Fracture clustering effect on amplitude variation with offset and azimuth analyses

Abstract

Traditional amplitude variation with offset and azimuth (AVOAz) analysis for fracture characterization extracts fracture properties through analysis of reflection AVOAz to determine anisotropic parameters (e.g., Thomsen's parameters) that are then related to fracture properties. The validity of this method relies on the basic assumption that a fractured unit can be viewed as an equivalent anisotropic medium. As a rule of thumb, this assumption is taken to be valid when the fracture spacing is less than λ∕10. Under the effective medium assumption, diffractions from individual fractures destructively interfere and only specular reflections from boundaries of a fractured layer can be observed in seismic data. The effective medium theory has been widely used in fracture characterization, and its applicability has been validated through many field applications. However, through numerical simulations, we find that diffractions from fracture clusters can significantly distort the AVOAz signatures when a fracture system has irregular spacing even though the average fracture spacing is much smaller than a wavelength (e.g., ≪ λ∕10). Contamination by diffractions from irregularly spaced fractures on reflections can substantially bias the fracture properties estimated from AVOAz analysis and may possibly lead to incorrect estimates of fracture properties. Additionally, through Monte Carlo simulations, we find that fracture spacing uncertainty inverted from amplitude variation with offset (AVO) analysis can be up to 10%-20% when fractures are not uniformly distributed, which should be the realistic state of fractures present in the earth. Also, AVOAz and AVO analysis gives more reliable estimates of fracture properties when reflections at the top of the fractured layer are used compared with those from the bottom of the layer.

Estimation of Fracture Compliance From Attenuation and Velocity Analysis of Full‐Waveform Sonic Log Data

“…The effects of fractures on seismic wave propagation strongly depend on the relation between the characteristic size of the fractures, their separation, and the prevailing seismic wavelengths (e.g., Fang et al, 2017). Many analytical and numerical models have been proposed to study seismic wave propagation in rocks containing cracks or fractures that are much smaller than the wavelengths (e.g., Chapman, 2003;Gurevich, 2003;Hudson, 1980;Rubino et al, 2013;Schoenberg & Douma, 1988;Sil, 2013).…”

Analysis of Spatially Distributed Fracture Attributes: Normalized Lacunarity Ratio

“…As shown in Section 3.1, it can identify the cluster width in the case where fractures occur in clusters and indicate if there is any correlation between the location of such clusters and large fractures. Determining fracture clusters and their properties are important in studying seismic signatures because they can cause AVAz/AVO responses to deviate from those predicted by effective medium theory (Fang et al, 2017). Barabosa et al (2018) suggested that the presence of fracture clusters can result in two frequency regimes of seismic attenuation and velocity dispersion.…”

Impact of poroelastic effects on the inversion of fracture properties from amplitude variation with offset and azimuth data in horizontal transversely isotropic media

“…Probably the most remarkable manifestation of the presence of fractures embedded in an otherwise isotropic background is the effective anisotropic behavior of seismic waves. This has made the analysis of reflection amplitude variations with offset and azimuth (AVOAz) a common and successful practice for retrieving practically important parameters of the fractured formation such as, for example, the density and the azimuthal orientation of the fractures, as well as the type of saturating pore fluid (e.g., Bakulin et al, 2000;Shen et al, 2002;Fang et al, 2016).…”