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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-78
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
21 Jul 2017
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).
On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity
Valentin S. Gischig1, Joseph Doetsch1, Hansruedi Maurer1, Hannes Krietsch1, Florian Amann1, Keith F. Evans1, Morteza Nejati1, Mohammadreza Jalali1, Benoît Valley2, Anne Obermann1, Stefan Wiemer1, and Domenico Giardini1 1Departement of Earth Sciences, ETH Zürich, Switzerland
2Centre for Hydrogeology and Geothermics (CHYN), Université de Neuchâtel, Switzerland
Abstract. To characterize the stress field at the Grimsel Test Site (GTS) underground rock laboratory a series of hydrofracturing test and overcoring test were performed. Hydrofracturing was accompanied by seismic monitoring using a network of highly sensitive piezo sensors and accelerometers that were able to record small seismic events associated with decimeter-sized fractures. Due to potential discrepancies between the hydro-fracture orientation and stress field estimates from overcoring, it was essential to obtain high-precision hypocenter locations that reliably illuminate fracture growth. Absolute locations were improved using a transverse isotropic P-wave velocity model and by applying joint hypocenter determination that allowed computation of station corrections. We further exploited the high degree of waveform similarity of events by applying cluster analysis and relative relocation. Resulting clouds of absolute and relative located seismicity showed a consistent east-west strike and 70° dip for all hydro-fractures. The fracture growth direction from microseismicity is consistent with the principal stress orientations from the overcoring stress tests provided an anisotropic elastic model for the rock mass is used in the data inversions. σ1 is significantly larger than the other two principal stresses, and has a reasonably well-defined orientation that is subparallel to the fracture plane. σ2 and σ3 are almost equal in magnitude, and thus lie on a circle defined by the standard errors of the solutions. The poles of the microseismicity planes also lie on this circle towards the north. The trace of the hydraulic fracture imaged at the borehole wall show that they initiated within the foliation plane, which differs in orientation from the microseismicity planes. Thus, fracture initiation was most likely influenced by a foliation-related strength anisotropy. Analysis of P-wave polarizations suggested double-couple focal mechanisms with both thrust and normal faulting mechanisms present, whereas strike-slip and thrust mechanisms would be expected from the overcoring-derived stress solution. The reasons for these discrepancies are not well understood, but may involve stress field rotation around the propagating hydrofracture. Our study demonstrates that microseismicity monitoring along with high-resolution event locations provides valuable information for interpreting stress characterization measurements.

Citation: Gischig, V. S., Doetsch, J., Maurer, H., Krietsch, H., Amann, F., Evans, K. F., Nejati, M., Jalali, M., Valley, B., Obermann, A., Wiemer, S., and Giardini, D.: On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity, Solid Earth Discuss., https://doi.org/10.5194/se-2017-78, in review, 2017.
Valentin S. Gischig et al.
Valentin S. Gischig et al.
Valentin S. Gischig et al.

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