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Preliminary stress characterization for an in-situ stimulation experiment at the Grimsel Underground Laboratory
Auteur(s)
Krietsch, Hannes
Doetsch, J.
Gischig, Valentin
Amann, Florian
Jalali, Mohammadreza
Madonna, Claudio
Evans, Keith F.
Giardini, Domenico
Wiemer, Stefan
Maurer, Hansruedi
Loew, Simon
Date Issued
2016-4-1
Abstract
A 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.
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.
Notes
, 2018
Event name
EGU General Assembly Conference Abstracts
Location
Vienna
Publication type
conference paper
