Study On Shale Rock Physics Modeling And Azimuthal Seismic Anisotropy

As one of unconventional sources,shale gas has great potential in becoming the alternative of conventional sources.In the progress of shale gas exploration,the importance of study on shale is noticeable for its role as gas reservoir.Geophysics techniques are the core of shale gas reservoir identification and characterization.While rock physics modeling and seismic forward modeling are the basic of relevant researches.Yet there is much room for improvement,because of the complexity in shale's microstructure,elasticity and strong seismic anisotropy in macroscopic scale.Therefor this thesis is focused on the shale rock physics modeling and its matching seismic forward modeling method.Due to intrinsic VTI related to preferred alignment of clay particles and the existence of high angle fractures,shales usually present orthorhombic anisotropy at least.In this thesis,an anisotropic rock physics model for shales at the seismic scale is built.And based on analysis of log data from shale formation in the Sichuan Basin of Southwest China,the scheme of modeling is applied.The model is built by using hybrid approaches and upscaling methods,and can be used to describe the azimuthal anisotropy of shale at the seismic scale under the combination of intrinsic anisotropy,layer-inducing anisotropy and fracture-inducing anisotropy.In the workflow,some rock physics experiments results are taken as the assumption about intrinsic anisotropy.Generalization of Backus averaging is adopted to the upscaling of logging data to get the equivalent VTI medium.Then,the theory of Linear Slip is used to insert vertical fractures into this VTI background medium for constructing the equivalent orthorhombic medium.Finally,generalization of Backus averaging is used again to achieve the equivalent orthorhombic model at the seismic scale.The workflow is universal for shale seismic rock physics modeling.In order to understand the relationship between physical properties of the rock physics model and seismic wave propagation by AVAZ analysis,I propose a seismic forward modeling method.Conventional AVO analysis method is based on Zoeppritz equations and their approximations.As its generalized technique,AVAZ analysis method mostly uses Rüger equation,or the linearization formulas conducted by Vavry?uk and P?en?ík.These studies are suitable for the model of a interface which separates two different layers.Wave propagation in anisotropic media can't be neglected,as the reservoirs are a stack of layers.In contrast to the case of a single interface,Schoenberg and Protázio proposed an approach which considered the case of a fractured layer.The designed formalism can be regarded as a simplification of the anisotropic reflectivity method mathematically.Herein we propose the simplified anisotropic reflectivity method by combining the anisotropic reflectivity method and their method.This algorithm can be seen as the pre-stack seismic AVAZ forwarding modeling method.This method can be used to calculate seismic amplitude variation with incident angles and azimuth from stratified media.Being based on the scheme of anisotropic reflectivity method shows it is accurate and robust,and the consideration about wave propagation effect includes the effects of reflection,transmission,mode conversion,interval multiples and tuning,is in agreement with real scenarios.It is verified by numerical modeling and comparison with linearization formulas.The AVAZ analysis is conducted on the seismograms of PP-wave,radial and transverse components of PS-wave from shale seismic rock physics model.The results show that overburden effects can't be ignored.The simplified anisotropic reflectivity method can also be the forwarding modeling part of the prestack waveform inversion which take the anisotropy and anistropic attenuation into consideration.