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The effect of fault architecture on slip behavior in shale revealed by distributed fiber optic strain sensing
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  • Chet Hopp,
  • Yves Guglielmi,
  • Antonio Pio Rinaldi,
  • Florian Soom,
  • Quinn Wenning,
  • Paul Cook,
  • Michelle Robertson,
  • Maria KAKURINA,
  • Alba Zappone
Chet Hopp
Lawrence Berkeley National Lab, Lawrence Berkeley National Lab

Corresponding Author:[email protected]

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Yves Guglielmi
Lawrence Berkeley National Lab, Lawrence Berkeley National Lab
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Antonio Pio Rinaldi
ETH Zurich,Lawrence Berkeley National Laboratory, ETH Zurich,Lawrence Berkeley National Laboratory
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Florian Soom
Lawrence Berkeley National Lab, Lawrence Berkeley National Lab
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Quinn Wenning
ETH Zurich, ETH Zurich
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Paul Cook
Lawrence Berkeley National Lab, Lawrence Berkeley National Lab
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Michelle Robertson
Lawrence Berkeley National Lab, Lawrence Berkeley National Lab
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Maria KAKURINA
University of Neuchâtel, University of Neuchâtel
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Alba Zappone
ETH Zurich, ETH Zurich
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Abstract

We use Distributed Strain Sensing (DSS) through Brillouin scattering measurements to characterize the reactivation of a fault zone in shale (Opalinus clay), caused by the excavation of a gallery at ∼400 m depth in the Mont Terri Underground Laboratory (Switzerland). DSS fibers are cemented behind casing in six boreholes cross-cutting the fault zone. We compare the DSS data with co-located measurements of displacement from a chain potentiometer and a three-dimensional displacement sensor (SIMFIP). DSS proves to be able to detect in- and off-fault strain variations induced by the gallery excavated 30-50 m away. The total permanent displacement of the fault is ∼200 microns at rates up to 1.5 nm/sec. DSS is sensitive to longitudinal and shear strain with measurements showing that fault shear is concentrated at the top and bottom interfaces of the fault zone with little deformation within the fault zone itself. Such a localized pattern of strain relates to the architecture of the fault that is characterized by a thick, weak layer, slipping at the edges, with no surrounding damage zone. Overall, DSS shows that slow slip may activate everywhere there is a weak fault within a shale series. Thus, our work demonstrates the importance of shear strain on faults caused by remote loading, highlighting the utility of DSS systems to detect and quantify these effects at large reservoir scales.
Jan 2022Published in Journal of Geophysical Research: Solid Earth volume 127 issue 1. 10.1029/2021JB022432