Azimuthal Seismic Inversion for Shale Gas Reservoir Characterisation

Abstract

Summary Azimuthal AVO analysis can be split into two parts: the Amplitude Versus Offset (AVO) analysis and the Amplitude Versus Azimuth (AVAz) analysis. The properties of the Fourier transform allow these problems to be treated separately. If we calculate a Fourier transform of AVAz PP reflectivity data at a particular angle of incidence, we obtain Fourier Coefficients (FCs) that parsimoniously describe the AVAz and anisotropy, similar to the near offset Rüger equation. It is possible to attach physical significance to each of the FCs using simple rock physics models that relate the anisotropy to the fractures. The different FCs may be combined in a non-linear fashion to estimate fundamental fracture parameters including fracture intensity and orientation. The FCs are calculated using a limited range of offsets or angles, which places less demands on the data acquisition. This may be advantageous for 3D land data sets, where near offsets are particularly under sampled. However, reflectivity-based fracture characterization techniques show some limitations in terms of interpretation of the results. These limitations can be removed by using the azimuthal elastic inversion which provides layer properties that are easier to interpret and relate more directly to the geology. Introduction Ikelle (1996) and then Sayers and Dean (2001) demonstrated that it is possible to express the azimuthal AVO reflectivity response in terms of a Fourier series as a function of azimuth. Due to the orthogonality of the Fourier transform, these Fourier Coefficients (FCs) represent independent information and may be used in combination or individually to make inferences about fracture-induced seismic anisotropy. Fracture properties can be calculated using just one offset or angle of incidence and we can obtain results similar to those obtained by the near offset Rüger equation using multi-offset/angle data. Combinations of FCs may be used to obtain unbiased estimates of both the anisotropic gradient and the fracture orientation. The FCs and near offset Rüger methods share the same limitation that amplitudes are treated as reflectivities, i.e. the wavelet effect is still part of the solution. Moreover, the estimates are interface properties which are more difficult to relate to the geology than interval properties. To overcome these issues, Downton and Roure (2010) introduced an azimuthal elastic inversion where azimuthal angle stacks are inverted simultaneously. This method was later extended to invert azimuthal Fourier Coefficients (Roure and Downton, 2012). The fracture properties estimated by the elastic inversion can be more easily used for interpretation.