PhD thesis defense of M. Atta-Ul Monem Ayaz, in english, entitled
"Experimental and numerical investigation of cluster morphologies and dynamics during biphasic flow in porous media".
The defense will take place on Monday, Dec. 16th, 2019, 14:00, at the Amphitheater Rothé, IPGS/EOST, 5 rue Descartes, Strasbourg. The thesis is a cotutelle (co-supervision) between the IPGS & LHYGES, EOST, University of Strasbourg and SFF PoreLab, Njord, Physics Dept, University of Oslo.
The jury will be composed of:
Pr. Insa Neuweiler, Leibniz Universität Hannover, Faculty of Civil Engineering and Geodetic Science - reviewer (rapporteuse)
Dr. Stéphane Santucci, CR CNRS - HDR, Laboratoire de Physique de l'ENS de Lyon UMR5672 CNRS/Université Claude Bernard Lyon I / ENS de Lyon- reviewer (rapporteur)
Dr. Philippe Ackerer, DR CNRS, LHYGES, UMR7517, Université de Strasbourg/CNRS (examiner)
Dr. Renaud Toussaint, DR CNRS, IPGS, UMR7516, Université de Strasbourg/CNRS (directeur de thèse)
Pr. Knut Jørgen Måløy, SFF PoreLab, The Njord Center, Physics Dept, University of Oslo (directeur de thèse)
Pr. Gerhard Schäfer, LHYGES, LHYGES, UMR7517, directeur de thèse, Université de Strasbourg/CNRS (directeur de thèse)
Abstract:
Porous materials widely exists around us and play a role in many aspects of our life. Natural substances such as rocks and soil can often be considered to be porous materials. It is thanks to their porosity that soil allows for precipitation to migrate from the surface to ground water aquifers. Similarly in rocks such as calcite the connected pores set up pathways for mineral water to percolate through stone, allowing for changes of the chemical composition over long time scales. Furthermore, as porous materials have low density of mass and high structural
rigidity they are among other found to take shape as skeletal structures. An example is the bones in our body.
The work presented in this thesis is concerned with improving our understanding of the underlying mechanisms by which fluids are transported in porous media and how dense suspensions transport through a confining geometry. Fluid flow in porous media is a research topic which has been extensively studied for the past decades. An improved understanding of such processes promises economical benefits such as enhanced oil recovery, and great environmental rewards, among other in terms of CO2 sequestration in sub-sea reservoirs and mapping and controlling of migrating ground water contaminants.
From a fundamental perspective flow in porous media is of interest as it displays complex behavior. Such system responds to a slowly changing external condition not in a smooth way but in the terms of a series of discrete events or bursts spanning a broad range of sizes. Such avalanches are observed for many different systems ranging from earthquakes occurring when slowly moving tectonic plate shear into each other, to the tearing of paper. Another example is the motion of a fluid front when it invades a porous media. Moreover the displacement
dynamics is seen to generate fractal structures for certain conditions.
In our first this study we experimentally, numerically and analytically study the influence of gravity and _finite-size effects on pressure-saturation measurements during slow drainage where the lighter fluid invades the system from the top. This relationship is much used by hydrologist and soil-scientists to give closure to the Darcy equations used in continuum modeling.
We experimentally perform tabletop experiments utilizing a quasi two-dimensional (2D) porous medium, made up by a monolayer of poly-disperse glass beads sandwiched between two confining plates. The boundaries are sealed of with layer of silicon glue with one side open to the invading phase (air). The relative pressure difference between the non-wetting and wetting phase is measured with pressure sensors while the saturation of the two phases is extracted by thresholding the grayscale map from images taken at fixed time lapse. The saturating fluid is withdrawn with a constant slow flow rate, making viscous effects negligible for the invasion process. Coupled with the experiments we perform numerical simulations using an invasion percolation algorithm. In such models the invasion is represented by a cluster growth process on a random lattice, dependent solely on the global homogeneous pressure in each phase and on the local capillary pressure thresholds in the pore throats along
the interface.
The effect of gravity is systematically varied by tilting the system relative to the horizontal configuration, causing the invasion to be gravity stabilized. The displacement structure that emerges is composed of clusters of defending that stay trapped after the invasion interface has moved through the system and are hence built by the motion of the interface. By exploiting the fractal nature of the displacement structure we are able to obtain a relationship between the final saturation, and the dimensionless capillary and Bond numbers Ca and Bo, characterizing the competition between the viscous, capillary and gravitational forces, using percolation theory. For the 2D case this relation was verified by simulation and experiments.
For the 3D case we compared our relation with measurement conducted by Nouri et al.. Here saturation and pressure were measured using a hanging column apparatus for four different soil types, showing compatibility with our obtained relationship. Moreover the saturation, pressure and Bond number are functionally related allowing for pressure-saturation curves to collapse onto a single master curve.
In our second study we investigate the connectivity of the secondary flow network that emerges during slow drainage. The network consists of trapped defending fluid clusters in connection to capillary bridges. An increased understanding of these pathways and mechanism by which they are disconnected is important for the understanding and ultimately controlling entrapment in porous media. We again conduct experiments in a two-dimensional synthetic porous medium.
