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| Titel |
Brittle Creep of Tournemire Shale: Orientation, Temperature and Pressure Dependences |
| VerfasserIn |
Zhi Geng, Audrey Bonnelye, Pierre Dick, Christian David, Mian Chen, Alexandre Schubnel |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141782
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| Publikation (Nr.) |
 EGU/EGU2017-5325.pdf |
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| Zusammenfassung |
| Time and temperature dependent rock deformation has both scientific and socio-economic
implications for natural hazards, the oil and gas industry and nuclear waste disposal. During
the past decades, most studies on brittle creep have focused on igneous rocks and porous
sedimentary rocks. To our knowledge, only few studies have been carried out on the brittle
creep behavior of shale.
Here, we conducted a series of creep experiments on shale specimens coming from the
French Institute for Nuclear Safety (IRSN) underground research laboratory located in
Tournemire, France. Conventional tri-axial experiments were carried under two
different temperatures (26˚ C, 75˚ C) and confining pressures (10 MPa, 80 MPa), for
three orientations (σ1 along, perpendicular and 45˚ to bedding). Following the
methodology developed by Heap et al. [2008], differential stress was first increased to ∼
60% of the short term peak strength (10−7/s, Bonnelye et al. 2016), and then in
steps of 5 to 10 MPa every 24 hours until brittle failure was achieved. In these
long-term experiments (approximately 10 days), stress and strains were recorded
continuously, while ultrasonic acoustic velocities were recorded every 1∼15 minutes,
enabling us to monitor the evolution of elastic wave speed anisotropy. Temporal
evolution of anisotropy was illustrated by inverting acoustic velocities to Thomsen
parameters. Finally, samples were investigated post-mortem using scanning electron
microscopy.
Our results seem to contradict our traditional understanding of loading rate
dependent brittle failure. Indeed, the brittle creep failure stress of our Tournemire shale
samples was systematically observed ∼50% higher than its short-term peak strength,
with larger final axial strain accumulated. At higher temperatures, the creep failure
strength of our samples was slightly reduced and deformation was characterized with
faster ‘steady-state’ creep axial strain rates at each steps, and larger final axial strain
accumulated.
At each creep step, ultrasonic wave velocities first decreased, and then increased
gradually. The magnitude of elastic wave velocity variations showed an important orientation
and temperature dependence. Velocities measured perpendicular to bedding showed increased
variation, variation that was enhanced at higher temperature and higher pressure. The case of
complete elastic anisotropy reversal was even observed for sample deformed perpendicular to
bedding, with a reduction amount of axial strain needed to reach anisotropy reversal at higher
temperature.
Our data were indicative of competition between crack growth, sealing/healing, and
possibly mineral rotation or anisotropic compaction during creep. SEM investigation
confirmed evidence of time dependent pressure solution and crack sealing/healing. Our
research not only has practical engineering consequence but, more importantly, can provide
valuable insights into the underlying mechanisms of creep in complex media like shale. In
particular, our study highlights that the short-term peak strength has little meaning in shale
material, which can over-consolidate importantly by ‘plastic’ flow. In addition, we
showed that elastic anisotropy can switch and even reverse over relatively short time
periods (<10 days) and for relatively small amount of plastic deformation (<5%). |
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