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| Titel |
Frequency, pressure, and pore fluid dependence of elastic wavespeeds: new laboratory measurements on Fontainebleau sandstone |
| VerfasserIn |
Emmanuel David, Jerome Fortin, Alexandre Schubnel, Yves Guéguen, Robert W. Zimmerman |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053612
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| Zusammenfassung |
| Measuring and modelling the frequency dependence of elastic moduli in the laboratory
has been proven to be a very difficult task. Elastic properties are generally only
accessible either in the very low-frequency regime (static), or the high frequency,
ultrasonic range (~1 MHz), but there are currently very few sets of data available
between the two limiting regimes. This is of crucial interest for correctly interpreting
measurements of wavespeeds in the field that are in the seismic (~1 Hz) or sonic (~1 kHz)
band.
We have carried out a series of experiments on two hydrostatically stressed samples of
Fontainebleau sandstone, having porosities of 4% and 17%, and permeabilities of around 1
milliDarcy and 1 Darcy, respectively, using a new apparatus recently commissioned at the
Ecole Normale Superieure of Paris. The samples were saturated with three different fluids
(argon, glycerine, water), with the pore pressure in each case held constant at 5
MPa.
Strain, pore volume change, permeability, and ultrasonic (1 MHz) velocities have been
recorded, as well as low-frequency values for the bulk modulus (K) in the range 10-3 to 1
Hz, as a function of hydrostatic confining pressure, up 95 MPa. The low-frequency
measurements of K were obtained using the stress-strain method (Adelinet et al., GRL,
2010), by oscillating the confining pressure. Very sensitive semiconducting strain gauges,
glued on the sample, allowed us to measure very low strain amplitudes (of the order 10-6,
and even down to 10-7), thereby avoiding issues such as inelasticity or amplitude
effects.
At a confining pressure of 10 MPa, on both Fontainebleau sandstone samples (4% and
17%), a piezoactuator was placed between the axial load piston and the end cap in
order to produce small axial stress oscillations of a few tens of kPa. The stress
oscillations amplitude was measured by an aluminium force gauge placed next
to the sample in the jacket, equipped with semiconducting strain gauges as well.
Again, the axial strain was measured directly on the rock (down to 10-7) using
the semiconducting strain gages. This new design worked successfully. In such
a way, we have obtained measurements of Young’s modulus E, in the complete
frequency range going from 10-3 to 200 Hz, and with three different saturation
fluids.
This combination of techniques allows us to measure a set of two elastic constants over a
large frequency range (low and high frequencies), and under pressure, which had never been
performed before in the laboratory.
Here we present these new experimental results, and discuss some possible
interpretations in terms of poroelasticity theory and pore pressure relaxation mechanisms. |
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