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- PublicationAccès libreHydraulic fracturing operations in mining: conceptual approach and DFN modeling example(2015-12-20)
;Katsaga, T ;Riahi, A ;DeGagne, DO ;Damjanac, B
- PublicationAccès libreStress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic MethodsAn 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.
- PublicationAccès libreDistinct element method simulation of an analogue for a highly interlocked, non-persistently jointed rockmass(2014)
;Bahrani, N ;Kaiser, P.KA grain-based distinct element model is used to reproduce the laboratory response of both intact and granulated Wombeyan marble. The term “granulated” refers to a heat treated marble where the cohesion of grain boundaries has been destroyed. The unconfined compressive strength granulated marble is less than 50% of that of intact marble, while the strength of the granulated marble increases to about 80% of that of the intact marble at higher confining stresses. An iterative calibration approach is developed to match the unconfined and confined strengths of the models to those of intact and granulated marble. The simulation test results of the models of intact and granulated marble including the transition in the failure mode, stress-strain response, and the evolution of inter- and intra-grain micro-cracks with increasing confinement are discussed. The observed rapid strengthening effect, in terms of increasing confinement, is interpreted to be due to the high degree of grain assembly geometric interlock, which arises from the tight fit geometric shape of the grains as well as the roughness of the grain boundaries. It is suggested that the granulated marble can be considered to be an analogue for a highly interlocked, non-persistently jointed rockmass. It is shown that when the generalized Hoek–Brown failure criterion and the Geological Strength Index (GSI) are used to match the strength of the granulated marble at zero confinement, the confined strength of the granulated marble is underestimated by as much as a factor of two. Therefore, the confined strength of a highly interlocked, non-persistently jointed rockmass, with strong, brittle rock blocks, could be significantly higher than that predicted by commonly adopted empirical approaches. This has practical implications for the design of highly confined pillars and abutments, which is discussed in this paper.
- PublicationAccès libreFractal 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. ; ;Bär, KristianGenter, AlbertThe 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.
- PublicationMétadonnées seulementStructure of the low permeable naturally fractured geothermal reservoir at Soultz(2010)
;Dezayes, Chrystel ;Genter, Albert
- PublicationAccès libreOn 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 ; ;Obermann, A. ;Wiemer, StefanGiardini, Domenico
- PublicationMétadonnées seulementGeologische und geotechnische Untersuchungen einer oberflächennahen Lockergesteinsrutschung bei Lutzenberg, AR(2003)
; ;Thuro, Kurosch ;Eberhardt, ErikRaetzo, Hugo
- PublicationAccès libreNumerical simulation of drilling-induced core damage and its influence on mechanical properties of rocks under unconfined condition(2015-9-1)
;Bahrani, N. ;Kaiser, P.K.At the early stages of deep underground construction projects, data are primarily obtained from boreholes from which strategic decisions are made for the design of underground infrastructures. For this purpose, the unconfined compressive strength (UCS) and the Young's modulus (E) of intact rock as well as the in situ stress state are obtained as fundamental engineering parameters during the course of geotechnical site characterization. Unfortunately, drilling in relatively high stress environments can induce disturbance, micro-cracking of the cores, which in turn may result in lower rock strength and Young's modulus measured in the laboratory1 compared to their intact values or affect stress measurement results2,3 . This may lead to erroneous estimates of design parameters.
- PublicationAccès libreIn situ observation of helium and argon release during fluid-pressure triggered rock deformationTemporal 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.
- PublicationAccès libreHydraulic 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 ; ;Evans, Keith F. ;Jalali, Mohammadreza ;Jordan, D. ;Kittilä, A. ;Ma, Xiadong ;Meier, Peter ;Nejati, M. ;Obermann, A. ;Plenkers, K. ;Saar, Martin O. ;Shakas, A.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