22 research outputs found
Hydrothermal coupling in a rough fracture
Heat exchange during laminar flow is studied at the fracture scale on the
basis of the Stokes equation. We used a synthetic aperture model (a self-affine
model) that has been shown to be a realistic geometrical description of the
fracture morphology. We developed a numerical modelling using a finite
difference scheme of the hydrodynamic flow and its coupling with an
advection/conduction description of the fluid heat. As a first step,
temperature within the surrounding rock is supposed to be constant. Influence
of the fracture roughness on the heat flux through the wall, is estimated and a
thermalization length is shown to emerge. Implications for the
Soultz-sous-For\^{e}ts geothermal project are discussed
Influence of asperities on fluid and thermal flow in a fracture: a coupled Lattice Boltzmann study
The characteristics of the hydro-thermal flow which occurs when a cold fluid
is injected into a hot fractured bedrock depend on the morphology of the
fracture. We consider a sharp triangular asperity, invariant in one direction,
perturbing an otherwise flat fracture. We investigate its influence on the
macroscopic hydraulic transmissivity and heat transfer efficiency, at fixed low
Reynolds number. In this study, numerical simulations are done with a coupled
lattice Boltzmann method that solves both the complete Navier-Stokes and
advection-diffusion equations in three dimensions. The results are compared
with those obtained under lubrication approximations which rely on many
hypotheses and neglect the three-dimensional (3D) effects. The lubrication
results are obtained by analytically solving the Stokes equation and a
two-dimensional (integrated over the thickness) advection-diffusion equation.
We use a lattice Boltzmann method with a double distribution (for mass and
energy transport) on hypercubic and cubic lattices. Beyond some critical slope
for the boundaries, the velocity profile is observed to be far from a quadratic
profile in the vicinity of the sharp asperity: the fluid within the triangular
asperity is quasi-static. We find that taking account of both the 3D effects
and the cooling of the rock, are important for the thermal exchange. Neglecting
these effects with lubrication approximations results in overestimating the
heat exchange efficiency. The evolution of the temperature over time, towards
steady state, also shows complex behavior: some sites alternately reheat and
cool down several times, making it difficult to forecast the extracted heat.Comment: In Journal of Geophysical Research B (2013) online firs
Hydrothermal coupling in a self-affine rough fracture
International audienceThe influence of the multi-scale fracture roughness on the heat exchange when a cold fluid enters a fractured hot solid is studied numerically on the basis of the Stokes equation and in the limit of both hydro- and thermo- lubrications. The geometrical complexity of the fracture aperture is modeled by small self-affine perturbations added to a uniform aperture field. Thermal and hydraulic properties are characterized via the definition of hydraulic and thermal apertures both at micro and macroscopic scales and obtained by comparing the fluxes to the ones of flat fractures. Statistics over a large number of fracture configurations provide an estimate of the average behavior and its variability. We show that the long range correlations of the fracture roughness induces strong channelling effects that significantly influence the hydraulic and thermal properties. An important parameter is the aspect ratio (length over width) of the fracture: we show for example that a downstream elongated rough fracture is more likely to inhibit the hydraulic flow and subsequently to enhance the thermal exchange. Fracture roughness might, in the opposite configuration, favor strong channeling which inhibits heating of the fluid. The thermal behavior is in general shown to be mainly dependent on the hydraulic one, which is expressed through a simple law
Hydraulic transmissivity and heat exchange efficiency of open fractures: a model based on lowpass filtered apertures
Natural open joints in rocks commonly present multi-scale self-affine
apertures. This geometrical complexity affects fluid transport and heat
exchange between the flow- ing fluid and the surrounding rock. In particular,
long range correlations of self-affine apertures induce strong channeling of
the flow which influences both mass and heat advection. A key question is to
find a geometrical model of the complex aperture that describes at best the
macroscopic properties (hydraulic conductivity, heat exchange) with the
smallest number of parameters. Solving numerically the Stokes and heat equa-
tions with a lubrication approximation, we show that a low pass filtering of
the aperture geometry provides efficient estimates of the effective hydraulic
and thermal properties (apertures). A detailed study of the influence of the
bandwidth of the lowpass filtering on these transport properties is also
performed. For instance, keeping the information of amplitude only of the
largest Fourier length scales allows us to reach already an accuracy of 9% on
the hydraulic and the thermal apertures.Comment: This is a draft version of an article published in Geophys. J. Int.;
10 pages, 15 figure
Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.
RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)
Hydro-thermal behavior of a fluid flowing in a rough fracture,modelling and application to fractured bedrocks
Dans de nombreux réservoirs géothermiques profonds, l'échangeur thermique est un massif de roches chaudes fracturées. Comment la morphologie des fractures influence-t-elle le flux hydraulique et le champ de température lorsque de l'eau froide est injectéeFor many deep geothermal systems, the heat exchanger consists in a hot fractured bedrock. How does the morphology of the fractures influence the hydraulic flow and the temperature field when cold water is injected into this medium? This question is numer
Comportement hydro-thermique d'un écoulement de fluide dans une fracture rugueuse: Modélisation et application à des massifs fracturés
For many deep geothermal systems, the heat exchanger consists in a hot fractured bedrock. How does the morphology of the fractures influence the hydraulic flow and the temperature field when cold water is injected into this medium? This question is numerically addressed at the fracture scale, using two methods: finite differences, in 2D, and lattice Boltzmann methods, in 3D. The finite difference computations show that a self-affine aperture induces space variability of the hydraulic flow and temperature field. It can create a channeling of the fluid, which reduces the efficiency of the heat transfer between the rock and fluid. These hydrothermal characteristics are mostly generated by the largest wavelengths of these self-affine apertures. A first application at large scales, dimensioned according to the characteristics of the geothermal site of Soultz-sous-Forêts (Alsace, France) is proposed. A second application aims at evaluating the permeability of the basement of Draix (French Alps). To do so, the geometry of fractures is precisely characterized using geological cores. The topography of the fractures, measured with a laser, is used to reconstruct the morphology of fracture apertures. These data are used to estimate the permeability, here of the order of 10^- 9-10^- 8 m^2, which is a relatively high value. Lattice Boltzmann methods are used to model the hydrothermal behavior in fractures whose topography is very steep. The hydrothermal behavior near a simple-shaped asperity is very different (recirculation) from the one in the remaining parts of the fracture. All these modelings suggest that it is necessary to take into account the morphology of the fractures to estimate the hydrothermal behavior for geothermal purposes.Dans de nombreux réservoirs géothermiques profonds, l'échangeur thermique est un massif de roches chaudes fracturées. Comment la morphologie des fractures influence-t-elle le flux hydraulique et le champ de température lorsque de l'eau froide est injectée dans ce milieu ? Cette question est abordée à l'échelle de la fracture, de manière numérique, à l'aide de deux méthodes : par différences finies, en 2D, et par méthode de Boltzmann sur réseau, en 3D. Les calculs en différences finies montrent qu'une ouverture auto-affine induit une variabilité spatiale du flux hydraulique et du champ de température. Dans certains cas, il se crée une chenalisation du fluide, qui réduit l'efficacité du transfert thermique entre la roche et le fluide. Ces caractéristiques hydro-thermiques sont essentiellement engendrées par les plus grandes longueurs d'ondes de ces ouvertures auto-affines. Une première application à grande échelle, dimensionnée suivant les caractéristiques du site géothermique de Soultz-sous-Forêts (Alsace, France), est proposée. Une deuxième application consiste à évaluer la perméabilité du sous-sol de Draix (Alpes, France). Pour cela, la géométrie de fractures présentes dans le sous-sol est caractérisée précisément à partir de carottes géologiques. La topographie des fractures, mesurée avec un laser, est utilisée pour reconstituer la morphologie de l'ouverture des fractures. Ces données sont utilisées pour quantifier la perméabilité, ici élevée -- de l'ordre de 10^-9-10^-8 m^2. À l'aide de méthodes de Boltzmann sur réseau, la modélisation du comportement hydro-thermique dans des fractures présentant une topographie avec de fortes pentes est abordée. Au voisinage d'une aspérité de forme simple, le comportement hydro-thermique modélisé est très différent (recirculation) de celui dans le reste de la fracture. L'ensemble de ces modélisations suggère qu'il est nécessaire de prendre en compte la morphologie des fractures pour estimer le comportement hydro-thermique en géothermie
Comportement hydro-thermique d'un écoulement de fluide dans une fracture rugueuse (Modélisation et application à des massifs fracturés)
