Propagation de fractures dans des roches sous sollicitations thermo-hydro-mécaniques

L'approche proposée pour l'étude de la propagation dans des roches, sous sollicitations thermo-hydro-mécaniques complexes, consiste à recréer en laboratoire des conditions aussi proches que possible de celles rencontrées sur le site. Cette mise en situation des champs de températures et de contraintes peut être envisagée soit globalement sur un domaine de roche suffisamment important pour représenter la réalité in situ, soit uniquement sur un sous-domaine, au voisinage de l'extrémité de fissure. On s'attachera essentiellement à la deuxième approche, seule praticable dans l'exemple emprunté au cadre historique de la géothermie en roches chaudes sèches.Après avoir rappelé les fondements théoriques de la thermo-mécanique de la rupture, on donnera la banque de résultats actuellement disponible dans des expériences de propagation de fractures, en situations thermo-hydro-mécaniques complexes. Enfin on passera en revue les difficultés actuelles pour prévoir, à partir des expériences de laboratoire, les conditions de rupture in situ

Transferts de chaleur dans la zone non saturée

On propose une méthode d'élaboration de modèles mathématiques permettant d'aborder les applications liées à l'exploitation thermique du sous-sol. Cette méthode, basée sur la thermodynamique des processus irréversibles, est illustrée dans le cas du stockage de chaleur dans les sols non saturés. On présente les résultats expérimentaux d'une aire d'essai de stockage de chaleur ; ces résultats sont utilisés pour : 1) Analyser les modifications provoquées par le stockage dans la couche de sol non saturée. 2) Valider le modèle mathématique et proposer un modèle simplifié du site de stockage

Atomic modelling of crystal/complex fluid/crystal contacts-Part I. The genetic iterative multi-species (GIMS) approach and case of kaolinite/brine/kaolinite

International audiencePredicting the impact of underground engineering on the environment requires the knowledge of natural media at different scales. In particular, understanding basic phenomena controlling the properties of rocks in the presence of complex fluids necessitates a detailed atomic description of the solid/fluid/solid contacts, the subject of Part I of the present study. First, building the solid interspace between two different crystals in a non-periodic situation is achieved using the ab initio and molecular mechanics code GenMol (TM). A description of the fluid confined within the interspace is then derived from the original genetic iterative multi-species (GIMS) algorithm implemented in the same code. This approach consists of equilibrating chemical potentials, cycle after cycle and species after species, between the confined fluid and the free natural fluid. An elementary iteration for a species k consists of different steps incrementing the number N-k of particles k, with other numbers N-k remaining constant. At each step, an optimum fluid composition is obtained by a genetic process distributing the fluid particles on a grid by stochastic shots, followed in fine by a refining process. The effectiveness of the GIMS approach is demonstrated in the case study of a fluid confined between two (0 0 1) kaolinite faces, with apertures h varying between 4 and 10 A. connected to a 9-species external solution [H2O, Cl-, Na+ CO2(aq), NaCl(aq), Ca2+, Mg2+, HCO3-, H3O+] where concentrations are ranging from 55 to 10(-4) mol/L. The results show a drastic variation in the solute/solvent and cations/ions ratios in the confined fluid when aperture h is lowered to less than 1 nm. These results obtained with a very rapid convergence of the iterative algorithm combined with a very competitive genetic optimizer are validated with high precision on a free solution. This description of contacts between crystals is original and unattainable by standard crystal interface approaches. It opens the way to understanding and/or predicting phenomena between crystals in a complex natural environment, such as adhesion or repulsion investigated in the Part II of this study

Mass and momentum interface equilibrium by molecular modeling. Simulating AFM adhesion between (120) gypsum faces in a saturated solution and consequences on gypsum cohesion

International audienceProperties of composite materials depend on interatomic phenomena occurring between binder crystals. Experimental information of Atomic Force Microscopy (A.F.M.) is of prime importance; however understanding is helped by molecular modeling. As underlined in Section 1, the present study is able to simulate crystal interfaces in presence of a solution within apertures less than 1 Nanometer at a full atomic scale. Section 2 presents the case study of a gypsum solution between (120) gypsum faces, with related boundary conditions and atomic interactions. Section 3 deals with the mass equilibrium of the solution within interfaces <5 angstrom, using the original Semi Analytical Stochastic Perturbations (SASP) approach. This information becomes in Section 4 the key for explaining the peak of adhesion obtained in A.F.M. around an aperture of 3 A and gives enlightenments on gypsum cohesion. In conclusion, this illustration shows the potentialities of full atomic modeling which could not be attained by any numerical approach at a mesoscopic scale

Changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous CO2 seepage

International audienceClaystone caprocks are often the ultimate seal for CO2 underground storage when residual CO2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO2-enriched brine and water vapor-saturated CO2 gas are investigated. Results show that brine flow induces a large porosity increase (up to 50%) in the vicinity of the fracture due to dissolution of calcite and quartz, while permeability remains unchanged. Conversely, cyclic flows of CO2-brine and CO2-gas increase the fracture aperture abruptly after each gas flow period, producing a progressive decrease of the caprock seal capacity. Aperture increase is controlled by decohesion of the clay framework within a micrometer-scale-thick layer induced by CO2-gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO2-brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods