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/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.