The mechanics of active clays circulated by salts, acids and bases: Comprehensive version

An elastic-plastic model that accounts for electro-chemo-mechanical couplings in clays, due to the presence of dissolved salts and acids and bases, is developed here for the first time. To the authors' best knowledge, no other comprehensive project to embody the effects of pH in the elastic-plastic behavior of geomaterials has been attempted so far. Chemically sensitive clays are viewed as two-phase multi-species saturated porous media circulated by an electrolyte. The developments are embedded in the framework of the thermodynamics of multi-phase multi-species porous media. This approach serves to structure the model, and to motivate constitutive equations. The present extension capitalize upon the earlier developments by Gajo et al. [2002] and Gajo and Loret [2004], which were devoted to modeling chemo-mechanical couplings at constant pH. Four transfer mechanisms between the solid and fluid phases are delineated in the model: (1) hydration, (2) ion exchange, (3) acidification, (4) alkalinization. Thus all fundamental exchanges at particle level are fully taken into account. Only mineral dissolution is neglected, since experimental observations indicates a negligible role of mineral dissolution for active clays at room temperature. In particular, the newly considered mechanisms of acidification and alkalinization directly affect the electrical charge of clay particles and thus have a key role in the electro-chemo-mechanical couplings. These four mechanisms are seen as controlling both elastic and elasto-plastic behaviors. Depending on concentrations and ionic affinities to the clay mineral, these mechanisms either compete or cooperate to modify the compressibility and strength of the clay and may induce swelling (volume expansion) or shrinking (volume contraction). The framework is rich enough to allow for the simulations of recently performed laboratory experiments on clay samples submitted to intertwined mechanical and chemical loading programmes, involving large changes in ionic strengths and pH. This work is the comprehensive version of the paper published on the Journal of the Mechanics and Physics of Solids, 55(8), 1762-180

The importance of local thermal non-equilibrium in the modeling of a fractured hot dry rock reservoir

International audienceThermal recovery from a HDR reservoir, viewed as a deformable fractured medium, is investigated with a focus on the assumption of local thermal non-equilibrium. The numerical model is used to investigate the coupled thermo-hydro-mechanical behavior of the Fenton Hill site. The time profile of the outlet fluid temperature displays a double-step pattern, a feature which is interpreted as characteristic of established local thermal non-equilibrium

A thermo‐hydro‐mechanical coupled model in local thermal non‐equilibrium for fractured HDR reservoir with double porosity

International audienceThe constitutive thermo-hydro-mechanical equations of fractured media are embodied in the theory of mixtures applied to three-phase poroelastic media. The solid skeleton contains two distinct cavities filled with the same fluid. Each of the three phases is endowed with its own temperature. The constitutive relations governing the thermomechanical behavior, generalized diffusion and transfer are structured by, and satisfy, the dissipation inequality. The cavities exchange both mass and energy. Mass exchanges are driven by the jump in scaled chemical potential, and energy exchanges by the jump in coldness. The finite element approximation uses the displacement vector, the two fluid pressures and the three temperatures as primary variables. It is used to analyze a generic hot dry rock geothermal reservoir. Three parameters of the model are calibrated from the thermal outputs of Fenton Hill and Rosemanowes HDR reservoirs. The calibrated model is next applied to simulate circulation tests at the Fenton Hill HDR reservoir. The finer thermo-hydro-mechanical response provided by the dual porosity model with respect to a single porosity model is highlighted in a parameter analysis. Emphasis is put on the influence of the fracture spacing, on the effective stress response and on the permeation of the fluid into the porous blocks. The dual porosity model yields a thermally induced effective stress that is less tensile compared with the single porosity response. This effect becomes significant for large fracture spacings. In agreement with field data, fluid loss is observed to be high initially and to decrease with time

