Study On Methodology Of Pre-stack Seismic Inversion For Fractured Medium

Studies have confirmed that most of the reservoirs found in the world have natural fractures,and natural fractures are favorable for fluid storage and flow.The development of fractures is related to the reserves and mining difficulties of reservoirs,so the identification of fractures is very important for oil and gas exploration.The vertically oriented fracture formation can be equivalent to a HTI medium,which is a transversely isotropic medium with a horizontal symmetry axis.In the HTI medium,the velocity of seismic wave and the amplitude of reflected wave vary with the incidence angle and azimuth angle.The development of fractures results in azimuthal anisotropy.Therefore,the pre stack seismic inversion method based on azimuth seismic data can be used to obtain reservoir fracture parameters and predict reservoir fractures.Based on the basic types of anisotropic medium and based on the rock physical parameters describing the fractured medium,the theoretical framework of seismic rock physical modeling in fractured medium is introduced.The approximate equation of reflection coefficient is the theoretical basis of pre stack seismic inversion.Based on the elastic matrix of fractured medium,based on the exact equation of the azimuth reflection coefficient,this paper introduces the deduction process of the approximate equation of the reflection coefficient of the characterization of the fracture density,and compares and analyzes the characteristics of the azimuth reflection coefficient of the four types of gas bearing sandstone models.Based on the approximate equation of the azimuth reflection coefficient,the author established the anisotropic elastic impedance formula based on the crack density,connected the anisotropic elastic impedance to the rock physical parameters of the crack,and combined with the pre stack azimuth seismic data to construct a pre stack seismic inversion method for the fractured medium.The model data and the actual data test show that the method is in good agreement with the true log value in the seismic inversion of the fracture density before the stack density,and reduces the accumulation error easily produced in the intermediate process.Because the fracture density can also be predicted by the ground stress,the ground stress field can be predicted by the crack weakness.Therefore,in view of the problem of the unstable inversion of the fracture parameters in the fractured medium,this paper presents a method for the simultaneous inversion of the crack weakness by using the Bayesian theory.Model test and actual data test show that the method has good stability and reliability.