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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
    A comparison of FBG- and Brillouin-strain sensing in the framework of a decameter-scale hydraulic stimulation experiment
    (: American Rock Mechanics Association, 2018-6-18)
    Krietsch, H.
    ;
    Gischig, V.
    ;
    Jalali, R.
    ;
    Doetsch, J.
    ;
    Valley, Benoît 
    ;
    Amann, F.
    In the framework of the In-situ Stimulation and Circulation (ISC) experiment Fiber-Bragg-Grating (FBG) and Brillouin strain sensing systems were installed to monitor deformation during six hydraulic shearing and six hydraulic fracturing experiments. Three boreholes were dedicated to strain monitoring. Both systems are installed in the same boreholes, offering a unique opportunity to compare these systems with respect to their applicability in hydraulic stimulation tests. A total of 60 FBG sensors with 1 m base length were installed across fractures, shear zones and intact rock. Along the entire borehole length, pre-stressed optical cables for Brillouin distributed strain (DBS) sensing were embedded in grout with two installation methods: a bare cable and a cable packed and fixed with glue every 0.65 m. The strain signals were compared as time series for a given borehole depth and as profiles along the borehole axis. The study reveals that the FBG system gives a high accuracy (0.04 µ-strain) and temporal resolution (>1s) with pointwise measurements. The bare DBS leg yield good quantitative strain data with poorer strain accuracy (>500 times poorer than FBG) and poorer temporal resolution (factor of >100). The packed DBS leg provide no meaningful information about the strain field.
  • Publication
    Accès libre
    Observations of fracture propagation during decameter-scale hydraulic fracturing experiments
    (: American Rock Mechanics Association, 2018-6-17)
    Dutler, Nathan 
    ;
    Valley, Benoît 
    ;
    Gischig, V.
    ;
    Jalali, M.
    ;
    Doetsch, J.
    ;
    Krietsch, H.
    ;
    Villiger, L.
    ;
    Amann, F.
    Various in-situ hydraulic fracturing experiments were carried out in the naturally fractured, crystalline rock mass of the Grimsel Test Site (GTS) in Switzerland. The purpose was to study the geometry of the newly created fractures and their interaction with the pre-existing fracture network using transient pressure and rock mass deformation observations. Under controlled conditions, six hydraulic fractures with similar injection protocols were executed in two sub-vertical injection boreholes. The rock mass is intersected by two E-W striking shear zones (S3), and two biotite-rich meta-basic dykes with a densely fractured zone in between. The S3 shear-zone intersecting the rock volume of interest acts as a high-permeability connection to the tunnel for the experiments executed south of it. Strong variation in injectivity enhancement, jacking pressure, break down pressure, instantaneous shut-in pressure and fluid flow recovery among the different injection intervals indicate different stress conditions north and south of S3.
  • Publication
    Accès libre
    Stress measurements in crystalline rock: Comparison of overcoring, hydraulic fracturing and induced seismicity results
    (: American Rock Mechanics Association, 2017-6-25)
    Krietsch, H.
    ;
    Gischig, V.
    ;
    Evans, K.
    ;
    Doetsch, J.
    ;
    Valley, Benoît 
    In preparation of a decameter-scale fault stimulation experiment at the Grimsel Test Site, Switzerland, a comprehensive rock stress characterization survey was conducted. The survey combines overcoring of CSIRO-HI and USBM probes with hydrofracture measurements with concomitant monitoring of the induced microseismicity. Impression packer surveys were run following the hydrofracture tests to determine the orientation of the induced fracture at the wellbore. The orientation of the fracture away from the wellbore was determined from the pattern of the microseismicity. The use of a transverse isotropic model for inverting the strains measured during overcoring was essential to obtain stress solutions that were consistent with the hydrofracture and microseismicity results…