The Effect of Fault Architecture on Slip Behavior in Shale Revealed by Distributed Fiber Optic Strain Sensing

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/s. 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 “layer” including an anastomosing network of polished surfaces where clay‐rich rock splits into progressively smaller flakes conferring the entire zone very little cohesion and friction (weak zone), and favoring slipping at the edges of the “layer”. 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.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

“…Comparing the in-situ gouge fabric at Mont Terri URL, formed by faulting of a slightly overconsolidated protolith (max. burial ~ 1350 m) at an overburden of ~ 1000 m (Nussbaum et al 2011(Nussbaum et al , 2017, with the experimentally developed microfabrics, we infer ductile, aseismic deformation behavior of the "Main Fault", in agreement with field studies (Laurich et al 2017(Laurich et al , 2018Hopp et al 2022).…”

“…During the construction and lifetime of a repository, structural discontinuities can destabilize by changes in the effective stress field and may potentially trigger seismic hazard and damage to underground facilities. In the context of deep geological storage of nuclear waste, changes in the stress field may be induced by construction measures (e.g., tunneling activity) that can lead to the failure of the intact rock and/or to reactivation of bedding planes and faults (Lisjak et al 2015;Amann et al 2018;Rinaldi and Urpi 2020;Hopp et al 2022). Furthermore, the alteration of in-situ pore pressure can change the effective stress state.…”

“…There, the "Main Fault" crossing the URL provides an opportunity to study the structural, petrophysical and hydro-mechanical properties of representative tectonic structures. The heterogeneous architecture of the up to 5 m thick fault zone constitutes scaly clay (Vannucchi et al 2003), slickensides on fractures, mineralized veins, anastomosing networks of µm-thin shear zones and fault gouge (Nussbaum et al 2011;Laurich et al 2014Laurich et al , 2017Laurich et al , 2018Jaeggi et al 2017), resulting in complex fault rupture behavior and modification of hydraulic properties when reactivated by gallery excavation and fluid injection (e.g., Guglielmi et al 2017Guglielmi et al , 2020Guglielmi et al , 2021Wenning et al 2021;Hopp et al 2022).…”

Frictional properties of Opalinus Clay: influence of humidity, normal stress and grain size on frictional stability

“…The risk of any damage to the Opalinus Clay formation caused by the mechanical re-activation of faults must be evaluated during the diverse life stages of the geological repository. During the repository construction phases, recent studies highlight how new mining activities can cause mechanical instability on nearby fault zones (Hopp et al 2022). Similar excavation damages can be expected all through the operational and closing phases of underground galleries (Nussbaum et al 2011;Popp et al 2008).…”

“…In addition to the effects on the near field (i.e. EDZ), a recent study by Hopp et al (2022) reported small displacements (∼200 μm) at slow rates (up to 1.5 nm s -1 ), tens of meters away from the excavation site of new tunnels. The Authors located these mechanical instabilities at the boundary of the fault zone, where frictionally weak clay flakes are abundant.…”