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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
    Depth-Dependent Scaling of Fracture Patterns Inferred from Borehole Images in GPK3 and GPK4 Wells at Soultz-sous-Forêts Geothermal Site
    (2021-4-19)
    Moein, M. J. A.
    ;
    Bär, Kristian
    ;
    Valley, Benoît 
    ;
    Genter, Albert
    ;
    Sass, Ingo
    Engineering an Enhanced Geothermal System (EGS) requires a proper understanding of the fracture network properties from small to large scales in order to create a reliable geological model for reservoir simulations. As deterministic identification of all fractures in a reservoir is practically impossible, stochastic approaches known as Discrete Fracture Networks (DFN) are used. This consists of parametrizing a statistical realization of fracture networks constrained by direct observations from borehole images and/or outcrop data, if available. DFN models can be used to study the thermo-hydro-mechanical (THM) properties of fractured rocks and to simulate the processes associated within: I) fluid circulation, II) flow and heat production as well as III) seismic response to hydraulic stimulations. Fractal DFNs are based on multiscale fracture network characteristics and are constrained by the scaling properties of fracture network attributes such as length (or size) and spatial distribution. The dual power-law model is a mathematical representation of fractures that parametrize fractal DFNs with two scaling exponents: 1) scaling of spatial distribution using two-point correlation dimension of fracture centers in three dimensions and 2) power-law exponent of fracture length distribution. Direct measurements of fracture length exponents from borehole images or cores are an unresolved challenge and the resolution of geophysical investigations is not sufficient to image the natural fracture networks. In contrast, the spatial distribution of fractures may be precisely characterized using borehole image logs and cores. Currently, the depth-dependence of spatial clustering of fracture patterns in the earth’s crust is not fully understood, although it may be required to anticipate deep reservoir conditions from shallower datasets. Here, we study such a depth dependency by using the two-point correlation dimension of fractures along the boreholes as a reliable estimate of the fractal dimension. We investigate the data stemming from two deep boreholes, GPK3 and GPK4, drilled into the crystalline basement rocks at the Soultz-sous-Forêts geothermal site. Recent analyses unraveled no systematic variation of fractal dimension with depth in any of the boreholes at the one standard deviation level of uncertainty. This conclusion may support the hypothesis of generating fracture network models with only a single correlation dimension using the stereological relationships in reservoirs up to 5 km depth in crystalline basements.
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    How a better characterization of a deep crystalline reservoir can contribute to improve EGS performance at Soultz
    (2007-10-26)
    Genter, Albert
    ;
    Cuenot, Nicolas
    ;
    Dezayes, Chrystel
    ;
    Sausse, Judith
    ;
    Valley, Benoît 
    ;
    Baumgärtner, J.
    ;
    Fritsch, D.
  • Publication
    Métadonnées seulement
    Main fracture zones in GPK3 and GPK4 and cross-hole correlations
    (2005)
    Valley, Benoît 
    ;
    Dezayes, Chrystel
    ;
    Genter, Albert
    ;
    Maqua, Emilien
    ;
    Syren, Guillaume
  • Publication
    Métadonnées seulement
    Natural fracture system of the Soultz granite based on UBI data in the GPK3 and GPK4 wells
    (2005)
    Dezayes, Chrystel
    ;
    Valley, Benoît 
    ;
    Maqua, Emilien
    ;
    Syren, Guillaume
    ;
    Genter, Albert