By optical monitoring of the invasion process we are able to identify capillary bridges and map their connectivity to trapped clusters of defending fluid. Increasing the gravitational component acting on the system sets up stabilizing forces which in effect changes how much and the sequence by which the pore-space is explored by the invading fluid. To answer the question of possible directional growth of these networks with respect to gravity we utilized a
bonding box which is the smallest rectangle that encloses the subnetwork. By studying the distribution of its extension in the lx direction transverse to the flow direction and lz parallel to the flow direction, we conclude that the effect of gravity on the structure of the network is not present.
Next we utilized the framework of network theory to transform the set of connected capillary bridges and clusters into a graph object. Each edge corresponds to a capillary bridge or the link to its neighboring island of defending fluid. We characterize the network by calculating measures typically associated to networks such as the clustering coefficient and efficiency.
During the invasion process liquid bridges were observed to sometimes rupture, also referred to as snap-off. The snap-off was measured to statistically occur more regularly for the longest capillary bridges, and most of the rupturing events took place close to the mean interface position. Furthermore the front width and rupture activity area behave in a coupled manner in space.
Many systems respond to a slowly changing external condition not in a smooth way but in terms of a series of discrete events or bursts spanning a broad range of sizes. Such avalanches are observed for many different systems, among which for drainage in the capillary regime.
Here the fluid pressure difference between is solely responsible for the displacement, such systems have displayed pressure evolution in terms of avalanches, also referred to in the literature as Haines jumps. The pressure signal is characterized by slow buildups followed by sudden drops in pressure.
Influence of gravity in such systems have shown to exhibit long range correlations along the interface, reducing the spread between the two phases, in effect spatially limiting the front of the invading phase. More-over, varying the effective gravity changes how much and how the pore-space is displaced. As its the ability of the interface to facilitate for the displaced volume during displacement which drives the avalanches. This is of importance for the dynamics as the number of pores to redistribute is decreased with an increase in the effective gravity.
During experiments the pressure in the phase is measured together with optical monitoring.
We show by correlating the two measured quantities that the intermittent evolution of the pressure signal can be related to the invasion of multiple pores in a burst like fashion.
Furthermore we investigate the influence of gravity on its statistics, and we found a linear relationship between the size of the bursts and invasion of multiple pores, and we are able to extract the average relaxation time of the pressure decay preceding a burst.
The study of multiphase flows in a cylindrical confinement is an important topic with a broad range of interest from geophysical, biological and engineering perspective, from transport of blood cells through arteries and veins to flow of oil and gas by pipelines. Even the simple case of two-phase air and water flow through tubes and capillaries displays complex flow behavior, with commonly observed transitions between stratified, bubbly, slug, and annular
flow depending on the flow rates of the respective phases. Three-phase grain-liquid-gas flows have shown to generate complex flows. For studies where the characteristic length scale of the system is at the capillary length, meaning there exist an active meniscus between the confining plates. Such systems have shown to generate patterns caused by the interplay between capillary, viscous and the solid frictional forces, the latter existing through graingrain interactions setting up a force network witch branches onto the confining geometry. For
a quasi 2D confinement it has been showed that when such system are driven with an external flow rate out of equilibrium they generate a variety of patterns.
In this study we a further investigate a simplified confining geometry: Namely a narrow tube with a well defined capillary meniscus. We look at the structures that arise from the displacement of a dense suspension consisting of a granular bed of glass beads immersed in water. As the meniscus travels through the tube it either evacuates the grains from the whole cross-section of the tube or it seen to leave behind a structure of granular material
characterized by its granular plugs followed by gaps.
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Bonjour,
nous avons le plaisir de vous inviter à la soutenance dethèse de M. Atta-Ul Monem Ayaz,
en anglais, intitulée "Etude expérimentale et numérique de la morphologie et dynamique d'amas durant des écoulements biphasiques en milieux poreux".
Elle aura lieu le lundi 16 décembre 2019, 14:00, en Amphithéâtre Rothé, IPGS/EOST, 5 rue Descartes, Strasbourg. La thèse est une cotutelle de l'Université de Strabourg et l'Université d'Oslo.
Le jury sera composé de:
Pr. Insa Neuweiler, Leibniz Universität Hannover, Faculty of Civil Engineering and Geodetic Science - rapporteuse
Dr. Stéphane Santucci, CR CNRS - HDR, Laboratoire de Physique de l'ENS de Lyon UMR5672 CNRS/Université Claude Bernard Lyon I / ENS de Lyon - rapporteur
Dr. Philippe Ackerer, DR CNRS, LHYGES, UMR7517, Université de Strasbourg/CNRS - examinateur
Dr. Renaud Toussaint, DR CNRS, IPGS, UMR7516, Université de Strasbourg/CNRS - directeur de thèse
Pr. Knut Jørgen Måløy, SFF PoreLab, The Njord Center, Physics Dept, University of Oslo - directeur de thèse
Pr. Gerhard Schäfer, LHYGES, LHYGES, UMR7517, directeur de thèse, Université de Strasbourg/CNRS - directeur de thèse
Merci, Vielen Dank og Tusen Takk !