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Valley, Benoît

Nom
Valley, Benoît
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
benoit.valley@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/943
_
0000-0003-4632-0504

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  • Publication
    Accès libre
    Modeling heat and fluid flow in discrete fracture networks: application to fluid-driven seismicity and engineered geothermal systems
    (2018)
    Jansen, Gunnar
    ;
    Miller, Stephen A.
    ;
    Valley, Benoît 
    La compréhension de la dynamique des systèmes naturels et des réservoirs fracturés en termes des écoulements, des transferts de la chaleur ou de stabilité de fractures (p. ex. les tremblements de terre) est importante pour diverses applications telles que les systèmes géothermiques et la séismicité naturelle induite par des fluides. Aujourd’hui, beaucoup de codes numériques existent pour simuler certains aspects de ces dynamiques, mais en général, ils sont très couteux ou alors leur utilisation n’est possible que par leurs développeurs eux-mêmes. Dans cette thèse, je présente une solution open-source MATLAB efficace pour des simulations numériques de processus couplés dans les systèmes fracturés. J’utilise le modèle des fractures discrètes intégrées.
    Dans une étude numérique thermoélastique, je cherche à comprendre l’effet de l’injection de fluides froids dans un réservoir plus chaud et plus spécifiquement les changements des contraintes associées aux variations de température. Ces changements induisent des ruptures en cisaillement dans le réservoir. J’ai observé une influence très forte des propriétés hydrauliques du réservoir sur l’évolution des contraintes thermiques. De plus, j’ai trouvé que le changement des contraintes thermiques peut conduire à du cisaillement sur des fractures orientées non optimalement pour la rupture. Ces résultats suggèrent que les changements des contraintes thermiques devraient être pris en compte dans tous les modèles lors d’injections de fluides de longue durée dans des réservoirs fracturés.
    J’utilise le modèle développé pour comprendre une séquence de tremblements de terre qui prit place à l’Ouest de Reno, Mogul, Nevada, aux États-Unis à la fin février 2008. Après 2 mois de séismes précurseurs répétés près d’une structure de faille inconnue préalablement, un tremblement de terre de magnitude 4.9 a eu lieu. Ici, je montre que la séquence de séismes précurseurs a pu être engendrée par l’intrusion de fluides à haute pression dans la structure de faille préexistante et j’ai trouvé une forte corrélation entre le front de pression et l’hypocentre des événements sismiques.
    Finalement, je présente un algorithme simple et efficace pour générer des maillages hexahedraux à partir d’un modèle géologique utilisable par n’importe quels simulateurs numériques. En utilisant la méthode de «binary space partitioning» et d’«octree refinement» sur la géométrie importée, un maillage précis est créé. L’algorithme fournit une nouvelle méthode pour la création de maillages hexahedraux pour toutes configurations géologiques., Understanding the dynamics of naturally fractured systems and fractured reservoirs in terms of flow, heat transport and fracture stability (e.g. earthquakes) is important for a range of applications including geothermal systems, waste water injection and natural fluid-driven seismicity. Many numerical codes exist that can simulate aspects of these dynamics, but in general require either expensive licences or their utility is limited mostly to the developers. In this thesis I present an open source MATLAB package for efficient numerical simulations of the coupled processes in fractured systems. I take advantage of the embedded discrete fracture model that accounts for discrete fractures that are computationally efficient.
    In a numerical thermo-elasticity study, I investigate the effect of cold fluid injection on the reservoir and the resulting thermal stress change on potential shear failure in the reservoir. I observe a strong influence of the hydraulic reservoir properties on thermal stress propagation. I also find that thermal stress change can lead to induced shear failure on non-optimally oriented fractures. The results suggest that thermal stress changes should be taken into account in all models for long-term fluid injections in fractured reservoirs.
    Furthermore, I use the developed model to investigate an earthquake sequence that occurred in Mogul west of Reno, Nevada, USA in late February 2008. It culminated in a magnitude 4.9 main-shock after a foreshock-rich period of approximately 2 months on a previously unidentified fault structure. Here I show that the foreshock sequence may have been driven by a fluid pressure intrusion into this preexisting structure and find a strong correlation between high fluid pressure fronts and foreshock hypocenters.
    Finally, I present a simple and efficient algorithm to generate hexahedral meshes from a geological model to be used in numerical simulation tools. Using binary space partitioning of the input geometry and octree refinement on the grid, a successive increase in accuracy of the mesh is achieved. The algorithm provides a new method for hexahedral mesh generation in geological settings. It generates high accuracy discretizations with cell counts suitable for state-of-the-art subsurface simulators.
  • Publication
    Accès libre
    Development of connected permeability in massive crystalline rocks through hydraulic fracture propagation and shearing accompanying fluid injection
    (Hoboken, New Jersey, uSA: John Wiley & Sons Ltd, 2014-2)
    Preisig, Giona 
    ;
    Eberhardt, E.
    ;
    Gischig, V.
    ;
    Roche, V.
    ;
    Van der Baan, M.
    ;
    Valley, Benoît 
    ;
    Kaiser, P.K.
    ;
    Duff, D.
    ;
    Lowther, R.
    The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 1017–1015 m2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high-stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct-element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow-packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.
  • Publication
    Accès libre
    Numerical modeling of strain transfer from rock mass to a fibre optic sensor installed inside a grouted borehole
    (: American Rock Mechanics Association, 2012-6-24)
    Madjdabadi, B.
    ;
    Valley, Benoît 
    ;
    Dusseault, M.B.
    ;
    Kaiser, P.K.
    Strain measurements in underground excavation are usually done locally, with extensometers or similar devices placed within 10-15 meters of adit or stope faces, mainly to gage development of the EDZ (excavation damaged zone) and assess its evolution and impact on local safety (rock falls, rapid deterioration of wall condition...). However, the calibration of three-dimensional stress analysis models used to assess excavation geometry and sequencing requires strain (displacement) measurements in those parts of the rock mass that are in the elastic domain for some or all of their active design life. Recently developed distributed fibre optic sensors are now being used to measure local linear displacements continuously in a large rock mass volume in real mining conditions in Canada. Grouted inside a borehole and therefore encased in a material of far lower stiffness that the rock mass, an optical fibre may register strains different from those actually occurring in the rock mass. A number of factors affect the process of rock mass strain conveyance through the grout to the fibre. This paper reports a study that simulates the borehole-grout-fibre interaction to find how the strain transfer takes place and whether there are any issues serious enough to warrant alterations in installation procedures and grout materials.