Stress Measurements for an In Situ Stimulation Experiment in Crystalline Rock: Integration of Induced Seismicity, Stress Relief and Hydraulic Methods
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Date
2019-02-08
Publication Type
Journal Article
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yes
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Abstract
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.
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published
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Journal / series
Volume
52 (2)
Pages / Article No.
517 - 542
Publisher
Springer
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Edition / version
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Date collected
Date created
Subject
Overcoring; Hydraulic fracturing; Integration of stress relief and hydraulic methods; Transversely isotropic media; Stress inversion
Organisational unit
03465 - Löw, Simon (emeritus) / Löw, Simon (emeritus)
09494 - Saar, Martin O. / Saar, Martin O.
03953 - Robertsson, Johan / Robertsson, Johan
02282 - SCCER-SoE / SCCER-SoE
Notes
It was possible to publish this article open access thanks to a Swiss National Licence with the publisher.
Funding
169178 - A decameter-scale reservoir stimulation experiment - the full hydro-mechanical response of a fault zone to high-pressure water injection (SNF)