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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, C.
    ;
    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 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
    Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock
    (2020-4-28)
    Villiger, Linus
    ;
    Gischig, Valentin
    ;
    Doetsch, J.
    ;
    Krietsch, Hannes
    ;
    Dutler, Nathan 
    ;
    Jalali, Mohammadreza
    ;
    Valley, Benoît 
    ;
    Selvedurai, P. A.
    ;
    Mignan, Arnaud
    ;
    Plenkers, K.
    ;
    Giardini, Domenico
    ;
    Amann, Florian
    ;
    Wiemer, Stefan
    We performed a series of 12 hydraulic stimulation experiments in a 20 m×20 m×20 m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.
  • 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
    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 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
    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.