SEG San Antonio 09/27/ Interaction of seismic background noise with oscillating pore fluids causes spectral modifications of passive seismic measurements at low frequencies Marcel FrehnerETH Zurich, Switzerland, Stefan M. SchmalholzETH Zurich, Switzerland Yuri PodladchikovUniversity of Oslo, Norway

SEG San Antonio 09/27/ Motivation: Observed spectral variations above oil  Long-time continuous passive seismic measurements  Fourier transformation MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |  Characteristic spectral variations can be used to detect oil (Spectraseis). Voitsdorf area, Austria, 2005 Spectraseis survey for RAG

SEG San Antonio 09/27/ 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 MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ So far so good…… but  What is the physical mechanism that causes the observed spectral modification at low frequencies? MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Two potential mechanisms  Resonant scattering (patchy saturation)  Resonant amplification (surface tension) MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ 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. MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ 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. MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ 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 center of mass of oil blob  Resonance curve like that of a harmonic oscillator 00 MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ But still…  How can these oscillations be transferred to the earth surface?  Coupling micro-scale oscillations with macro-scale elastic wave propagation? MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Coupling oscillations with elastic rock  4 contributions to total energy  MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Coupling oscillations with elastic rock  Hamilton’s variational principle  Equations of motion MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Numerical model setup  Explicit 1D finite differences  Staggered grid in space (Virieux, 1986)  Predictor-corrector method in time  Non-reflecting or rigid boundaries (Ionescu & Igel, 2003) MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Eigenvalues This study Anderson and Hampton, 1980 J. of Acoustical Society of America Gas bubbles in water MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Energy conservation and transfer Solid velocity Fluid velocity MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Incident elastic wave MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Incident elastic wave MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Spectra over time MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Conclusions  Presented wave propagation-oscillation model shows  To initiate fluid oscillations, energy is taken from the solid at resonance frequency (Trough in solid spectrum)  As fluid continues to oscillate, energy is transferred back from fluid 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) MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Open questions / Future work  Scale of oscillations?  Influence of saturation level?  Complex pore geometry: non-linearities  More complete physical model e.g. 3-phase mixture model MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ 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 in Hilpert, 2007 J. Colloid and Interface Science MotivationOscillationCouplingNumericsResultsConclusions||||||| Outlook |

SEG San Antonio 09/27/ Acknowledgement  Spectraseis AG, Switzerland for providing passive seismic data and financial support  Swiss Commission for Technology and Innovation KTI for financial support

SEG San Antonio 09/27/ Thank you