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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
    Poroelasticity Contributes to Hydraulic-Stimulation Induced Pressure Changes
    (2021-2)
    Dutler, Nathan 
    ;
    Valley, Benoît 
    ;
    Amann, F.
    ;
    Jalali, M.
    ;
    Villiger, L.
    ;
    Krietsch, H.
    ;
    Gischig, V.
    ;
    Doetsch, J.
    ;
    Giardini, D.
    High-pressure fluid injections cause transient pore pressure changes over large distances, which may induce seismicity. The zone of influence for such an injection was studied at high spatial esolutions in six decameter-scaled fluid injection experiments in crystalline rock. Pore pressure time series revealed two distinct responses based on the lag time and magnitude of pressure change, namely, a near- and far-field response. The near-field response is due to pressure diffusion. In the far-field, the fast response time and decay of pressure changes are produced by effective stress changes in the anisotropic stress field. Our xperiments confirm that fracture fluid pressure perturbations around the injection point are not limited to the near field and can extend beyond the pressurized zone.
  • Publication
    Accès libre
    In situ observation of helium and argon release during fluid-pressure triggered rock deformation
    (2020-10)
    Roques, Clément 
    ;
    Weber, U. W.
    ;
    Brixel, B.
    ;
    Krietsch, H.
    ;
    Dutler, Nathan 
    ;
    Brennwald, M. S.
    ;
    Villiger, L.
    ;
    Doetsch, J.
    ;
    Jalali, M.
    ;
    Gischig, V.
    ;
    Amann, F.
    ;
    Valley, Benoît 
    ;
    Klepikova, M.
    ;
    Kipfer, R.
    Temporal changes in groundwater chemistry can reveal information about the evolution of flow path connectivity during crustal deformation. Here, we report transient helium and argon concentration anomalies monitored during a series of hydraulic reservoir stimulation experiments measured with an in situ gas equilibrium membrane inlet mass spectrometer. Geodetic and seismic analyses revealed that the applied stimulation treatments led to the formation of new fractures (hydraulic fracturing) and the reactivation of natural fractures (hydraulic shearing), both of which remobilized (He, Ar)-enriched fluids trapped in the rock mass. Our results demonstrate that integrating geochemical information with geodetic and seismic data provides critical insights to understanding dynamic changes in fracture network connectivity during reservoir stimulation. The results of this study also shed light on the linkages between fluid migration, rock deformation and seismicity at the decameter scale.
  • Publication
    Accès libre
    Hydromechanical insight of fracture opening and closure during in-situ hydraulic fracturing in crystalline rock
    (2020-9)
    Dutler, Nathan 
    ;
    Valley, Benoît 
    ;
    Gischig, V.
    ;
    Jalali, M.
    ;
    Brixel, B.
    ;
    Krietsch, H.
    ;
    Roques, Clément 
    ;
    Amann, F.
    Six hydraulic fracturing (HF) experiments were conducted in situ at the Grimsel Test Site (GTS), Switzerland, using two boreholes drilled in sparsely fractured crystalline rock. High spatial and temporal resolution monitoring of fracture fluid pressure and strain improve our understanding of fracturing dynamics during and directly following high-pressure fluid injection. In three out of the six experiments, a shear-thinning fluid with an initial static viscosity approximately 30 times higher than water was used to understand the importance of fracture leak-off better. Diagnostic analyses of the shut-in phases were used to determine the minimum principal stress magnitude for the fracture closure cycles, yielding an estimate of the effective instantaneous shut-in pressure (effective ISIP) 4.49±0.22 MPa. The jacking pressure of the hydraulic fracture was measured during the pressurecontrolled step-test. A new method was developed using the uniaxial Fibre-Bragg Grating strain signals to estimate the jacking pressure, which agrees with the traditional flow versus pressure method. The technique has the advantage of observing the behavior of natural fractures next to the injection interval. The experiments can be divided into two groups depending on the injection location (i.e., South or North to a brittle-ductile S3 shear zone). The experiments executed South of this zone have a jacking pressure above the effective ISIP. The proximity to the S3 shear zone and the complex geological structure led to near-wellbore tortuosity and heterogeneous stress effects masking the jacking pressure. In comparison, the experiments North of the S3 shear zone has a jacking pressure below the effective ISIP. This is an effect related to shear dislocation and fracture opening. Both processes can occur almost synchronously and provide new insights into the complicated mixedmode deformation processes triggered by high-pressure injection.
  • Publication
    Accès libre
    Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods
    (2018-9)
    Krietsch, H.
    ;
    Gischig, V.
    ;
    Evans, K. F.
    ;
    Doetsch, J.
    ;
    Dutler, Nathan 
    ;
    Valley, Benoît 
    ;
    Amann, F.
    An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18–24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1–14.4 MPa for σ1, 9.2–10.2 MPa for σ2, and 8.6–9.7 MPa for σ3 with σ1 plunging to the east at 30–40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past.
  • Publication
    Accès libre
    On the link between fracture toughness, tensile strength, and fracture process zone in anisotropic rocks
    (2018-8)
    Dutler, Nathan 
    ;
    Nejati, M.
    ;
    Valley, Benoît 
    ;
    Amann, F.
    ;
    Molinari, G.
