Marcel Frehner ETH Zurich, Switzerland, frehner@erdw.ethz.ch

1. Interaction of seismic background noise with oscillating pore fluids causes spectral modifications of passive seismic measurements at low frequencies Marcel Frehner ETH Zurich, Switzerland, frehner@erdw.ethz.ch Stefan M. Schmalholz ETH Zurich, Switzerland Yuri Podladchikov University of Oslo, Norway

2. Voitsdorf area, Austria, 2005Spectraseis survey for RAG | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Motivation: Observed spectral variations above oil • Long-time continuous passive seismic measurements • Fourier transformation • Characteristic spectral variations can be used to detect oil (Spectraseis).

3. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Motivation: Time reverse modeling • Time reverse modeling of elastic wave propagation using measured ground motion velocities • Low-frequency source signals within known reservoirs. Steiner et al., submitted

4. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook So far so good…… but • What is the physical mechanism that causes the observed spectral modification at low frequencies?

5. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Two potential mechanisms • Resonantscattering(patchy saturation) • Resonantamplification(surface tension)

6. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Resonance of trapped oil blobs:Resonance is important Hilpert et al, Geophysics, 2000 We investigate the excitation by sound waves of capillary trapped oil blobs. […] We derive approximate, analytical expressions for the resonance of oil blobs in capillary tubes […]. Based upon these simple model systems, we conclude that resonance of oil blobs is significant for coarse-grained but not fine-grained media.

7. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Resonance of trapped oil blobs:Oil in a pore can be treated as oscillator Beresnev, Geophysics, 2006 Quantitative dynamics of a non-wetting ganglion of residual oil entrapped in a pore constriction and subjected to vibrations of the pore wall can be approximated by the equation of motion of an oscillator moving under the effect of the external pressure gradient, inertial oscillatory force, and restoring capillary force.

8. w0 | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Resonance of trapped oil blobs: Numerical simulation Holzner et al., Comm. in Nonlinear Science and Numerical Simulation, 2007 • Full Navier-Stokes equations • Surface tension taken into account • One simulation for each frequency • Calculate response of centerof mass of oil blob • Resonance curve like that of a harmonic oscillator

9. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook But still… • How can these oscillations be transferred to the earth surface? • Coupling micro-scaleoscillations with macro-scaleelastic wave propagation?

10. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Coupling oscillations with elastic rock • 4 contributions to total energy

11. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Coupling oscillations with elastic rock • Hamilton’s variational principle • Equations of motion

12. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Numerical model setup • Explicit 1D finite differences • Staggered grid in space(Virieux, 1986) • Predictor-correctormethod in time • Non-reflecting orrigid boundaries(Ionescu & Igel, 2003)

13. Anderson and Hampton, 1980J. of Acoustical Society of AmericaGas bubbles in water | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Eigenvalues This study

14. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Energy conservation and transfer Solid velocity Fluid velocity

15. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Incident elastic wave

16. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Incident elastic wave

17. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Spectra over time

18. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Conclusions • Presented wave propagation-oscillation model shows • To initiate fluid oscillations, energy is taken from thesolid at resonance frequency (Trough in solid spectrum) • As fluid continues to oscillate, energy is transferred back fromfluid to solid at resonance frequency (Peak in solid spectrum) • After their initialization, oscillations decay differently for different reservoir thicknesses (Thickness information) • Implications • Solid spectrum is not expected to always show a peak at resonance frequency (Implication for data analysts: long time signals) • Current model requires transient pulses to initiate oscillations(e.g. discrete pulses, continuous pulses with varying amplitudes)

19. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Open questions / Future work • Scale of oscillations? • Influence of saturation level? • Complex pore geometry:non-linearities • More complete physical modele.g. 3-phase mixture model

20. | Motivation | Oscillation | Coupling | Numerics | Results | Conclusions | | Outlook Ongoing related research • Markus Hilpert, Johns Hopkins University, USA: Lattice-Bolzmann modeling, mobilization of trapped oil blobs • Holger Steeb, Saarland University, Germany:3D 3-phase mixture theory, inclusion of surface tension effects Snapshots published inHilpert, 2007 J. Colloid and Interface Science

21. Acknowledgement • Spectraseis AG, Switzerland forproviding passive seismic dataand financial support • Swiss Commission forTechnology and Innovation KTIfor financial support

22. Thank you