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| Titel |
Susceptibility of experimental faults to pore pressure increase: insights from load-controlled experiments on calcite-bearing rocks |
| VerfasserIn |
Elena Spagnuolo, Marie Violay, Stefan Nielsen, Chiara Cornelio, Giulio Di Toro |
| 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 |
250153198
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| Publikation (Nr.) |
 EGU/EGU2017-18143.pdf |
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| Zusammenfassung |
| Fluid pressure has been indicated as a major factor controlling natural (e.g., L’Aquila, Italy,
2009 Mw 6.3) and induced seismicity (e.g., Wilzetta, Oklahoma, 2011 Mw 5.7). Terzaghi’s
principle states that the effective normal stress is linearly reduced by a pore pressure (Pf)
increase σeff=σn(1 − αPf), where the effective stress parameter α, may be related to the
fraction of the fault area that is flooded. A value of α =1 is often used by default, with Pf
shifting the Mohr circle towards lower normal effective stresses and anticipating failure on
pre-existing faults. However, within a complex fault core of inhomogeneous permeability, α
may vary in a yet poorly understood way. To shed light on this problem, we conducted
experiments on calcite-bearing rock samples (Carrara marble) at room humidity
conditions and in the presence of pore fluids (drained conditions) using a rotary
apparatus (SHIVA). A pre-cut fault is loaded by constant shear stress τ under constant
normal stress σn=15 MPa until a target value corresponding roughly to the 80 %
of the frictional fault strength. The pore pressure Pf is then raised with regular
pressure and time steps to induce fault instability. Assuming α=1 and a threshold for
instability τp_eff=μp σeff, the experiments reveal that an increase of Pf does not
necessarily induce an instability even when the effective strength threshold is largely
surpassed (e.g., τp_eff=1.3 μpσeff). This result may indicate that the Pf increase did
not instantly diffuse throughout the slip zone, but took a finite time to equilibrate
with the external imposed pressure increase due to finite permeability. Under our
experimental conditions, a significant departure from α=1 is observed provided that
the Pf step is shorter than about < 20s. We interpret this delay as indicative of
the diffusion time (td), which is related to fluid penetration length l by l = √ κtd-,
where κ is the hydraulic diffusivity on the fault plane. We show that a simple cubic
law relates td to hydraulic aperture, pore pressure gradient and injection rate. We
redefine α as the ratio between the fluid penetration length and sample dimension L
resulting in α = min(√ktd,L)
L. Under several pore pressure loading rates this relation
yields an approximate hydraulic diffusivity κ ∼10−8 m2 s−1 which is compatible,
for example, with a low porosity shale. Our results highlight that a high injection
flow rate in fault plane do not necessarily induce seismogenic fault slip: a critical
pore penetration length or fluid patch size is necessary to trigger fault instability. |
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