Methodologies Of Pre-stack Seismic Inversion For Shale Gas Play

Shale gas is a kind of natural gas that is trapped within mudstone or shale formations,as an important alternative for conventional oil and gas resources,it is of great significance for economic development and energy security of our country.The characteristics of shale-gas plays,which make research on the theory and application more difficult,are organic-rich,lowporosity in geology and strong seismic anisotropy,weak ductility under stress in physics.Prestack seismic inversion is an effective means of identification and evaluation of shale gas plays,also remains challenging because of limited understanding of seismic response to the organic matter,significant seismic anisotropy,multi-parameter problem.The research of pre-stack seismic inversion for shale-gas play has theoretical and practical value for exploring and developing the shale-gas resource.Clay mineral,compliant pores and organic matter in shale-gas play are the intrinsic sources of seismic anisotropy.Starting with the geological and geophysical characteristics of shale-gas play,and under the calibration of experiments and well log data,this paper builds an effective model for solid matrix of shale,and then incorporates the effect of organic matter on the macro elastic properties of shale by employing anisotropic effective field theory to develop a rock physics model of organic-rich shale-gas play.Finally,rock-physics templates which provide the foundation for predict the petrophysical properties of shale-gas play,are built.Anisotropic Young's modulus and Poisson's ratio,which reflect the rock mechanical properties of shale-gas play,are the important parameters for identifying the engineering �sweet spot�.Beginning with the constitutive equations of transversely isotropic shale media,the anisotropic constitutive matrix has been decomposed into an isotropic background and anisotropic perturbation by perturbation theory,and then the approximate formulae of Young's modulus and Poisson's ratio are derived.Accordingly,the quantitative relation between anisotropic modulus and Thomsen-style anisotropy parameters is established.The accuracy of the approximate formulae derived has been tested by comparison with experimental measurements.The approximate in-situ stress equations in terms of intrinsic anisotropy parameters and fracture weakness are proposed by generalized orthorhombic rock mechanical model and linear slip effective medium of fractured rock provides the fundamental of prediction of anisotropic modulus and evaluation of fracability for shale-gas play.Rock modulus and anisotropy parameters are crucial to identify and evaluate for the favorable zone of the shale-gas resource.This paper initially derives an approximate reflection formulae of plane wave reflects the changes of rock modulus and anisotropy parameters,and then builds a linear modeling operator.Under the framework of Bayesian pre-stack seismic inversion,the multi-parameter seismic inversion method via azimuthal reflection coefficients has been achieved.Considering the instability for inverting the anisotropy parameters from reflection coefficients,this paper proposes a directly and simultaneously seismic inversion method based on azimuthal elastic impedances in logarithm domain for orthorhombic media,the stability of multi-parameter inversion has been enhanced,the field data processing tests proved the validity and the effectiveness of proposed methods,which provides an effective means to prediction and evaluation for shale-gas play.It is a leading edge subject to build multi-parameter seismic inversion methods for shalegas play by making use of wide-azimuth pre-stack seismic data.This paper initially studied the rock physics model,and explored the characterization of rock mechanical properties and in-situ stress state,analyzed the reflection coefficients of a plane wave in an effective orthorhombic media of shale-gas play,then developed the multi-parameter pre-stack seismic inversion methods.Theoretical model tests and application demonstrate the developed methods has a positive role in prediction and evaluation of the shale-gas play.