The Study In Azimuthal Pre-stack Seismic Inversion Method Based On Orthorhombic Anisotropy

The fractures in the reservoirs are important in the storage and movement of oil and gas.Nowadays,with the increasement of difficulty of seismic exploration and the development of wide azimuth seismic acquisition and processing,accurate fracture prediction in unconventional reservoirs becomes the focus of recent research.For the unconventional reservoir with two series of orthogonally nearly vertical fractures,it is necessary to carry out the targeted azimuth prestack attribute analysis and seismic inversion method,so as to obtain more reliable fractures prediction resultsThis paper begins with the basic theory of seismic anisotropy,and it introduces us several most fundamental types of anisotropic media.According to the actual situation,the reservoirs with two series of orthogonally nearly vertical fractures can be regarded as orthorhombic anisotropy media.The approximate formula of reflection coefficient of OA media can be transformed into new approximation suitable for the prestack attribute analysis of reservoirs according to the traditional AVO technique.Here introduces the method of extraction of anisotropic gradient related to fractures and the orientation of fractures and the method can quickly find the location of fractures in the reservoirs.Beginning with the basic theory of prestack seismic inversion,according to the OA reflection coefficient approximation this paper derives AVAZ inversion equation containing two terms of anisotropy parameters based on the Bayesian theory framework.Considering the anisotropic parameters estimated from fractured rock physics modeling and azimuthally prestack seismic data of unconventional reservoirs,we study the direct inversion method gain anisotropic parameters related to fractures.Finally,we apply the AVAZ inversion method in real work area,and we get reliable results which well agree with the well data.Because of the low SNR and poor quality of gathers of azimuthally prestack seismic data in some work area,this paper puts forward the azimuthally elastic impedance inversion method with two fracture parameters.The method improves the SNR by partly stack the prestack seismic data in every azimuth.Model test results show that the azimuthally elastic impedance inversion method is stable and reliable,and has high noise immunity.The inversion method is well used in real work area,and can accurately depicts the fractures in the unconventional reservoirs.