Poromechanics of Microporous Carbons: Application to Coal Swelling during Carbon Storage

International audienceCoal seams are naturally filled with natural gas. Enhanced Coal Bed Methane recovery (ECBM) is a technique which consists in injecting carbon dioxide (CO2) in coal seams in order to enhance the recovery of the methane (CH4) present in the coal seams. A major issue for the industrial development of this technique is the loss of permeability of the reservoirs during injection. In a coal bed, most of the transport of fluids occurs in a network of natural frac- tures. The loss of permeability is attributed to the closure of the fractures induced by the swelling of the coal ma- trix during the progressive replacement of CH4 by CO2. Since both fluids are mostly adsorbed in the microporous matrix of coal, this particular problem raises the funda- mental question of how adsorption impacts the mechanics of a microporous solid. In this work, we present a porome- chanical modeling valid for microporous solids under ad- sorption and we apply this modeling to the specific case of ECBM. The first section presents the theoretical derivation of general constitutive equations of poromechanics which are valid for generic pore sizes and morphologies. In the second section, we apply this general poromechanics to the specific case of CH4 adsorption in coal. We use molecu- lar simulations to calibrate the derived constitutive laws. In the third section we validate this calibration by analyz- ing results of adsorption experiments in unjacketed condi- tions. The fourth section is dedicated to the case of CO2 adsorption in coal. Finally in the last section, we use this modeling to predict the swelling of coal in the context of ECBM

Are Deicing Salts Necessary to Promote Scaling in Concrete?

International audienceThe main purpose of the present study is to investigate the role of the material parameters such as permeability, thermal diffusivity, and pore size distribution on the mechanical behavior of cementitious structures submitted to frost action, such as surface scaling. An experimental device, in which a cement paste specimen is exposed to freezing-thawing cycles under a thermal gradient, has been developed. The experimental results show that under high thermal gradient (up to 1.5°C/mm), skin damage can occur without a saline layer in contact with the frozen surface. This can be explained and quantified in the framework of poromechanics. The model is based on the coupling between liquid-ice crystal thermodynamic equilibrium, liquid water transport, thermal conduction, and elastic properties of the different phases that form the saturated porous material. It eventually predicts that a less permeable sample is more susceptible to damage by surface defacement, which explains the observed experimental result

Investigation of water to ice phase change in porous media by ultrasonic and dielectric measurements

International audienceThe main objective of this paper is to study the evolution of the ice content of porous media submitted to sub-zero temperatures by dielectric and ultrasonic measurements. Dielectric measurements are made by a capacitive sensor-based apparatus. The amount of ice formed within the tested sample is estimated from the global dielectric constants of the sample and of all the phases that form the tested composite material. On the other hand, ultrasonic measurements are based on the evolution of the ultrasonic wave velocity through the tested sample during a freezing-thawing cycle. These two methods lead to very close results and appear to be cheaper alternatives to low temperature calorimetry. The ice content curves are analyzed with the help of thermoporometry concepts in order to characterize the pore size distribution. Results appear to be complementary to mercury intrusion porosimetry ones. Moreover the commonly observed hysteresis of the ice content during a freezing-thawing cycle is investigated with respect to material microstructure

Comportement au gel-dégel d'une structure en ciment

Ce travail vise à l'étude du comportement au gel-dégel des matériaux cimentaires. Le model poromécanique développé dans cette optique permet d'étudier l'influence sur la mécanique du couplage entre le changement de phase eau/glace, les transports en milieu poreux, la conduction thermique et le comportement de chacune des phases en présence. Nous pouvons ainsi prédire une pression de pore maximale au voisinage de la surface soumise au gel-dégel pouvant induire un endommagement localisé superficiel. Par la suite, l'influence relative des différents phénomènes physiques responsables de l'endommagement au gel-dégel est étudiée, permettant d'identifier les pressions hydrauliques de Powers comme prépondérantes

Accelerating Effect of Heating Treatment on the Damage of Cement Paste under Sulfate Attack

The ettringite (AFt) formation and damage process are investigated for cement paste subjected to external sulfate attack with and without heating treatment during curing. Slice and disc specimens were exposed to 10 g/L Na2SO4 + pH of 13. The evolutions of AFt formation, pore structure, and expansion on slice specimens were characterized through 27Al NMR, MIP, and micrometer. The surface cracks on disc specimens were observed through an optical microscope. The experimental results show that: (1) heating treatment dissolves part of AFt and accelerates the ettringite formation and material expansion; and (2) heating treatment initiates the microcracks, thus promoting the material cracking