Thermal recovery from a fractured medium in local thermal non‐equilibrium

International audienceThermal recovery from a hot dry rock (HDR) reservoir viewed as a deformable fractured medium is investigated with a focus on the assumption of local thermal non-equilibrium (LTNE). Hydraulic diffusion, thermal diffusion, forced convection and deformation are considered in a two-phase framework, the solid phase being made by impermeable solid blocks separated by saturated fractures. The finite element approximation of the constitutive and field equations is formulated and applied to obtain the response of a generic HDR reservoir to circulation tests. A change of time profile of the outlet fluid temperature is observed as the fracture spacing increases, switching from a single-step pattern to a double-step pattern, a feature which is viewed as characteristic of established LTNE. A dimensionless number is proposed to delineate between local thermal equilibrium (LTE) and non-equilibrium. This number embodies local physical properties of the mixture, elements of the geometry of the reservoir and the production flow rate. All the above properties being fixed, the resulting fracture spacing threshold between LTNE and LTE is found to decrease with increasing porosity or fluid velocity. The thermally induced effective stress is tensile near the injection well, illustrating the thermal contraction of the rock, while the pressure contribution of the fracture fluid is negligible during the late period

Enhanced Geothermal Reservoirs with two Fluid Cavities and Unequal Solid and Fluid Temperatures

International audienceThermo-hydro-mechanical (THM) constitutive equations and generalized diffusion and transfer constitutive relations are developed in a comprehensive, coupled and unified framework, assuming a deformable rock formation. Particular attention is laid on both mass and energy exchanges between the cavities which are controlled by the out-of-balances of the chemical potentials and by the out-of-balances of the coldnesses, respectively. Emphasis is laid a) on the mass exchanges between the pore system and the fracture network, which are endowed each with their own pressure, and mainly; b) on the energy exchanges between the rock, the pore network and the fracture network, which are endowed each with their own temperature. Thermo-hydro-mechanical (THM) constitutive equations and generalized diffusion and transfer constitutive relations are developed in a comprehensive, coupled and unified framework, assuming a deformable rock formation. Particular attention is laid on both mass and energy exchanges between the cavities which are controlled by the out-of-balances of the chemical potentials and by the out-of-balances of the coldnesses, respectively. The model is applied to simulate circulation tests using a domestic finite element code. The parameters are calibrated from the thermal outputs of the Fenton Hill and Rosemanowes reservoirs. At variance with a double porosity model with Local Thermal Equilibrium (LTE), the Local thermal Non Equilibrium model (LTNE) displays the characteristic two step time profile that is reported for these two reservoirs. In agreement with field data, fluid loss is observed to be high initially and decreases with time. A sensitivity analysis is performed to determine the influences of the internal length scales, namely fracture spacing and crack aperture, in the complete framework of the dual porosity (2 pressures 2P) and local thermal non equilibrium (3 temperatures 3T) . The fine description of the effective stress, pore and fracture pressures, and solid, pore and fracture temperatures of the most general format (2P-3T) is essentially unchanged when the model is specialized to (2P-2T) with equal pore and solid temperatures. At variance, the quality of the description is degraded for the (1P-2T) model that omits the permeability contribution of the pores, and for the (1P-1T) standard single porosity LTE model. The progressive transition is quantified during circulation tests at the Fenton Hill HDR

Borehole stability analysis in a thermoporoelastic dual-porosity medium

International audienceThe problem of diffusion and mass transfer in dual porous media is considered in a threephase framework. The solid phase is assumed to contain two distinct cavities filled with fluid. The porous mixture is composed of two overlapping media: the porous blocks and the fissure network. The fluid can transfer between the cavities due to fluid pressure difference. In addition, hydraulic and thermal diffusions take place through the mixture. A global understanding of mass transfer, diffusion and deformation is provided. The governing equations associated with these phenomena are presented for a mixture in thermal equilibrium. The finite element approximation of the governing equations is formulated and applied to the stability analysis of a vertical borehole. A parametric analysis is carried out to evaluate the influence of mass transfer on the pressure profiles of the fluids around the borehole. Permeable and a semipermeable boundary conditions are compared to predict the potential for failure of the wellbore under drained and partially undrained conditions

Géothermie profonde (stimulation de la perméabilité par fracturation hydraulique dans un cadre thermo-poroélastique)

Ce travail concerne l'utilisation de la technique de Fracturation Hydraulique (FH) pour exploiter l'énergie géothermique des réservoirs profonds de roches sèches chaudes (HDR). La fracturation hydraulique est réalisée par injection de fluides géothermiques dans des réservoirs partiellement fracturés de faible perméabilité. Les fluides à haute pression sont destinés à faire évoluer les fissures et leur connectivité. Les valeurs de débit/pression auxquelles les fluides géothermiques doivent être pompés, ainsi que le calendrier de pompage pour initier la fracturation hydraulique, dépendent principalement des conditions géostatiques existantes (contraintes géostatiques, pression fluide et température initiales de l'HDR) ainsi que des propriétés des fissures de l'HDR (longueur, épaisseur, densité et distribution directionnelle initiales moyennes de fissures). Tous ces éléments, en sus de leurs effets sur la stabilité des forages, sont analysés dans cette recherche. Des modèles de fracturation, qui sont capables de suivre l'évolution des fissures dans toutes les orientations spatiales possibles, sont utilisés pour obtenir le tenseur anisotrope de perméabilité. Ces modèles sont intégrés dans un code domestique d'éléments finis qui est développé pour résoudre des problèmes aux limites thermo-poroélastiques. Pour supprimer/diminuer les oscillations qui accompagnent les solutions paraboliques et/ou hyperboliques lors de la convection forcée, plusieurs techniques de stabilisation ont dû être implémentées.The application of the Hydraulic Fracturing (HF) technology to exploit geothermal energy from Hot Dry Rocks (HDR) reservoirs is addressed. HF is achieved by extensively pumping geothermal fluids to already existing fractured HDR reservoirs of low permeability. High fluid pressures are expected to drive cracks to evolve and connect. The newly created burgeoning hydraulic conduits should supposedly enhance the permeability of the existing HDR reservoirs. The flow rate/pressure values at which geothermal fluids should be pumped, as well as the pumping schedule to initiate HF, depend primarily on the existing geostatic conditions (geostatic stresses, initial HDR pressure and temperature) as well as on HDR fracture properties (initial mean fracture length, mean fracture aperture, density and orientational distribution of fractures). While these components, in addition to their effects on borehole stability, are scrutinized in this research, focus is on the evolution during circulation processes of the fracture properties. A fracturing model that is capable of tracking fracture evolution in all possible spatial orientations is used to obtain the time course of the anisotropic permeability tensor. This evolving property is integrated into a domestic finite element code which is developed to solve thermo poroelastic BVPs: emphasis is laid on the efficiency of the doublet flow technique where a fluid gains thermal energy by circulating through the HDR reservoir from the injection well to the production well. The spurious oscillations in the hyperbolic solutions of the approximated finite element approach that are commensal with the phenomenon of forced heat convection are healed/mitigated through several stabilization approaches.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Anatomical, functional, physiological and behavioural aspects of the development of mastication in early childhood

Mastication efficiency is defined as the efficiency of crushing food between the teeth and manipulating the resulting particles to form a swallowable food bolus. It is dependent on the orofacial anatomical features of the subject, the coordination of these anatomical features and the consistency of the food used during testing. Different measures have been used to indirectly quantify mastication efficiency as a function of children's age such as observations, food bolus characterisation, muscle activity measurement and jaw movement tracking. In the present review, we aim to describe the changes in the oral physiology (e.g. bone and muscle structure, teeth and soft tissues) of children and how these changes are associated with mastication abilities. We also review previous work on the effect of food consistency on children's mastication abilities and on their level of texture acceptance. The lack of reference foods and differences in testing methodologies across different studies do not allow us to draw conclusions about (1) the age at which mastication efficiency reaches maturity and (2) the effect of food consistency on the establishment of mature mastication efficiency. The effect of food consistency on the development of children's mastication efficiency has not been tested widely. However, both human and animal studies have reported the effect of food consistency on orofacial development, suggesting that a diet with harder textures enhances bone and muscle growth, which could indirectly lead to better mastication efficiency. Finally, it was also reported that (1) children are more likely to accept textures that they are able to manipulate and (2) early exposure to a range of textures facilitates the acceptance of foods of various textures later on. Recommending products well adapted to children's mastication during weaning could facilitate their acceptance of new textures and support the development of healthy eating habit