Study On Azimuthal Anisotropy Seismic Forward Modeling And Inversion Of Fractured Shale Reservoir

As a special kind of oil and gas reservoir,shale oil and gas has many characteristics such as many occurrence modes,complex reservoir structure and strong heterogeneity.The exploration and evaluation of shale oil and gas requires higher requirements than conventional oil and gas reservoirs.There are many fractures in the shale reservoir,which are affected by factors such as the complex structure of the shale reservoir.The fractures are mostly vertical or nearly vertical,and the horizontal seams are relatively small.Fractures developed in shale can increase the effective porosity of shale oil and gas reservoirs,improve the connectivity of reservoir space and increase reservoir permeability,and provide important channels for hydrocarbon migration.Therefore,it is important to study the propagation characteristics of seismic waves in fractured shale reservoirs and to use seismic data to invert fracture parameters for seismic prediction of reservoirs.The theory of elastic wave in anisotropic media is the basis for studying fractured reservoirs,which can effectively explore the propagation characteristics of seismic waves in anisotropic media.For shale reservoirs with vertical fractures,the reservoir can be equivalent to horizontal transversely isotropic(HTI)medium when considering only the effect of vertical fractures on anisotropy;if the effect of vertical fractures and horizontal bedding on anisotropy is considered simultaneously,the reservoir can be equivalent to an orthotropic medium.In this paper,the seismic amplitude variations versus azimuth(AVAZ)inversion of anisotropic parameters is studied based on the two media models and the theory of rock physics.For HTI and orthotropic medium models,the approximate formulas of reflection coefficients of PP or PS waves are introduced,and the variation characteristics of reflection coefficients with incident angle and azimuth angle are analyzed.At the same time,based on rock physics theory,the effects of fracture attributes(fracture orientation,fracture density,fracture fluid,etc.)on seismic AVAZ response and anisotropic parameters are discussed,which lays a foundation for inversion of anisotropic parameters of shale reservoirs.When only vertical fractures are considered in shale reservoirs,this paper proposes to invert Thomsen anisotropic parameters of fractured reservoirs using simulated annealing particle swarm optimization(SA-PSO)based on HTI medium rock physical model and approximate formula of reflection coefficients of PP and PS waves.At the same time,before inversion,the Fourier series method is used to estimate the fracture orientation,and the estimated value is used to correct the fieldl data to provide more accurate input data.In the theory of seismic AVAZ inversion with anisotropic parameters,the sensitivity of inversion parameters can be improved and more accurate elastic parameters of fractured reservoirs can be obtained by using the correlation between Thomsen's anisotropic parameters or the method of joint inversion of PP wave and PS wave.The validity of seismic AVAZ inversion theory is verified by theoretical model test,and the field seismic data are applied.The results show that Thomsen anisotropic parameters obtained by inversion can be used to evaluate the degree of fracture development and provide a basis for fracture identification in shale reservoirs.When considering both vertical fractures and horizontal bedding in shale reservoirs,this paper studies seismic AVAZ inversion method of fracture parameters in shale reservoirs based on orthotropic rock physical model,and predicts elasticity and fracture parameters of shale reservoirs under vertical transversely isotropic(VTI)background.According to the geological structure of shale gas reservoirs in the study area,a single interface theoretical model is designed to verify the effectiveness of inversion method based on VTI constraints and Bayesian framework,and the inversion method is applied to the field azimuth seismic data of Longmaxi shale gas reservoir.According to the analysis of rock physics theory,the inverted fracture parameters not only reflect the degree of fracture development,but also correlate with mineral content.The larger the anisotropic parameters,the higher the content of brittle minerals such as quartz.This indicates that there is a correlation between brittle minerals and fracture development,which is consistent with the basic geological theory and can be used to describe the distribution of fracture and brittleness characteristics of reservoirs..