Durabilité sous percolation et/ou cristallisation confinée en milieu poreux

In civil engineering the use of geomaterials can raise durability problems for which some thermodynamic, hydraulic, physical, chemical, and mechanical phenomena are highly coupled. Here the following issues are tackled: (i) hydraulic permeability of cementitious materials; (ii) electro-osmotic permeability of clays; (iii) mechanical behaviour of concrete under freezing and thawing.On aborde quelques problèmes de durabilité posés par l'utilisation de géomatériaux en génie civil dans lesquels interviennent de forts couplages thermo-hydro-physico-chimio-mécaniques : (i) perméabilité à l'eau liquide des matériaux cimentaires ; (ii) perméabilité électro-osmotique dans les argiles ; (iii) comportement mécanique des bétons au gel/dégel

Analyse micromécanique des variations dimensionnelles de matériaux alvéolaires — Application au polystyrène expansé

This work aims at improving the manufacturing process of expanded polystyrene (EPS) slabs by Lafarge-Plâtre. It consists in better controlling their after-shrinkage which is the dimensional shrinkage that occurs with time from the temperature stabilisation at 20°C and that is responsible for the warping, and possibly, the breaking of EPS/plaster sandwiches. To do so, this work has investigated the physico-mechanical causes of such a phenomenon by means of multi-scales mechanics and experiments: it is shown how the EPS after-shrinkage can result from peculiar macromolecular movements called β secondary transitions.Ce travail vise à aider à l'optimisation du procédé d'élaboration de plaques de polystyrène expansé (PSE) produites par Lafarge-Plâtre en vue d'une meilleure maîtrise de leur post-retrait, défini comme le retrait dimensionnel différé dans le temps à partir de la stabilisation de la température à 20 °C, source du gauchissement, voire de la rupture, de complexes de doublage PSE/plâtre.À cette fin, mon travail a essentiellement consisté à comprendre l'origine physico-mécanique d'un tel phénomène, en la validant expérimentalement. C'est ainsi que, grâce à la collaboration de Jean-Louis Halary et d'André Dubault du Laboratoire de Physico-Chimie Structurale et Macromoléculaire de l'ESPCI, j'ai pu expliquer comment le post-retrait du PSE peut provenir d'une mobilité moléculaire de transition secondaire, dite β, en combinant : — la mise en évidence, par un essai de flexion 3 points à 20 °C, d'une flèche viscoélastique du polystyrène constitutif du PSE d'amplitude équivalente à celle du post-retrait (entre 0,3% et 1% sur 6 semaines), — la détermination de la nature physique des mouvements de transition impliqués dans la mobilité moléculaire du polystyrène plastifié par du pentane (agent porogène dont la vaporisation conduit à la microstructure alvéolaire du PSE) par la mise au point d'un dispositif de suivi d'expansion de billes servant à fabriquer le PSE, — une méthode de changement d'échelles par champs moyens dans un cadre thermoviscoélastique linéaire, isotrope, non-vieillissant, et anisotherme pour prendre en compte la nature du milieu poreux à porosité fermée dont la phase connexe solide est constituée par du polystyrène amorphe sur laquelle s'exerce une pression représentant l'action des pores au cours de toute l'histoire thermique suivant la fin du moulage du PSE

Durabilité sous percolation et/ou cristallisation confinée en milieu poreux

In civil engineering the use of geomaterials can raise durability problems for which some thermodynamic, hydraulic, physical, chemical, and mechanical phenomena are highly coupled. Here the following issues are tackled: (i) hydraulic permeability of cementitious materials; (ii) electro-osmotic permeability of clays; (iii) mechanical behaviour of concrete under freezing and thawing.On aborde quelques problèmes de durabilité posés par l'utilisation de géomatériaux en génie civil dans lesquels interviennent de forts couplages thermo-hydro-physico-chimio-mécaniques : (i) perméabilité à l'eau liquide des matériaux cimentaires ; (ii) perméabilité électro-osmotique dans les argiles ; (iii) comportement mécanique des bétons au gel/dégel