Seismic Response And Inversions For Orthorhombic Shale Gas Reservoir

The seismic response of shale generally has strong anisotropy,which is dominated by VTI(Vertical Transverse Isotropy,transverse isotropy with a vertical axis of symmetry).The layer with high-angle fractures shows horizontal transverse isotropy property(HTI).The azimuthal anisotropy of seismic can be used for fracture description and in-situ stress prediction of shale reservoirs with high-angle fractures.The development of fractures is of great significance to the storage and development of shale gas.The presence of high-angle fractures make the shale show complex orthorhombic properties(strong VTI + weak HTI).Existing rock-physics models,anisotropic seismic inversion and sweet spot prediction methods are failed to be applied in the shale reservoir with complex and strong anisotropy characteristics.This paper conducts research on orthorhombic rock-physics modeling,orthorhombic seismic inversion and sweet spot seismic prediction methods for fractured shale reservoirs.Seismic interpretation of sweet spots in fractured shale reservoirs is applied in the field application.First,a rock physical model for fractured shale with orthorhombic characteristics considering the distribution of fracture dip is established by combining the anisotropic equivalent medium theory with the linear slip theory.The rock physical model can accurately model strong VTI and weak HTI azimuthal anisotropy of Wufeng-Lomgmaxi shale.The sensitive parameters of fracture characteristic parameters are analyzed and discussed,and provide a theoretical basis for the research of orthorhombic inversion method and sweet spot prediction method.Second,a nonlinear seismic inversion method based on hybrid quantum genetic algorithm(HQGA)is developed for anisotropy assumption with many parameters to be determined.By introducing adaptive rotation strategy,quantum revolving gate and quantum catastrophe,an accurate and efficient multi-parameter global inversion is developed.Third,an approximation formula of orthorhombic reflection coefficient for fractured shale reservoirs is proposed based on VTI background medium parameters and fracture-related sensitive parameters,and the trade-off between parameters are eliminated and the parameters to be inverted are reduced.The stepwise inversion strategy for stable inversion is further introduced: first,invert the parameters that are not related to the azimuth,and then invert the fracture-related parameters through the difference of seismic data in different azimuths,so as to conduct the seismic inversion in orthorhombic anisotropic media.The reliability of the method is verified by the application of azimuth seismic data in the synthetic seismic data and actual field.Finally,combined with the methods mentioned above,the seismic prediction of fracture parameters,TOC,lithofacies and brittleness is carried out for shale gas reservoirs.Through analyzing the rock-physics characteristics of the logging data,the elastic parameters and anisotropy parameters that are sensitive to the sweet spot of the shale reservoir are clarified.The statistical rock-physics method and support vector regression algorithm enable sweet spots(fracture density,TOC,lithofacies and brittleness)to be predicted at the seismic scale,and finally the seismic characterization of fractured shale reservoir sweet spots is conducted in the synthetic and field application.