Research On Pre-stack Seismic Inversion Method Of Shale Oil And Gas Reservoirs

This paper based on the basic theory of seismic anisotropy,taking into consideration the presence of vertical fractures and horizontal bedding of shale oil and gas reservoirs at the same time.When only considering vertical fractures,shale reservoirs can be equivalent to transversely isotropic media with a horizontal axis of symmetry(HTI media);When only considering horizontal bedding,shale reservoir can approximately equivalent to transversely isotropic media with a vertical axis of symmetry(VTI media).By combined the definition of Thomsen anisotropy parameters of HTI media and VTI media,the relationship were constructed between the anisotropy parameters and vertical fractures and horizontal bedding,respectively,and lay the foundation for establish the shale rock physics model.Comprehensive considering the anisotropy characterization of vertical fracture and horizontal bedding of shale,established an anisotropic shale rock physics model for shale oil and gas reservoirs first.Use the shale rock physics model,and combined with conventional logging data(porosity,clay content,water saturation,density and acoustic time,etc.)can be calculated elastic matrix of underground rock.According to the obtained elastic matrix,not only can predict the S-wave velocity of shale reservoirs,but also can estimate anisotropy parameters.When only considering the impact of vertical fracture to anisotropy parameters,the formation can be equivalent to HTI media.Combined with shale rock physics model,from YPD(Young's modulus-Poisson's ratio-density)reflection coefficient approximation formula derived the YPD elastic impedance equation,using this equation direct inversion azimuthal Young's modulus.Then improve the traditional least-squares ellipse fitting algorithm and gat a robust least squares ellipse fitting algorithm,using the improved algorithm to the azimuthal Young's modulus,the density of fracture can be denoted by ellipticity,the strike of the fracture can be denoted by the long axis of the ellipse,thus achieve the formation fracture strike and fracture density prediction.When only considering the impact of horizontal bedding to anisotropy parameters,the shale can be equivalent to VTI media.From Ruger approximate formula,in order to avoid the weak contribution of anisotropy parameters to the reflection coefficient,derived elastic impedance equation that eliminate the isotropic background media influence,only retaining the part of the anisotropic elastic impedance,“increase” the contribution of the anisotropy parameters to the reflection coefficient.Based on this idea,a new method of direct inversion anisotropic parameters was proposed,is more accurate,and combines with the relationship of anisotropic parameters and horizontal bedding,finally achieve pre-stack seismic inversion of polarization anisotropic,and realize the purposes of evaluate the shale formation horizontal bedding.