Rock physics for frequency-dependent anisotropy
Frequency-dependent geophysical properties have attracted more and more interests from the research communities and the industry, which potentially links the subsurface seismic response to permeability and the dominant scale length of the heterogeneities in the subsurface. The situation regarding this subject is comparable to that of research on seismic anisotropy in the 80s: the potential is high yet more work needs to be done towards both the understanding of the phenomenon and its implications.
During the 5th Phase, we have developed a dynamic rock physical model accounting for changes in both pore pressure and saturation. In the 6th Phase, a range of observable effects was predicted, including the frequency dependence of shear-wave time delays, differential attenuation between fast and slow shear-waves and P-wave attenuation anisotropy. A series of VSP datasets have been analyzed in order to evaluate these effects, including multicomponent VSPs, walkway VSPs and time-lapse VSPs. These studies have confirmed that these effects are detectable in field data and can be related to fracture properties such as dominant scale length and fluid saturation, using robust modelling and inversion procedures.
Phase 7 developed these ideas across a spectrum of activities, encompassing further theoretical development, laboratory calibration, development of processing techniques and application to field data. The goal of the research was to use the modern ideas in spectral decomposition and poroelastic theory to establish a robust link between seismic anisotropy and the petrophysical properties of rock, particularly the anisotropic variation of permeability. We analysed VSP surveys including time-lapse VSP data, to consolidate our understanding.
Phase 8, the new direction was the emphasis on applying our ideas to laboratory measurement and field seismic data. Laboratory measurements have been carried out to understand the anisotropic response of fractured rocks for different fluids and sensitivity of shear-wave splitting to fluid saturation.
A Frequency-dependent AVO inversion scheme was also proposed to estimate seismic dispersion from pre-stack data. The inversion scheme extended Smith and Gidlow(1987)' two-term AVO approximation to be frequency-dependent by using modern spectral decomposition techniques.
At the end of Phase 8 (2012), We further developed Baysian inversion scheme to estimate gas saturation from pre-stack data based on frequency-dependent rock physics model.
Fluid mobility has an infuence on azimuthal AVO, particularly if we consider variations with frequency. The on going study is focused on estimation of fluid mobility from frequency-dependent azimuthal AVO.
Areas of interest and deliverables for the 9th phase
o improved rock physics models, calibrated against field and laboratory data, which relate the frequency dependent anisotropic response to fracturing, fluid saturation and stress state;
o finite difference modeling for wave propagation through a discrete fracture network, to establish the effect of scattering from fractures of different sizes on the anisotropic response;
o use of attenuation measurements for reservoir characterisation, in particular a comprehensive theory for interpretation of azimuthal AVO in the presence of frequency dependent anisotropy;
o Analysis of time-lapse multi-azimuth VSP data to establish the relationship between the anisotropic seismic response and pressure and saturation variations.
o a fully tested comprehensive rock physics subroutine library
Datasets available for the 9th Phase:
o Laboratory measurements of synthetic fractured rocks with known fracture geometries
o Multi-azimuths VSPs from the Wyoming Basin
o Time lapse 3D VSPs from Weyburn
o Time lapse VSPs from Quarnalam/PDO.
o FAVO analysis from Vienna Basin data.