Study On Methodology Of Pre-stack Seismic Inversion For Azimuthal Anisotropic Medium

In unconventional reservoirs,fracture can not only be the migration channels of hydrocarbon,but also can be the reservoir space of oil and gas.So,azimuthal anisotropic inversion in fracture reservoirs plays an important role in estimating reservoir production and selecting favorable fracture zone.Fracture development will make the reservoir exhibits azimuthal anisotropic,which reflected in the pre-stack seismic data is that different azimuthal seismic have different amplitude and other attributes.Therefore,the pre-stack inversion based on azimuthal seismic data can obtain relevant parameters about fracture reservoir,and then those parameters can be used to reservoir prediction.This paper begins with the basic method of azimuthal anisotropic medium.We summed up several basic types of anisotropic medium,introduces the petrophysical parameters describe the azimuthal anisotropic,and gives the conversion relationship between the anisotropic parameters and fracture parameters.As the well-log of anisotropic parameters is hard to obtain,we present an method to calculate the well-log of anisotropic parameters based on the relationship between anisotropic parameters and the velocity of fast and slow shear wave.Reflection coefficient approximation equation is the basis theory of pre-stack seismic inversion.Combined with precise azimuthal reflection coefficient equation,this paper describes the derivation of the azimuthal reflection coefficient approximation equation.Then we analyzes the sensitivity of the approximation equation,and gives the root reason why the inversion of anisotropic parameters is instability.Meanwhile,we note a new method to solve the instability of anisotropic parameters inversion.Based on the azimuthal reflection coefficient approximation equation,combined with the azimuthal pre-stack seismic data of fracture reservoir,this paper constructs a pre-stack seismic inversion process for azimuthal anisotropic reservoir.Model and real data test show that the anisotropic parameters inversion and the degree of anisotropy prediction have a high similar on the real well-log.Real data tests show that this method can reach a good effect on anisotropic parameters inversion and the degree of reservoir's anisotropy in fracture reservoir.According to the unique property of the anisotropic parameters in the sensitivity analysis of azimuthal reflection coefficient approximation equation,combined with perturbation theory,we propose a fracture parameter inversion method based on perturbation elastic.The model test shows that this method has good stability and reliability.Furthermore,we compared the fracture parameters inversion results of this method and the method based on azimuthal elastic impedance,and found that the results based on perturbation elastic impedance is much more accurate and reliable for the fracture reservoir description.This method has a broader applicability.