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| Titel |
Determining the frequency dependence of elastic properties of fractured rocks |
| VerfasserIn |
Benedikt Ahrens, Jörg Renner |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128210
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| Publikation (Nr.) |
 EGU/EGU2016-8176.pdf |
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| Zusammenfassung |
| In the brittle crust, rocks often contain joints or faults on various length scales that have a
profound effect on fluid flow and heat transport, as well as on the elastic properties of rocks.
Improving the understanding of the effect of fractures and the role of stress state and
heterogeneity along the fractures on elastic properties of rocks is potentially important for the
characterization of deep geothermal reservoirs. Seismic surveys, typically covering a
frequency range of about 1 to 1000 Hz, are a valuable tool to investigate fractured
rocks but the extraction of fracture properties remains difficult. The elementary
frequency-dependent interaction between fractured rock matrix and viscous pore
fluids and the resulting effects on wave propagation require well-founded dispersion
analyses of heterogeneous rocks. In this laboratory study, we investigate the stress
dependence of the effective elastic properties of fractured reservoir rocks over a broad
frequency range. To assess the effect of faults on the effective elastic properties, we
performed cyclic axial loading tests on intact and fractured samples of Solnhofen
limestone and Padang granodiorite. The samples contained an idealized fault, which
was created by stacking two sample discs on top of each other that experienced
various surface treatments to vary their roughness. The dynamic loading tests were
conducted with frequencies up to 10 Hz and amplitudes reaching 10% of the statically
applied stress. Simultaneously, P- and S-wave measurements were performed in
the ultrasonic frequency range (above 100 kHz) with a total of 16 sensors, whose
positioning above and below the samples guarantees a wide range of transmission and
reflection angles. Preliminary results of static and dynamic elastic properties of
intact Padang granodiorite show a pronounced increase in Young’s moduli and
Poisson’s ratio with increasing axial stress. Stress relaxation is accompanied by a
decrease of the modulus and the Poisson’s ratio. The pressure sensitivity of the elastic
parameters reduces with increasing normal stress, likely reflecting the successive
closure of pre-existent microcracks. At constant strain, both parameters exhibit
a small frequency-dependent increase (in the frequency range from 10−2 to 100
Hz). This trend is consistently continued by the ultrasonic measurements (>100
kHz) that yield the highest moduli. Further investigations will focus on the effect of
faults on the stress and frequency dependence of the elastic properties. The chosen
ultrasonic sensor arrangement will allow us to characterize the fault state by calculating
transmission and reflection coefficients and their dependence on incident angle. |
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