    This paper presents experimental results on the anisotropy of the fracture toughness, Brazilian tensile strength, and the fracture process zone (FPZ) in granodiorite samples. The fracture toughness is measured using semi-circular bending tests, while Brazilian disk tests were conducted to measure the tensile strength indirectly. Digital image correlation (DIC) was employed to obtain full-field surface deformation associated with the fracture propagation and identify the FPZ. An averaging scheme is proposed to determine the length and width of the FPZ from the strain field. The DIC results confirm a semi-elliptical FPZ developing ahead of the crack tip, with an average length-to-width ratio of approximately two. The results also indicate that the theoretical models such as Irwin and strip-yield with uniform traction, which are based on plastic deformation near the crack tip, underestimate the extent of the inelastic zone, while the stripyield model with a linear cohesion stress distribution overestimate the length of the process zone. The anisotropy ratio of the FPZ length obtained from the models, however, agrees very well with the ratio obtained from the DIC measurements. This evidence supports the basis of the theoretical models that predict the FPZ length to be proportional to the square of fracture toughness over tensile strength.
  • 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
    The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment
    (2018-2)
    Amann, F.
    ;
    Gischig, V.
    ;
    Evans, K.
    ;
    Doetsch, J.
    ;
    Jalali, R.
    ;
    Valley, Benoît 
    ;
    Krietsch, H.
    ;
    Dutler, Nathan 
    ;
    Villiger, L.
    ;
    Brixel, B.
    ;
    Klepikova, M.
    ;
    Kittilä, A.
    ;
    Madonna, C.
    ;
    Wiemer, S.
    ;
    Saar, M.O.
    ;
    Loew, S.
    ;
    Driesner, T.
    ;
    Maurer, H.
    ;
    Giardini, D.
    In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.
  • Publication
    Accès libre
    On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity
    (2017-7)
    Gischig, V.
    ;
    Doetsch, J.
    ;
    Maurer, H.
    ;
    Krietsch, H.
    ;
    Amann, F.
    ;
    Evans, K.
    ;
    Nejati, M.
    ;
    Jalali, R.
    ;
    Valley, Benoît 
    ;
    Obermann, A.
    ;
    Wiemer, S.
    ;
    Giardini, D.
    To characterize the stress field at the Grimsel Test Site (GTS) underground rock laboratory a 10 series of hydrofracturing test and overcoring test were performed. Hydrofracturing was accompanied by seismic monitoring using a network of highly sensitive piezo sensors and accelerometers that were able to record small seismic events associated with decimeter-sized fractures. Due to potential discrepancies between the hydro-fracture orientation and stress field estimates from overcoring, it was essential to obtain high-precision hypocenter locations that reliably illuminate fracture growth. 15 Absolute locations were improved using a transverse isotropic P-wave velocity model and by applying joint hypocenter determination that allowed computation of station corrections. We further exploited the high degree of waveform similarity of events by applying cluster analysis and relative relocation. Resulting clouds of absolute and relative located seismicity showed a consistent east-west strike and 70° dip for all hydro-fractures. The fracture growth direction from microseismicity is consistent with 20 the principal stress orientations from the overcoring stress tests provided an anisotropic elastic model for the rock mass is used in the data inversions. σ1 is significantly larger than the other two principal stresses, and has a reasonably well-defined orientation that is subparallel to the fracture plane. σ2 and σ3 are almost equal in magnitude, and thus lie on a circle defined by the standard errors of the solutions. The poles of the microseismicity planes also lie on this circle towards the north. The trace of the 25 hydraulic fracture imaged at the borehole wall show that they initiated within the foliation plane, which differs in orientation from the microseismicity planes. Thus, fracture initiation was most likely influenced by a foliation-related strength anisotropy. Analysis of P-wave polarizations suggested double-couple focal mechanisms with both thrust and normal faulting mechanisms present, whereas strike-slip and thrust mechanisms would be expected from the overcoring-derived stress solution. The 30 reasons for these discrepancies are not well understood, but may involve stress field rotation around the propagating hydrofracture. Our study demonstrates that microseismicity monitoring along with high-resolution event locations provides valuable information for interpreting stress characterization measurements.
  • Publication
    Accès libre
    Near-surface principal stress trajectories in Alpine valleys (Grimsel region, Switzerland) as inferred from exfoliation fracture plumose structure axes and numerical modeling
    (: American Rock Mechanics Association, 2014-6-1)
    Ziegler, M.
    ;
    Loew, S.
    ;
    Valley, Benoît 
    ;
    Amann, F.
    In this study we will show how fractographic (i.e., fracture surface morphology) data of exfoliation fractures (i.e., fractures subparallel to landscape surfaces limited to near-surface rock masses) and three-dimensional numerical modeling can be used to infer orientations of principal rock mass stresses in topographically complex Alpine areas. Analysis of exfoliation fracture plumose axes, i.e., fractographic features that indicate main fracture propagation directions, and supposed local maximum compressive principal stress (σ1) orientations at the time of fracture formation, suggests complex directional trends of near-surface σ1 within trough valleys of the Grimsel region (central Swiss Alps). We investigated near-surface stress tensors with a threedimensional, elastic numerical model to 1. deduce evidence that plumose axes form parallel to σ1 in an overall compressive (farfield) stress field, and to 2. increase our knowledge of near-surface stress orientations in Alpine settings. Model results illustrate that superposition of topographic stresses with realistic horizontal strains reveals complex near-surface σ1 trajectories that widely follow the patterns of plumose axes. The model results demonstrate large variations of stress orientations, which cannot be captured by small numbers of classical stress measurements. In-situ stress measurements support exfoliation fracture formation under compression and principal stress directions as inferred from our numerical model.