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- PublicationAccès libreA 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. ;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.
- PublicationMétadonnées seulementPreliminary stress characterization for an in-situ stimulation experiment at the Grimsel Underground Laboratory(2016-4-1)
;Krietsch, Hannes ;Doetsch, J. ;Gischig, Valentin ;Amann, Florian ;Jalali, Mohammadreza ;Madonna, Claudio ;Evans, Keith F. ; ;Giardini, Domenico ;Wiemer, Stefan ;Maurer, HansruediLoew, SimonA decameter-scale in-situ stimulation experiment is currently being executed at the Grimsel Test Site in Switzerland, spanning from hydraulic fracturing to controlled fault-slip experiments. For the feasibility of this project the in-situ stress tensor is of foremost importance. Therefore a unique stress characterization campaign combining stress relief methods (overcoring of USBM and CSIRO-HI probes) with hydraulic fracturing (HF) and hydraulic testing on pre-existing fractures (HTPF) in three boreholes was conducted in a first phase of this project. During all hydraulic stress measurements, micro-seismicity was monitored and localized in real time utilizing a dense network of piezo-electric sensors. In this contribution, we present preliminary results of the stress characterization and compare the derived stress tensor with previous estimates of the stress state. The stress characterization campaign was conducted in three boreholes, one sub-vertical and two sub-horizontal boreholes, assuming that the sub-vertical and one sub-horizontal are parallel to a principal stress component. A major task in this contribution is the integration of the different stress characterization methods. Our results of the different methods (overcoring and HF) are largely consistent, but disagree with some of the previous stress orientation estimates. From the new campaign the overcoring measurements indicate a sub-horizontal sigma1 of 17.3 MPa with a strike of 145°, a sigma2 of 9.7 MPa with 241°/69° and a sigma3 of 8.3 MPa with 055°/21° using an isotropic approach for inversion calculation. Whereas the USBM-Probe measures a projection of the principal stresses in a plane normal to borehole axis, the CSIRO-HI Probe provides the real 3D stress tensor. The HF and HTPF measurements indicate a far-field minimum horizontal stress between 8.7 and 9.1 MPa, consistent with the overcoring. Principal stresses, estimated by location of micro-seismic events during HF and HTPF, suggest that the maximum horizontal stress strikes EW, the minimum horizontal stress strikes NS and sigma2 stress direction is sub-vertical dipping towards south. One sub-horizontal borehole dedicated to stress characterization penetrates one of the fault zones targeted for a future fault-slip experiment. Results reveal a significant drop in the minimum stress component towards the fault zone. This stress information will be critical for the planning of the stimulation phase of the project.
- PublicationAccès libreStress measurements in crystalline rock: Comparison of overcoring, hydraulic fracturing and induced seismicity results(: American Rock Mechanics Association, 2017-6-25)
;Krietsch, H. ;Gischig, V. ;Evans, K. ;Doetsch, J.In preparation of a decameter-scale fault stimulation experiment at the Grimsel Test Site, Switzerland, a comprehensive rock stress characterization survey was conducted. The survey combines overcoring of CSIRO-HI and USBM probes with hydrofracture measurements with concomitant monitoring of the induced microseismicity. Impression packer surveys were run following the hydrofracture tests to determine the orientation of the induced fracture at the wellbore. The orientation of the fracture away from the wellbore was determined from the pattern of the microseismicity. The use of a transverse isotropic model for inverting the strains measured during overcoring was essential to obtain stress solutions that were consistent with the hydrofracture and microseismicity results…
- PublicationAccès libreObservations of fracture propagation during decameter-scale hydraulic fracturing experimentsVarious 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.
- PublicationAccès libreComparing 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 ; ;Evans, Keith F. ;Saar, Martin O.Amann, Florian
- PublicationMétadonnées seulementReservoir Stimulation Experiments at the Grimsel Test Site: Stress Measurements using Hydraulic fracturing, Hydraulic Tests on Pre-existing Fractures and Overcoring(2015-12-1)
;Doetsch, J. ;Gischig, Valentin ;Amann, Florian ;Madonna, Claudio ;Jalali, Mohammadreza ;Evans, Keith F.
- PublicationMétadonnées seulementStress Measurements in Crystalline Rock: Comparison of Overcoring, Hydraulic Fracturing and Induced Seismicity Results(2017-8-28)
;Krietsch, Hannes ;Gischig, Valentin ;Jalali, Mohammadreza ;Amann, Florian ;Evans, Keith F. ;Doetsch, J.
- PublicationAccès librePoroelasticity Contributes to Hydraulic-Stimulation Induced Pressure ChangesHigh-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.
- 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 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