Dans de nombreux réservoirs géothermiques profonds, l'échangeur thermique est un massif de roches chaudes fracturées. Comment la morphologie des fractures influence-t-elle le flux hydraulique et le champ de température lorsque de l'eau froide est injectée dans ce milieu ? Cette question est abordée à l'échelle de la fracture, de manière numérique, à l'aide de deux méthodes : par différences finies, en 2D, et par méthode de Boltzmann sur réseau, en 3D. Les calculs en différences finies montrent qu'une ouverture auto-affine induit une variabilité spatiale du flux hydraulique et du champ de température (chenalisation). Souvent, ceci réduit l'efficacité du transfert thermique entre la roche et le fluide. Le comportement hydro-thermique est essentiellement contrôlé par les plus grandes longueurs d'ondes des ouvertures. Deux application à grande échelle sont proposées: à Soultz-sous-Forêts (France) et à Draix (France). À Draix, la géométrie de fractures du sous-sol est caractérisée précisément à partir de carottes géologiques, en utilisant un laser. Ces données sont utilisées pour quantifier la perméabilité. À l'aide de méthode de Boltzmann sur réseau, la modélisation du comportement hydro-thermique dans des fractures présentant une topographie avec de fortes pentes est abordée. Au voisinage d'une aspérité de forme simple, le comportement hydro-thermique modélisé est très différent (recirculation) de celui dans le reste de la fracture. L'ensemble de ces modélisations suggère qu'il est nécessaire de prendre en compte la morphologie des fractures pour estimer le comportement hydro-thermique en géothermie.For many deep geothermal systems, the heat exchanger consists in a hot fractured bedrock. How does the morphology of the fractures influence the hydraulic flow and the temperature field when cold water is injected into this medium? This question is numerically addressed at the fracture scale, using two methods: finite differences, in 2D, and lattice Boltzmann methods, in 3D. The finite difference computations show that a self-affine aperture induce space variability of the hydraulic flow and temperature field. It can create a channeling of the fluid, which reduces the efficiency of the heat transfer between the rock and fluid. These hydrothermal characteristics are mostly generated by the largest wavelengths of these self-affine apertures. A first application at large scales, dimensioned according to the characteristics of the geothermal site of Soultz-sous-Forêts (France) is proposed. A second application aims at evaluating the permeability of the basement of Draix (French Alps). To do so, the geometry of fractures is precisely characterized using geological cores. Using laser measurements, the morphology of fracture apertures is reconstructed. These data are used to estimate the permeability, shown to be relatively high. Lattice Boltzmann methods are used to model the hydrothermal behavior in fractures whose topography is very steep. The hydrothermal behavior near a simple-shaped asperity is very different (recirculation) from the one in the remaining parts of the fracture. All these modelings suggest that it is necessary to take into account the morphology of the fractures to estimate the hydrothermal behavior for geothermal purposes.STRASBOURG-EOST (674822249) / SudocSudocFranceF
Rugosité de fracture et échange thermique : étude de cas à Soultz-sous-Forêts
International audienceHeat exchange during laminar flow in an open fracture is studied numerically on the basis of the Stokes equation in the limit of hydro-thermal lubrication. We examine the influence of fracture roughness on hydraulic permeability and heat flux through the fracture sides when a cold fluid is injected into a homogeneous hot host rock. Spatial temperature fluctuations inside the fluid are studied assuming the temperature of the rock to be constant and the fracture aperture to be self-affine. An application to the case study at the deep geothermal reservoir of Soultz-sous-Forêts, France, is presented. Finally, a thermal model based on sparse spatial information of the geometrical aperture is successfully proposed to reproduce the response of the fracture.L'échange de chaleur en régime laminaire est étudié numériquement dans une fracture ouverte sur la base de l'équation de Stokes, dans la limite de l'hypothèse de lubrification hydro-thermique. Nous observons l'influence de la rugosité sur la perméabilité hydraulique ainsi que sur le flux de chaleur à travers les parois de la fracture quand un fluide froid est injecté dans une roche mère ayant une température chaude homogène. Les fluctuations de la température du fluide sont étudiées en supposant que la température de la roche est constante et la fracture auto-affine. Une application au cas d'étude du réservoir de géothermie profonde à Soultz-sous-Forêts, France, est présentée. Finalement, nous proposons un modèle thermique basé sur la connaissance spatiale réduite de l'ouverture géométrique, qui reproduit bien la réponse de la fracture