Seismic inversion of high-angle fracture indicator for shale gas reservoirs

It is difficult to use a vertical transverse isotropy (VTI) and a horizontal transverse isotropy (HTI) model to characterize fractured shale gas reservoirs with horizontal bedding and several groups of parallel high-angle fractures. However, an orthorhombic anisotropy (OA) model is a feasible alternative. Based on the anisotropic characteristics of high-angle fractures described using the reflectivity equation, an indicator that represents the intensity of high-angle fractures is proposed. We derive an azimuthal elastic impedance equation containing a fracture indicator for OA media from the approximate equation of OA reflection coefficient and an equivalent OA parameter. We also establish a method to invert fracture indicator and fracture azimuthal elastic impedance. To improve the stability of the inversion, the derived azimuthal elastic impedance equation is rewritten in a two-parameter form. A model-based iterative least-squares inversion is used to achieve the robust estimation of fracture indicator and fracture azimuth. A model test demonstrates the reliable estimation of fracture indicator and fracture azimuth even at the signal-to-noise ratio of 2. A practical application also yields reliable predictions. This method is a useful tool to predict shale gas reservoirs with high-angle fractures.