Research On Fluid Identification Method Based On Two-phase Media

With the rapid growth of offshore oil and gas exploration,China pays more and more attention to offshore oil and gas exploration and development.In recent years,China has made good achievements in the shallow water area of the northern South China Sea,and many large-scale oil and gas fields have been identified,but oil and gas exploration in the deep sea area needs to be developed urgently,and the cost of deepwater drilling is very expensive.how to improve the success rate of oil and gas identification in deep water area by geophysical means and reduce the risk of exploration and development well drilling is also the focus and difficulty of the current research.Fluid identification is a very important and effective means of oil and gas exploration.With the development of seismic exploration,considering the complexity of underground reservoir,the actual reservoir is composed of solid and liquid phases,and the single-phase medium theory can not well describe the characteristics of underground media.We use the two-phase medium theory to more truly describe the propagation of seismic waves in strata.This paper intends to study and apply the fluid identification method based on the asymptotic analysis theory of two-phase media.Based on the previous theoretical studies of classical biphasic media,the asymptotic expressions of reflection and transmission coefficients of normal incident P waves in seismic frequency band and the asymptotic solutions of velocity dispersion and attenuation coefficients of fast and slow P waves are obtained by asymptotic analysis theory.Based on the asymptotic expression of normal incident P wave reflection coefficient obtained by asymptotic analysis,combined with the seismic data of the South China Sea study area,a sandstone permeable two-phase medium model is established.It is found that the reflection coefficient dispersion on the interface is obvious and sensitive to permeability.By analyzing the propagation and attenuation characteristics of waves in permeable interfaces,homogeneous layers,heterogeneous layers and multi-layer media,it is found that there are fast-slow-fast P waves(converted P waves)in multi-layer media.That is to say,at low frequency,when seismic waves pass through relatively thin layered permeable reservoirs,there will bemany conversions of fast and slow P waves,and the time delay caused by the changes of slow P wave velocity and amplitude energy will cause low frequency shadow phenomena.however,the high frequency attenuation and velocity dispersion of conventional P waves can not be reasonably explained.Therefore,based on the asymptotic analysis theory in two-phase media,the forward analysis of the dispersion and attenuation of fast and slow P waves in the seismic frequency band proves that the fast P waves will not produce dispersion and attenuation in the seismic frequency band.dispersion and attenuation are mainly caused by slow longitudinal waves.By calculating the time delay of low-frequency companion shadow to find the azimuth of low-frequency companion shadow,the complexity of the previous artificial frequency scanning method is improved,and the gas content recognition method of low-frequency companion shadow formed by fast-slow P-wave conversion mode is obtained.The normal reflection coefficient obtained based on the theory of progressive analysis of two-phase media can effectively reflect the variation of reflection coefficient with frequency on the permeable interface in the seismic frequency band,which provides the possibility for the fluid identification application of normal incident reflection coefficient on the gas-water interface.The fluid identification factor is constructed,and the attribute of fluid factor is extracted by least square inversion based on generalized S transform.Finally,the fluid factor and low frequency shadow are applied to the study area to verify the effectiveness of fluid factor and low frequency shadow in fluid identification,which has important utilization value for the exploration and development of oil and gas resources in the deep water area of the South China Sea.a set of fluid identification method based on asymptotic analysis of two-phase medium is formed.