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Discussion papers
https://doi.org/10.5194/se-2019-159
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-159
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 05 Nov 2019

Submitted as: research article | 05 Nov 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Influence of reservoir geology on seismic response during decameter scale hydraulic stimulations in crystalline rock

Linus Villiger1, Valentin Samuel Gischig2, Joseph Doetsch3, Hannes Krietsch3, Nathan Oliver Dutler4, Mohammadreza Jalali5, Benoît Valley4, Paul Antony Selvadurai1, Arnaud Mignan1,6, Katrin Plenkers1, Domenico Giardini1, Florian Amann5, and Stefan Wiemer1 Linus Villiger et al.
  • 1Swiss Seismological Service, ETH Zurich, Zurich, Switzerland
  • 2CSD Ingenieure, Bern, 3097, Switzerland
  • 3Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
  • 4CHYN, University of Neuchâtel, Neuchâtel, Switzerland
  • 5Department of Engineering Geology & Hydrogeology, RWTH Aachen, Aachen, Germany
  • 6Institute of Risk Analysis, Prediction and Management, Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, China

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20 × 20 × 20 m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: i) ductile ones with intense foliation and ii) brittle-ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydro-mechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in-situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b-values, spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle-ductile shear zones, while injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Linus Villiger et al.
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Microseismicity observed during hydraulic stimulations at the Grimsel Test Site L. Villiger, V. Gischig, J. Doetsch, H. Krietsch, M. Jalali, N. Dutler, and F. Amann https://doi.org/10.3929/ethz-b-000276170

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Short summary
Hydraulic stimulation summarizes fracture initiation or reactivation due to high pressure fluid injection. Several borehole intervals covering intact rock and pre-existing fractures were target for high-pressure fluid injections within a decameter scale rock mass. The observed induced seismicity strongly de-pends on target geology, nearby fracture density and deformation mode. The severity of the induced seismicity per experiment counter correlates the observed transmissivity enhancement.
Hydraulic stimulation summarizes fracture initiation or reactivation due to high pressure fluid...
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