A Novel Seismic Anisotropic Dispersion Attribute Inversion Method for Fracture Characterization in Orthorhombic Shale Gas Reservoirs

The detection of vertical fractures using seismic methods is crucial for characterizing shale gas reservoirs. When vertical fractures are aligned within a vertically transverse isotropic (VTI) shale formation, the medium behaves as an orthorhombic system. Seismic waves propagating through such fluid-saturated fractured shales exhibit anisotropic dispersion and attenuation, effects often not adequately considered by conventional fracture detection methods. This paper addresses this limitation by proposing an azimuthal frequency-dependent inversion method to compute anisotropic dispersion attributes for fracture characterization in orthorhombic shales. The method employs an azimuthal amplitude difference scheme to highlight anisotropy’s influence on the reflection coefficient by removing the VTI background effects, enhancing the robustness in anisotropic parameter estimation. A frequency-dependent inversion framework fully utilizes seismic reflection frequency information, while a frequency-scanning scheme improves mathematical rigor over traditional approaches. Numerical examples based on a viscoelastic orthorhombic shale model demonstrate the feasibility of the proposed dispersion attributes for fracture characterization. Synthetic data tests validate their effectiveness in assessing vertical fracture density, and field data applications confirm their reliability through good correlations with FMI images. Finally, a fracture indicator integrating the anisotropic dispersion attributes is developed to characterize vertical fractures in the studied shale reservoirs. This methodology holds potential for advancing the detecting of more complex fracture systems in hydrocarbon reservoirs.