are presented, a new method for the labelling of connected components, with significant advantages over the classical methods, and a local configuration encoding scheme with characteristics which render it useful for different applications. Their advantages and applicability are discussed, together with some examples of application in fields out of the scope of the specific problem which originated them, namely computer vision, image coding, and image synthesis

Local capillary trapping and permeability-retarded accumulation during geologic carbon sequestration

Safe storage of CO2 in saline aquifers depends on CO2 migration rate, accumulation, and trapping inside saline aquifers that have intrinsic heterogeneity. This heterogeneity can be in both capillary entry pressure and permeability. The former heterogeneity causes local capillary trapping while the latter results in permeability-retarded accumulation. A main objective of this dissertation is to understand how both local capillary trapping and permeability-retarded accumulation secure CO2 storage. We establish a fast simulation technique to model local capillary trapping during CO2 injection into saline aquifers. In this technique, modeling efforts are decoupled into two parts: identifying trapping in a capillary entry pressure field and simulating CO2 flow in a permeability field. The former fields are correlated with the latter using the Leverett j-function. The first part describes an extended use of a geologic criterion originally proposed by Saadatpoor (2012). This criterion refers to a single value of ‘critical capillary entry pressure’ that is used to indicate barrier or local traps cells during buoyant flow. Three issues with the criterion are the unknown physical critical value, the massive overestimation of trapping, and boundary barriers. The first two issues are resolved through incorporating viscous flow of CO2. The last issue is resolved through creating periodic boundaries. This creation enables us to study both the amount and clusters of local capillary traps in infinite systems, and meanwhile the effects of reservoir heterogeneity, system size, aspect ratio, and boundary types are examined. In the second part, we adapt a connectivity analysis to assess CO2 plume dynamics. This analysis is then integrated into the geologic criterion to evaluate how injection strategies affect local capillary trapping in reservoirs. We demonstrate that reservoir heterogeneity affects the optimal injection strategies in terms of maximizing this trapping. We conduct analytical and numerical modeling of CO2 accumulations caused by both permeability hindrances and capillary barriers. The analytical model describes CO2 buoyant migration and accumulation at a low permeability region above a high-permeability region. In the limiting case of zero capillary pressure, the model equation is solved using the method of characteristics. The permeability-retarded accumulation is illustrated through CO2 saturation profiles and time-distance diagrams. Capillary trapping is subsequently accounted for by graphically incorporating the capillary pressure curve and capillary threshold effect. The relative importance of these two types of accumulations is examined under various buoyant source fluxes and porous media properties. Results demonstrate that accumulation estimate that account for only capillary trapping understates the amount of CO2 accumulated beneath low permeability structures during significant periods of a sequestration operation.Petroleum and Geosystems Engineerin

Fractal analysis of laplacian pyramidal filters applied to segmentation of soil images

The laplacian pyramid is a well-known technique for image processing in which local operators of many scales, but identical shape, serve as the basis functions. The required properties to the pyramidal filter produce a family of filters, which is unipara metrical in the case of the classical problem, when the length of the filter is 5. We pay attention to gaussian and fractal behaviour of these basis functions (or filters), and we determine the gaussian and fractal ranges in the case of single parameter ?. These fractal filters loose less energy in every step of the laplacian pyramid, and we apply this property to get threshold values for segmenting soil images, and then evaluate their porosity. Also, we evaluate our results by comparing them with the Otsu algorithm threshold values, and conclude that our algorithm produce reliable test results

The importance of pore throats in controlling the permeability of magmatic foams

Vesiculation of hydrous melts at 1 atm was studied in situ by synchrotron X-ray tomographic microscopy at the TOMCAT beamline of the Swiss Light Source (Villigen, Switzerland). Water-undersaturated basaltic, andesitic, trachyandesitic, and dacitic glasses were synthesized at high pressures and then laser heated at 1 atm. on the beamline, causing vesiculation. The porosity, bubble number density, size distributions of bubbles, and pore throats, as well as their tortuosity and connectivity, were measured in three-dimensional tomographic reconstructions of sample volumes, which were also used for lattice Boltzmann simulations of viscous permeabilities. Connectivity of bubbles by pore throats varied from ~\u2009100 to 105 mm 123, and for each sample correlated with porosity and permeability. Consideration of the results of this and previous studies of the viscous permeabilities of aphyric and crystal-poor magmatic samples demonstrated that at similar porosities permeability can vary by orders of magnitude, even for similar compositions. Comparison of the permeability relationships from this study with previous models (Degruyter et al., Bull Vulcanol 72:63\u201374, 2010; Burgisser et al., Earth Planet Sci Lett 470:37\u201347, 2017) relating porosity, characteristic pore-throat diameters, and tortuosity demonstrated good agreement. Modifying the Burgisser et al. model by using the maximum pore-throat diameter, instead of the average diameter, as the characteristic diameter reproduced the lattice Boltzmann permeabilities to within 1 order of magnitude. Correlations between average bubble diameters and maximum pore-throat diameters, and between porosity and tortuosity, in our experiments produced relationships that allow application of the modified Burgisser et al. model to predict permeability based only upon the average bubble diameter and porosity. These experimental results are consistent with previous studies suggesting that increasing bubble growth rates result in decreasing permeability of equivalent porosity foams. This effect of growth rate substantially contributes to the multiple orders of magnitude variations in the permeabilities of vesicular magmas at similar porosities