Barometric pumping of a fractured porous medium

International audienceBarometric pumping plays a crucial role in the release of trace gases from fractured porous media to the atmosphere, and it requires a rigorous and complete modeling in order to go beyond the approximate schemes available in the literature. Therefore, a coupled set of convection and convection-diffusion equations for a slightly compressible fluid in unsteady conditions should be solved. The numerical methodology is presented, and it is applied to conditions close to the ones of the Roselend Natural Laboratory (France). The precision of the code is assessed and the mechanism of barometric pumping is explained. The usual schematization by simple vertical fractures is shown to be only qualitative. Finally, barometric pumping is shown to be efficient in a narrow range of parameter values; its efficiency is a decreasing function of the matrix porosity and of the fracture density

Foam patterning in porous media

We consider a model of patterning of one-dimensional foam-bubble chain confined in a bamboolike capillary. The discrete model of such a foam describes a distribution of foam films - lamellae that, like "bridges," span a capillary. This model is a kind of Ulam map, which admits many metastable distributions of lamellae in a bamboolike capillary as governing parameters (external pressure drop, lattice parameter, lamella tension, and gas compressibility) overcome certain barriers. In particular, some random distributions of bubble sizes over the chain are suited to solutions of the proposed discrete deterministic model. Randomization of lamella positions speaks in favor of the possibility of the glasslike patterning of foam in a bamboolike capillary. For such "chaotic" foam structures, the admissible pressure drop that the bubble chain can sustain, i.e., the so-called start-up, yield pressure drop, rises. We show that the start-up pressure drop depends upon the length of the chain nonlinearly. Only for short chains does it linearly depend upon the number of bubbles in the chain. For infinitely long chains, a saturation effect is observed; i.e., the critical pressure drop becomes independent of the chain length

Foam patterning in porous media

We consider a model of patterning of one-dimensional foam-bubble chain confined in a bamboolike capillary. The discrete model of such a foam describes a distribution of foam films - lamellae that, like "bridges," span a capillary. This model is a kind of Ulam map, which admits many metastable distributions of lamellae in a bamboolike capillary as governing parameters (external pressure drop, lattice parameter, lamella tension, and gas compressibility) overcome certain barriers. In particular, some random distributions of bubble sizes over the chain are suited to solutions of the proposed discrete deterministic model. Randomization of lamella positions speaks in favor of the possibility of the glasslike patterning of foam in a bamboolike capillary. For such "chaotic" foam structures, the admissible pressure drop that the bubble chain can sustain, i.e., the so-called start-up, yield pressure drop, rises. We show that the start-up pressure drop depends upon the length of the chain nonlinearly. Only for short chains does it linearly depend upon the number of bubbles in the chain. For infinitely long chains, a saturation effect is observed; i.e., the critical pressure drop becomes independent of the chain length

Le Sport : organe officiel de l'Aviron bayonnais

06 novembre 19101910/11/06 (N57)-1910/11/06.Appartient à l’ensemble documentaire : Aquit

Dynamic simulation of the formation of granular media and study of their properties

Colloque avec actes et comité de lecture. Internationale.International audienceA numerical model is presented which describes the evolution of a system containing a large number of deformable spherical grains based on Newton's second law. Starting from an initial state with fixed positions, velocities and grain characteristics, the system evolution is simulated by successive steps. The acceleration of each grain results from the application of an external force and from interactions with other particles. These contact forces are evaluated as functions of the grain deformations during the collisions considered as elastic. The grain bed can be deposited between vertical walls as well as with periodical conditions in the lateral directions. The properties of these packings submitted to mechanical stresses are characterized by using numerical codes which operate on unstructured tetrahedral grids on the scale of the individual grains

Macroscopic permeability of three-dimensional fracture networks with power-law size distribution

International audienceFracture network permeability is investigated numerically by using a three-dimensional model of plane polygons uniformly distributed in space with sizes following a power-law distribution. Each network is triangulated via an advancing front technique, and the flow equations are solved in order to obtain detailed pressure and velocity fields. The macroscopic permeability is determined on a scale which significantly exceeds the size of the largest fractures. The influence of the parameters of the fracture size distribution—the power-law exponent and the minimal fracture radius—on the macroscopic permeability is analyzed. Eventually, a general expression is proposed, which is the product of a dimensional measure of the network density, weighted by the individual fracture conductivities, and of a fairly universal function of a dimensionless network density, which accounts for the influences of the fracture shapes and of the parameters of their size distribution. Two analytical formulas are proposed which successfully fit the numerical data over a wide range of network densities

Modélisation à l'échelle microscopique de transports avec réaction en milieu poreux (combustion en lit fixe)

La combustion en milieu poreux est traitée par le biais de simulations numériques directes et détaillées, à la microéchelle, dans une extension du travail de Debenest (2003, 2005) qui porte principalement sur un enrichissement du modèle chimique. On considère plus particulièrement la combustion en lit fixe de particules solides, avec comme première application le brûlage de schistes bitumineux. Les processus de transport (convection, diffusion, conduction) et les réactions chimiques sont explicitement décrits à l'échelle des pores, ce qui permet d'exhiber leurs couplages et de révéler les phénomènes locaux qui déterminent les comportements globaux. Les simulations sont conduites principalement dans deux configurations bidimensionnelles, milieu stratifié ou réseau de cylindres, en examinant les effets des réactions pyrolytiques (cracking du kérogène et calcination des carbonates), et avec un schéma d'oxydoréduction qui fait intervenir jusqu'à quatre réactions. Une typologie phénoménologique est établie, incluant notamment l'existence de deux régimes principaux, avec ou sans flamme dans les pores. Des plages de fonctionnement sont identifiées, suivant les paramètres opératoires. On peut en rationaliser les tendances à l'aide de considérations théoriques, et montrer qu'une description macroscopique peut nécessiter des formulations différentes, selon les situations.Combustion in porous media is addressed by means of direct, detailed numerical simulations, on the microscale, in an extension of the work of Debenest (2003) where the main improvements are related to the chemical model. More specifically, fixed bed combustion of solid particles is considered, with application to the burning of oil shales. The transport processes (convection, diffusion, conduction) and the chemical reactions are explicitely described on the pore scale, which allows to account for their local couplings and to identify the small-scale phenomena which control the global behaviors. The simulations are conducted mainly in two two-dimensional configurations, a stratified medium and a network of cylinders. The effect of pyrolytic reactions (kerogen cracking and calcination of the carbonates) are examined, as well as the oxydative processes, with a chemical scheme involving four species and up to four reactions. A phenomenological typology is established, which features for instance two main regimes, with or without a flamme in the pore space. Ranges of functioning modes are identified, according to the operating parameters. Their trends can be rationalized by theoretical considerations, and it is shown that different situations may require different formulations in a macroscopic descriptionPOITIERS-BU Sciences (861942102) / SudocSudocFranceF