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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
    Fractal characteristics of fractures in crystalline basement rocks: Insights from depth-dependent correlation analyses to 5 km depth
    (2022-5-27)
    Afshari Moein, Mohammad Javad
    ;
    Evans, Keith F.
    ;
    Valley, Benoît 
    ;
    Bär, Kristian
    ;
    Genter, Albert
    The scaling laws describing the spatial arrangement of fractures along six deep boreholes penetrating the crystalline rocks in the Rhine Graben were derived using a correlation analysis. Five of the wells, two to 5 km depth, were located at the Soultz geothermal site and one well to 5 km depth was located at Basel, some 150 km from Soultz. Five datasets were derived from borehole imaging logs, whilst one stemmed from the analysis of 810 m of continuous core at Soultz. The two differed inasmuch as the core dataset included essentially all fractures, whereas the image log dataset had few fractures narrower than 1–3 mm. The results of the analysis for all image datasets showed that the spatial arrangement of fractures followed fractal behavior at all scales from meters to several hundred meters, the largest scale amenable to assessment, and that the fractal dimensions were confined to the narrow range 0.85–0.9. However, the core dataset showed significant deviation from fractal behavior, the best-fit fractal dimension of 0.8 being somewhat lower than values obtained from imaging logs in neighboring wells. Eliminating fractures with apertures less than 1 mm from the core dataset to improve comparability led to even lower fractal dimension estimates, indicating the discrepancy was not due to imaging log resolution. Analysis of successive depth sections of the core log suggested the discrepancy was due to the presence of a localized zone between 1750 and 2070 m where the fractal organization is disturbed or takes a lower dimension than elsewhere. Aside from this zone, no systematic variation of fractal dimension with depth was observed in any dataset, implying that a single exponent together with intensity adequately describes the arrangement of fractures along the entire length of the boreholes. The results are relevant to the parameterization of DFN models of deep rock masses.
  • Publication
    Accès libre
    Hydromechanical processes and their influence on the stimulation effected volume: observations from a decameter-scale hydraulic stimulation project
    (2020-9-4)
    Krietsch, Hannes
    ;
    Gischig, Valentin
    ;
    Doetsch, J.
    ;
    Evans, Keith F.
    ;
    Villiger, Linus
    ;
    Jalali, Mohammadreza
    ;
    Valley, Benoît 
    ;
    Loew, Simon
    ;
    Amann, Florian
    Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment fractures associated with one out of two shear zone types were hydraulically reactivated. The two shear zone types differ in terms of tectonic genesis and architecture. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in 11 boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to 3 orders in magnitude. With one exception, jacking pressures were unchanged by the stimulations. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone architectures govern the seismo-hydromechanical response. The elevated fracture fluid pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15–20 m from the injection point is characterized by variable extensional and compressive strain produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which strain magnitudes decrease with increasing distance. These compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. Our hydromechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud and can be subdivided into a primary stimulated and secondary effected zone.
  • Publication
    Accès libre
    Hydraulic stimulation and fluid circulation experiments in underground laboratories: Stepping up the scale towards engineered geothermal systems
    (2020-1-2)
    Gischig, Valentin
    ;
    Giardini, Domenico
    ;
    Amann, Florian
    ;
    Hertrich, Marian
    ;
    Krietsch, Hannes
    ;
    Loew, Simon
    ;
    Maurer, Hansruedi
    ;
    Villiger, Linus
    ;
    Wiemer, Stefan
    ;
    Bethmann, Falco
    ;
    Brixel, Bernard
    ;
    Doetsch, J.
    ;
    Gholizadeh, Nima
    ;
    Driesner, Thomas
    ;
    Dutler, Nathan 
    ;
    Evans, Keith F.
    ;
    Jalali, Mohammadreza
    ;
    Jordan, D.
    ;
    Kittilä, A.
    ;
    Ma, Xiadong
    ;
    Meier, Peter
    ;
    Nejati, M.
    ;
    Obermann, A.
    ;
    Plenkers, K.
    ;
    Saar, Martin O.
    ;
    Shakas, A.
    ;
    Valley, Benoît 
    The history of reservoir stimulation to extract geothermal energy from low permeability rock (i.e. so-called petrothermal or engineered geothermal systems, EGS) highlights the difficulty of creating fluid pathways between boreholes, while keeping induced seismicity at an acceptable level. The worldwide research community sees great value in addressing many of the unresolved problems in down-scaled in-situ hydraulic stimulation experiments. Here, we present the rationale, concepts and initial results of stimulation experiments in two underground laboratories in the crystalline rocks of the Swiss Alps. A first experiment series at the 10 m scale was completed in 2017 at the Grimsel Test Site, GTS. Observations of permeability enhancement and induced seismicity show great variability between stimulation experiments in a small rock mass body. Monitoring data give detailed insights into the complexity of fault stimulation induced by highly heterogeneous pressure propagation, the formation of new fractures and stress redistribution. Future experiments at the Bedretto Underground Laboratory for Geoenergies, BULG, are planned to be at the 100 m scale, closer to conditions of actual EGS projects, and a step closer towards combining fundamental process-oriented research with testing techniques proposed by industry partners. Thus, effective and safe hydraulic stimulation approaches can be developed and tested, which should ultimately lead to an improved acceptance of EGS
  • Publication
    Accès libre
    On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity
    (2018-7-1)
    Gischig, Valentin
    ;
    Doetsch, J.
    ;
    Maurer, Hansruedi
    ;
    Krietsch, Hannes
    ;
    Amann, Florian
    ;
    Evans, Keith F.
    ;
    Nejati, M.
    ;
    Jalali, Mohammadreza
    ;
    Valley, Benoît 
    ;
    Obermann, A.
    ;
    Wiemer, Stefan
    ;
    Giardini, Domenico
  • Publication
    Accès libre
    Comparing Simulations and Experiments for Hydraulic Fracture Stimulations Performed at the Grimsel Test Site, Switzerland
    (2017-1-26)
    Vogler, Daniel
    ;
    Settgast, R.
    ;
    Sherman, C.
    ;
    Gischig, Valentin
    ;
    Doetsch, J.
    ;
    Jalali, Mohammadreza
    ;
    Valley, Benoît 
    ;
    Evans, Keith F.
    ;
    Saar, Martin O.
    ;
    Amann, Florian