Research On Fluid Attribute Extraction Method Based On Prestack Inversion

With the increasing development of oil and gas regions,dispersion information and attenuation attributes have always been essential characterization tools for reservoir prediction and fluid identification.In this paper,we do some critical research on two properties from three perspectives: time-frequency analysis,poststack Q-value extraction,and prestack frequency dependence inversion.Firstly,the time-frequency analysis method is the foundation for Q-value extraction and dispersion estimation.This paper proposes an improved method based on unshaled S-transform and an improved method based on W-transform,which address the issue of insufficient temporal resolution of conventional S-transform in lowfrequency regions.In this way,the novel methods can improve the temporal resolution in the low-frequency regions,ensure reliable frequency distribution,and achieve adaptive regulation of time-frequency resolution.Secondly,poststack Q-value extraction technology is an essential means of quantitatively characterizing attenuation features.Based on different wavelet shapes,this article proposes different methods to improve the accuracy of Q-values.Under the assumption of the Ricker wavelet,this paper proposes an improved method based on the peak frequency shift method to address the robustness and accuracy issues of directly selecting peak frequency.The reliable peak frequencies are obtained through a constructed function and the corresponding inversion algorithm.A formula based on the variation of peak frequencies is formed to calculate the interlayer Q-value.In the face of non-Ricker wavelets,this paper proposes an FWE frequency shift method based on standard deviation parameters,which can avoid the problem of optimal symmetry index and obtain stable and effective Q-values.Then,traditional prestack inversion methods need the consideration of pore structure.In response to these issues,this paper proposes an AVO approximation based on multiple pore structures,avoiding the high pore assumption of conventional Gassmann approximation.In order to obtain stable inversion results,A step-by-step Gaussian Newton iterative inversion strategy is applied here,which solves the objective function with multiple prior constraints to obtain stable and reliable inversion results.Furthermore,in order to visually represent the fluid region,this paper proposes an analytical formula for the dispersion term based on frequency-dependent fluid modulus.Combining the AVO approximation based on pore morphology and attenuation model,the relationship between the dispersion term of the volume modulus of the variable fluid and the elastic impedance is established,and the corresponding inverse process is established to more intuitively indicate the fluid.Finally,this paper uses the viscoelastic reflectivity method to analyze the interlayer dispersion characteristics.This paper proposes an analytical formula for frequencydependent fluid terms based on the viscoelastic reflectivity method to characterize this feature.The analytical formula combines the analytical formula of the Russell fluid term and attenuation model.The new method can avoid inappropriate assumptions and high computational complexity of frequency variation inversion based on the viscoelastic reflectivity method.In this way,the new method can quickly and effectively indicate the fluid region.The forward and inversion results of synthetic records and real seismic data demonstrate the feasibility and effectiveness of the new method proposed in this paper in indicating fluids.