The Crack Prediction In Pre-stack Seismic Inversion Based On Orthorhombic Anisotropy

The fracture is not only the reservoir space in the underground reservoir,but also the main transport channel.With the improvement of meticulous standards of the seismic exploration and development of unconventional oil and gas,and the rapid improvement of the wide range acquisition and processing technology,the description of reservoirs fracture is playing an increasingly important role in shale gas exploration.For the high angle fracture(vertical cracks)of shale reservoir,it is necessary to do pre-stack azimuth seismic attributes analysis technology and study the azimuth pre-stack seismic inversion method,which can improve the fracture prediction accuracy and provide a reliable basis for the exploration.First,this paper introduces the basic theory of seismic anisotropy,and the shale reservoirs with high-angle fractures are approximately equivalent to orthotropic media.The vertical fracture is decomposed into two orthogonal symmetric planes.Sensitivity analysis is made on the formula of the approximate longitudinal wave reflection coefficient of the orthotropic media deduced by the predecessors.According to the sensitivity of the azimuth,the incidence and the parameters of the formula,the formula is further deduced into the expression of the approximate P-wave reflection coefficient for the high-angle fractures.And it is given the computing method of the azimuth of fracture,before the method of extraction of anisotropic gradient in two different symmetric surfaces of the orthotropic media,which explains the anisotropy gradient can be used to determine the degree of fracture development.Second,this paper introduces the basic theory of pre-stack seismic inversion.According to the orthogonal anisotropic P-wave reflection coefficient approximate formula,the linear inversion method of parameter de-correlation is realized by using rock physical model to estimate the vertical and horizontal wave impedances and the anisotropic parameters which can reflect fracture development.A single well test and a practical work area test have been carried it out successively.Then according to the orientation data of this area with bad quality,this paper proposed azimuth elastic impedance inversion with Cauchy-constraint,based on the Bayesian theory,and the anisotropic parameters related to the development degree of cracks are obtained by multivariate linear fitting.Both the well data test and the actual work area test are conducted successively to show that the inversion method of azimuth elastic impedance with Cauchy-constraint for orthotropic media has higher Stability and reliability,which can more accurately reflect the shale reservoir fractures.