Low Frequency Shadow Signal Analysis Technology And Application

The study of low-frequency shadow has always been an important research direction in the field of reservoir prediction.in the past few decades,low-frequency shadow has made an important contribution to identifying the location of oil and gas reservoirs,and has been widely concerned by researchers.However,there have been various controversies about the principle and mechanism of low frequency shadow association.geophysicists have nearly ten interpretation methods,such as "gas chimney effect" and "high reflection amplitude",which are produced by geophysicists in the course of their research on the principle mechanism of low frequency shadow association.The formation mechanism of low frequency shadow theory has become one of the frontiers and difficult problems in geophysics.In this paper,the mechanism of low frequency shadow phenomenon is simulated on the basis of the theory of twophase medium,and a series of research work is carried out,which has certain theoretical significance and application value.The asymptotic analysis of pore elastic Biot wave propagation model shows that there is obvious and abnormal fast and slow P wave conversion in thin interbed gas reservoir.In this paper,we first combine Hook's law,momentum and mass conservation equations and Darcy's law.Based on the low frequency asymptotic analysis of the pore elastic model of two phase media,the explicit expressions of the low frequency characteristic frequency and the reflection and transmission coefficients of the thin interlayer permeable medium interface are derived.Then,the wave field simulation based on isotropic wave equation in two-phase medium is carried out,and the characteristics of wave field and the law of fast and slow longitudinal wave propagation in two-phase medium are analyzed by using the simulation results.Then,based on the elastic modulus of solid skeleton,permeability of reservoir rock,fluid viscosity and reservoir thickness,a low frequency shadow model of dual phase medium reservoir is established,which is composed of 50 thin interbeds.The results of the model directly show the relationship between the fast and slow wave reflection coefficient and the twoway travel time from the reservoir surface.These converted P waves undergo high frequency correlation attenuation,and the slow P waves of some converted waves do not decay in the case of thin interlayer.It has a certain delay relative to the fast P wave,and these delays are superimposed to form the low frequency shadow area below the oil and gas reservoir.Thus,the characteristic frequency and delay related to the low frequency shadow are obtained from the fast-slow-fast P-wave model,and the prediction method of the gas content of the low-frequency shadow based on the fastslow P-wave conversion mode is derived.Finally,a systematic low frequency shadow reservoir prediction method based on two-phase medium wave field simulation,asymptotic equation low frequency analysis,time delay calculation and characteristic frequency determination method combined with GST time-frequency analysis is formed.In this paper,the actual data of a work area in the South China Sea are processed by the above low frequency shadow method,and the characteristic frequency of low frequency shadow and the time delay of reflection amplitude of low frequency seismic data are successfully predicted.The characteristic frequency and time delay are verified by GST method in seismic profile,and good application results are obtained.