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| Titel |
The Role of the Intermediate Principal Stress in the Brittle-Ductile Transition of Porous Sandstones |
| VerfasserIn |
Xiaodong Ma, Bezalel Haimson |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107219
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| Publikation (Nr.) |
 EGU/EGU2015-6914.pdf |
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| Zusammenfassung |
| Conventional triaxial experiments reveal that failure mode in some porous sandstones evolves
from brittle, dilatant, localized high-angle shear band at low Ïă2 = Ïă3, to ductile localized
failure in the form of compaction bands at high Ïă2 = Ïă3, to delocalized cataclastic
flow at even higher Ïă2 = Ïă3 (Wong and Baud, 2012). Brittle ductile transition is
characterized by multiple conjugate low-angle shear bands (Paterson and Wong,
2005).
True triaxial experiments (Ïă1 ≈¥ Ïă2 ≈¥ Ïă3) enable the direct observation of the
intermediate principal stress, Ïă2, effect on rock mechanical behavior, including
failure. We conducted a series of such tests on two quartz-rich porous sandstones,
Coconino and Bentheim, and ascertained the effect of Ïă2 on failure as Ïă3 was gradually
raised between tests from 0 to 150 MPa. In Coconino sandstone (17% porosity),
the single, steeply inclined shear band developed at low Ïă3 (= Ïă2), turned into
multiple conjugate, low angle, failure planes, signifying brittle-ductile transition, as Ïă3
(= Ïă2) reached about 100 MPa. However, by raising Ïă2from test to test for the
same Ïă3 (= 100 MPa), the number of conjugate planes diminished and eventually
reduced to one steeper shear band, failure stress rose, and the volumetric strain
associated with failure changed from compactant at Ïă2= Ïă3 to dilatant at Ïă2 > Ïă3.
These observations demonstrate that Ïă2 embrittles Coconino sandstone and retards
failure.
On the other hand, the brittle-ductile transition threshold in Bentheim sandstone (24%
porosity), which occurs at about Ïă3 (= Ïă2) = 60 MPa, is characterized by increased
compaction with the rise in Ïă2, contrary to the Coconino sandstone behavior. A reasonable
conjecture is that the apparent discrepancy in rock deformability as Ïă2 rises above the
constant Ïă3is related to the difference in porosity (and hence stiffness) between the two rocks.
As Ïă2 is raised, the mean stress increases independently of rock type. In the higher porosity
Bentheim sandstone, this brings about inelastic compaction sufficient to overwhelm the
dilatant deformation due to the change in the deviatoric stress. In the Coconino,
however, the lower porosity reduces compaction, so that dilatant deformation prevails. |
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