Abstract. The success of geological carbon storage depends on the
assurance of permanent containment for injected carbon dioxide
(CO2) in the storage formation at depth. One of the critical elements
of the safekeeping of CO2 is the sealing capacity of the caprock
overlying the storage formation despite faults and/or fractures, which may
occur in it. In this work, we present an ongoing injection experiment
performed in a fault hosted in clay at the Mont Terri underground rock
laboratory (NW Switzerland). The experiment aims to improve our
understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction
of the CO2 with the neighboring intact rock, and (iii) the impact of
the injection on the transmissivity in the fault. To this end, we inject
CO2-saturated saline water in the top of a 3 m thick fault in the
Opalinus Clay, a clay formation that is a good analog of common caprock
for CO2 storage at depth. The mobility of the CO2 within the fault
is studied at the decameter scale by using a comprehensive monitoring system.
Our experiment aims to close the knowledge gap between laboratory
and reservoir scales. Therefore, an important aspect of the experiment is
the decameter scale and the prolonged duration of observations over many
months. We collect observations and data from a wide range of monitoring
systems, such as a seismic network, pressure temperature and electrical
conductivity sensors, fiber optics, extensometers, and an in situ mass
spectrometer for dissolved gas monitoring. The observations are complemented
by laboratory data on collected fluids and rock samples. Here we show the
details of the experimental concept and installed instrumentation, as well
as the first results of the preliminary characterization. An analysis of
borehole logging allows for identifying potential hydraulic transmissive
structures within the fault zone. A preliminary analysis of the injection
tests helped estimate the transmissivity of such structures within the
fault zone and the pressure required to mechanically open such
features. The preliminary tests did not record any induced microseismic
events. Active seismic tomography enabled sharp imaging the fault zone.
A novel electron backscatter diffraction (EBSD) ‐based finite‐element (FE) wave propagation simulation is presented and applied to investigate seismic anisotropy of peridotite samples. The FE model simulates the dynamic propagation of seismic waves along any chosen direction through representative 2D EBSD sections. The numerical model allows separation of the effects of crystallographic preferred orientation (CPO) and shape preferred orientation (SPO). The obtained seismic velocities with respect to specimen orientation are compared with Voigt‐Reuss‐Hill estimates and with laboratory measurements. The results of these three independent methods testify that CPO is the dominant factor controlling seismic anisotropy. Fracture fillings and minor minerals like hornblende only influence the seismic anisotropy if their volume proportion is sufficiently large (up to 23%). The SPO influence is minor compared to the other factors. The presented FE model is discussed with regard to its potential in simulating seismic wave propagation using EBSD data representing natural rock petrofabrics.
Abstract. Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply eroded orogens like the Scandinavian Caledonides allow us to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications of a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can constrain the origin of this reflectivity. To this end, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. For some intervals of the COSC-1 borehole, the core and downhole velocities deviate by up to 2 km s−1. These differences in the core and downhole velocities are
most likely the result of microcracks mainly due to depressurization.
However, the core and downhole velocities of the intervals with mafic rocks
are generally in close agreement. Seismic anisotropy measured in laboratory
samples increases from about 5 % to 26 % at depth, correlating with a transition from gneissic to schistose foliation. Thus, metamorphic foliation has a clear expression in seismic anisotropy. These results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.
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