Stress Measurements in Crystalline Rock: Comparison of Overcoring, Hydraulic Fracturing and Induced Seismicity Results
- H. Krietsch (Geological Institute) | V. Gischig (Geological Institute) | M. R. Jalali (Geological Institute) | F. Amann (Geological Institute) | K. F. Evans (Geophysical Institute ETH) | J. Doetsch (Geophysical Institute ETH) | B. Valley (University of Neuchâtel)
- Document ID
- American Rock Mechanics Association
- 51st U.S. Rock Mechanics/Geomechanics Symposium, 25-28 June, San Francisco, California, USA
- Publication Date
- Document Type
- Conference Paper
- 2017. American Rock Mechanics Association
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- 65 since 2007
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ABSTRACT: 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. The indicate that the minimum principal stress, σ3, is sub-horizontal and oriented north-south. The maximum principal stress, σ1, dips at 30-40° to the east and has a magnitude of 13-17 MPa. The magnitude of σ2, is similar to σ3, and lies in the range 8-10 MPa.
To accelerate the development of Enhanced Geothermal System (EGS) technology, the In-situ Stimulation and Circulation (ISC) experiment is being conducted at the Grimsel Test Site (GTS), Switzerland to investigate fundamental processes activated by injections into preexisting faults and shear zones. This decameter scale experiment aims to address open questions concerning induced seismicity, permeability enhancement, and hydro-mechanical fault response during deep EGS reservoir stimulation operations. Further information can be found in Amann et al. (2017).
Knowledge of the in-situ stress situation within the rock mass is crucial to the design of the experiment and the eventual interpretation of the data, since stress dictates whether slip will occur on pre-existing discontinuities in response to the elevated pressure of the injections, and also controls the magnitude and direction. Thus, a stress measurement campaign that combined stress relief and hydraulic methods was conducted in three differently oriented boreholes (Figure 1). Borehole SBH-1 was drilled with a slight inclination to vertical of 15° towards N260°E, the best-estimate for the direction of σ3 suggested by Pahl et al., (1989) and Konietzky (1995). The SBH-3 borehole was drilled sub-horizontally towards the “best-estimate” direction of σ2 with the goal of obtaining a direct measure of the sub-vertical stress from the hydrofracture stress tests. SBH-1 and SBH-3 were drilled into nearly undisturbed rock to observe the stress conditions in the ‘far-field’ of the shear zone. The SBH-4 borehole was sub-horizontal and drilled orthogonal to SBH-3 so as to penetrate one of the shear zones that were the target for the stimulations with the objective of identifying potential stress changes towards the fault.
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