3D FINITE ELEMENT MODEL FOR THERMO-POROMECHANICAL DEFORMATION IN SEDIMENTARY BASINS

Sedimentary basins form when an appreciable amount of sediments are deposited along geological time and transformed into rock through natural phenomena known as diagenesis. Compaction of sediments, fluid and thermal flows are fundamental coupled processes in sedimentary basin modelling. Purely mechanical phenomena prevail in the upper layers involving pore fluid expulsion and rearrangement of solid particles, while chemomechanical compaction resulting from Intergranular Pressure-Solution (IPS) dominates for deeper burial as stress and temperature increase. The thermal evolution of the basin may substantially affect both processes as heat modifies fluid viscosity and physicochemical properties of minerals, thus affecting fluid flow and mineral stability. The aim of the present contribution is to provide a comprehensive 3D framework for constitutive and numerical modelling of thermo-poro-mechanical deformation during diagenesis. Purely mechanical and chemo-mechanical deformations are respectively modelled by means of poroplastic and poroviscoplastic models. The numerical simulations are performed through the finite element method with a shared memory multiprocessing interface. The sedimentary basin is modelled as a fully saturated thermo-poro-elasto-visco-plastic material undergoing large strains. Special attention is given to temperature effects on the deformation history of the basin

Poroelasticity and Poroplasticity At Large Strains

This paper reviews some aspects of the formulation of the constitutive behavior of a saturated porous material in isothermal evolutions in the domain of large strains. First, the results concerning the nonporous solid are recalled. Then, the poroelastic and poro-elastoplastic behaviors at large strains are successively considered. When the solid phase is elastic at large strains, a micro-macro approach shows that the free energy of the skeleton is a macroscopic thermodynamic potential. The latter depends on the macroscopic strain of the skeleton and on the lagrangian porosity, which can be interpreted as macroscopic state variables. When the solid is elastoplastic at large strains, a theory of finite poro-elastoplasticity is proposed within a macroscopic thermodynamic framework. The homogenization techniques allow to clarify some aspects of the formulation of the macroscopic behavior. In particular, the validity of the effective stress principle in finite poroelasticity and poroplasticity is established when the solid phase is incompressible. Even if the macroscopic strain applied to an elementary volume is infinitesimal, the strain field at the microscopic scale may not be infinitesimal. Hence, the simulation of the macroscopic behavior of this elementary volume in the framework of a micro-macro approach must take into account possible large strains at the microscopic scale

Poroélasticité et poro-élastoplasticité en grandes déformations

International audienceThis paper reviews some aspects of the formulation of the constitutive behavior of a saturated porous material in isothermal evolutions in the domain of large strains. First, the results concerning the nonporous solid are recalled. Then, the poroelastic and poro-elastoplastic behaviors at large strains are successively considered. When the solid phase is elastic at large strains, a micro-macro approach shows that the free energy of the skeleton is a macroscopic thermodynamic potential. The latter depends on the macroscopic strain of the skeleton and on the lagrangian porosity, which can be interpreted as macroscopic state variables. When the solid is elastoplastic at large strains, a theory of finite poro-elastoplasticity is proposed within a macroscopic thermodynamic framework. The homogenization techniques allow to clarify some aspects of the formulation of the macroscopic behavior. In particular, the validity of the effective stress principle in finite poroelasticity and poroplasticity is established when the solid phase is incompressible. Even if the macroscopic strain applied to an elementary volume is infinitesimal, the strain field at the microscopic scale may not be infinitesimal. Hence, the simulation of the macroscopic behavior of this elementary volume in the framework of a micro-macro approach must take into account possible large strains at the microscopic scale.Cet article examine quelques aspects de la formulation du comportement d'un milieu poreux saturé en évolution isotherme dans le domaine des transformations finies. Après avoir rappelé les résultats propres aux solides non poreux, on s'intéressera successivement aux comportements poroélastique et poro-élastoplastique. Si la phase solide qui constitue le squelette présente un comportement élastique dans le domaine des grandes déformations, un passage micro-macro démontrera que l'énergie libre du squelette constitue un potentiel thermodynamique pour le comportement macroscopique. Les arguments de ce potentiel sont la déformation macroscopique du squelette ainsi que la porosité lagrangienne. Ces deux quantités s'interprètent commes des variables d'état macroscopiques. Dans le cas où le comportement du solide est élastoplastique en grandes déformations, on propose un cadre thermodynamique permettant de formuler une théorie de la poroplasticité finie. Les techniques de changement d'échelle permettent de clarifier certains aspects de la formulation du comportement macroscopique. En particulier, la validité du concept de contrainte effective en poroélasticité et en poroplasticité finies est établie lorsque la phase solide est incompressible. Même lorsque la déformation macroscopique imposée à un élément de volume de milieu poreux est infinitésimale, le champ de déformation à l'échelle microscopique peut être non infinitésimal. De ce fait, la simulation du comportement macroscopique de ce volume élémentaire dans le cadre d'un passage micro-macro doit être effectuée en tenant compte de transformations finies à l'échelle microscopique. Ce point est illustré par l'exemple du chargement Sdométrique

Comportement élastique non linéaire macroscopique d'un matériau comportant un réseau de joints

S'appuyant sur la définition du comportement élastique en transformation finie d'un milieu multicouche à partir de son potentiel macroscopique, on aboutit par passage à la limite du modèle multicouche, à la formulation en transformation infinitésimale macroscopique d'une loi de comportement élastique non linéaire. Cette non-linéarité provient des grandes déformations que subit localement le matériau constitutif des joints dont on fait simultanément tendre l'épaisseur vers zéro. Ce modèle est appliqué aux massifs rocheux fracturés dont on fait clairement apparaître l'anisotropie élastique induite par la direction préférentielle des joints