Analysis of shear-wave response to fractures: A full waveform study of microseismic fracture imaging

Naturally fractured reservoirs are playing an important role in exploration geophysics. As fractures can control the permeability and pore pressure of the reservoir, it is crucial to study the fracture characterisation. The thesis is mainly including the estimated seismic anisotropy from shear-wave splitting (SWS) observations and the study of the S-wave scattering characteristics of fractured media as well. A suite of synthetic fractured media with a broad range of fracture parameters is generated. The range of fracture parameters was chosen based on the numerical simulation and also where there is a lack of research in the literature.
An automated approach of SWS analysis is performed which is suitable to cope with large volume of SWS measurements. The SWS analysis was automatically performed using cross-correlation and eigenvalue minimisation methods by using a cluster analysis technique. The automated quality measuring is obtained from the misfit calculation of both methods to estimate SWS measurements. This method leads to detect 7% and 4% high quality SWS of 6624 SWS measurements for the single and the double fracture sets models, respectively. This method is crucially beneficial as it reduces the number of inspection of SWS measurements. The SWS measurements are obtained from the receivers distribution at near-surface as well as four boreholes. The parametrisation study of SWS shows that the number of models with good SWS decreases with increasing fracture length size. Moreover, by increasing normal and tangential compliance by one order of magnitude while keeping compliance ratio constant leads to models with good SWS in most cases.
The simulation of synthetic microseismic event provides suitable S-wave sources that result in SWS measurements to image fracture parameters (i.e., fracture density and orientation). The δVS, the difference between the fast and slow shear-waves velocities along the raypath, varied between 0% and 14% which is influenced by the fracture density. As the discrete fractures are superimposed in an isotropic medium, so the anisotropy is interpreted in terms of the fracture strike and fracture density by implementing an inversion method based on the effective medium theory (EMT). The inversion was performed for a single fracture set (i.e., HTI) and double orthogonal fracture sets (i.e., orthorhombic symmetry system). The fracture strike inversion is more constrained than the fracture density due to the limited ray coverage and inversion algorithm assumptions.
In the subsequent part of the thesis, I confirm the general scale-dependence of seismic anisotropy and provide new results specific to SWS. I find that SWS develops under conditions when the ratio of wavelength to fracture size (λS /d) is greater than 3, where Rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model (EMM) theory is qualitatively valid. When 1 < λS /d < 3 there is a transition from Rayleigh to Mie scattering, where no effective anisotropy develops and hence the SWS measurements are unstable. When λS /d < 1 I observe geometric scattering and begin to see behaviour similar to transverse isotropy. I find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio. More importantly, I observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 to 2 wavelengths depending on the fracture density and compliance ratio.
Finally, I use different methods including the RMS envelope analysis, shear-wave polarisation distortion, differential attenuation analysis and peak frequency shifting to assess the scattering behaviour of parametrised models in which the propagation direction is either normal or parallel to the fracture surfaces. The quantitative measures show strong observable deviations for fractures size on the order of or greater than the dominant seismic wavelength within the Mie and geometric scattering regime for both propagation normal and parallel to fracture strike. The results suggest that strong scattering is symptomatic of fractures having size on the same order of the probing seismic wave.