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Title: Frequency-dependent anisotropy in partially saturated rocks
Author: Jin, Zhaoyu
ISNI:       0000 0004 8497 6456
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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Seismic wave propagation through fractured rocks is greatly influenced by the fracture system and fluid content. This has become increasingly apparent from recent laboratory measurements and field observations which have shown evidence that seismic anisotropy varies systematically with frequency and fluid saturation. Exploiting this behaviour for reservoir characterization relies on the availability of adequate theoretical rock physics models. Many previous theories on frequency-dependent anisotropy have been limited to the single fluid assumption, despite the fact that almost all reservoirs are partially saturated. This thesis focuses on improving the understanding of multiphase saturation effects on anisotropic seismic wave response in reservoirs containing aligned fractures. I do this by theoretically deriving expressions for the anisotropic frequency-dependent elastic constants. These depend on the relative mobilities of the saturating fluids and the coupled impact of 'squirt flow' and 'patch saturation' effects, which have previously been considered independently, on anisotropic seismic wave propagation. The effect of relative permeability is pronounced; fluid mobility can be lower in partially saturated rocks compared to the fully saturated case, and this can lead to a stiffening which dominates compressibility effects. This can result in unexpected non-monotonic relationships between moduli and water saturation, complicating attempts to invert saturation from seismic data. I test the theory against laboratory measurements carried out at the National Oceanography Centre, Southampton (NOCS) of S-wave splitting and P-wave anisotropy in synthetic fractured sandstones saturated by mixtures of both water/air and brine/CO2. I demonstrate that both squirt and patch mechanisms are significant for the interpretation of saturation effects on frequency-dependent anisotropy. I use the theory to predict the impact of anisotropic dispersion on azimuthal P-wave reflections based on an sand model with transverse isotropy with horizontal axis of symmetry (HTI) overlaid by shale with transverse isotropy with vertical axis of symmetry (VTI). Although VTI in the overburden does not lead to azimuthal anisotropy, its effect on angle dependence could significantly affect the azimuthal Amplitude-Versus-Offset (AVO) responses at far offsets. I show a modest effect on the amplitude and large effect on the phase, the latter of which could even be mistaken for azimuthal velocity variations. I present a Bayesian inversion based on a forward modelling technique aimed at recovering water saturation, fracture density and fracture length of an HTI sand. The results show potential of using frequency-dependent azimuthal AVO for the discrimination of large-scale fractures from micro-scale cracks.
Supervisor: Chapman, Mark ; Main, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: seismic anisotropy ; rock physics ; partial saturation ; squirt flow ; fractures ; AVO