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| Titel |
From dilatancy to contraction: Stress-dependent failure mode progression in two porous sandstones subjected to true triaxial testing |
| VerfasserIn |
Xiaodong Ma, Bezalel Haimson |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077869
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| Zusammenfassung |
| Porous sedimentary rocks such as sandstones are typical oil-bearing formations in which
failure due to high stress concentration is likely to occur during wellbore drilling and
subsequent operations. The objective of this research was to investigate the effect of Ïă2 on
strength, failure-plane angle, and failure mode under realistically simulated field conditions
(Ïă1 -¥Ïă2 -¥Ïă3). A series of true triaxial compression tests were conducted on two
representative porous sandstones: Coconino (17.5% porosity, 99% quartz, with rounded and
well-sorted 0.1 mm grains that are bonded by suturing and some quartz overgrowth), and
Bentheim (24% porosity, 95% quartz, with sub-rounded 0.3 mm grains that are bonded
exclusively by suturing).
Square cuboidal specimens (19 x 19 x 38mm) were subjected to independent loads in
three principal directions, using the University of Wisconsin testing apparatus, creating a true
triaxial state of stress (Ïă1 -¥Ïă2 -¥Ïă3). In all tests, Ïă3 and Ïă2 were maintained constant at
predetermined levels, while Ïă1 was raised monotonically until failure occurred. The
magnitude of Ïă3 varied between 0 and 150 MPa, covering the range of brittle behavior,
brittle-ductile transition, and the threshold to the ductile zone in the weaker Bentheim
sandstone. It was found that in both rocks the compressive strength (Ïă1,peak) for a
given Ïă3 increases as the preset Ïă2 is raised between tests, and reaches a peak (15%
over Ïă1,peak when Ïă2 = Ïă3 in the Coconino, and less than 10% in the Bentheim),
beyond which it gradually drops, such that when Ïă2 - Ïă1,peak, the strength is
approximately the same as when Ïă2 = Ïă3. This strengthening effect is considerably
lower than that in previously tested crystalline rocks, such as Westerly granite and
KTB amphibolite (more than 50%, Haimson, 2006). Plotting the test data in the
Ïoct vs. Ïăoct domain, where the two stress invariants Ïoct, the octahedral shear
stress, and Ïăoct,the mean normal stress, are both taken at failure), Coconino shows
a monotonically rising Ïoct with increasing Ïăoct. In the Bentheim, on the other
hand, the initially rising Ïoct forms a ‘cap’ at about Ïăoct = 200 MPa, an indication
of localized compaction, followed by a sharp decrease for larger magnitudes of
Ïăoct.
Failure characteristics in Coconino sandstone subjected to Ïă3= Ïă2evolve from a dilatant
mode, expressed by a single shear band, or fault, dipping steeply from 80Ë at Ïă3= 0 MPa, to
60Ë at Ïă3= 100 MPa; to a diminishing dilatant mode, signaled by multiple parallel and
conjugate shear bands (a characteristic of brittle-ductile transition) dipping at nearly 50Ë , at
Ïă3 = 120-150 MPa. At low preset Ïă3, failure plane dip increases by as much as 15Ë as Ïă2 is
raised, but the increase gradually drops at higher Ïă3. The role of Ïă2in retarding the
progression of the failure mode is noted, for example, at Ïă3 = 100 MPa, where characteristics
of brittle-ductile transition are gradually reversed as Ïă2is raised. The more porous Bentheim
sandstone is generally weaker. When subjected to Ïă3= Ïă2 between 0 to 60 MPa, its
failure mode is dilatant, leading to a single fault dipping 80Ë at Ïă3 = 0 MPa, to
50Ë at Ïă3 = 60 MPa. For a constant Ïă3, failure-plane angle rises with Ïă2, but the
increase is typically under 10Ë , somewhat less than in the Coconino. In the range
of 60 |
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