Influence Of Fluid Distribution In Stored Rock On Seismic Wave Dispersion And Attenuation

Based on Biot theory,Analysis of the fast P-wave and the slow P-wave,the effects of fast P-wave and slow P-wave on pore fluid are compared.Three methods for studying mesoscopic wave-induced fluid flow are presented : Periodic layer model,double-porosity dual-permeability theory and statistical approach.Because the double-porosity doual-permeability theory is an absolutely accurate method,therefore,the shape of the attenuation curve in the seismic frequency band is determined by using the double-porosity doual-permeability theory.The Periodic layer model is used to analyze the factors affecting wave dispersion and attenuation at the mesoscopic scale.At mesoscopic scale,the heterogeneous of rock frame has little influence on wave dispersion and attenuation.The main influencing factor is displacement of pore fluid relative to rock frame.Therefore,rock permeability,viscosity of pore fluid and saturation are the main reasons for attenuation.The Dynamic-Equivalent-Medium formula is derived,but the original DEMA formula is not suitable for the case of high fluctuation media.The analysis shows that the measured P-wave modulus is in agreement with the measured value of Gassmann-Hill model at high measuring frequency.The results at low frequencies agree with those measured by the Gassmann-Wood model.Therefore,by linking these two boundaries with DEMA formula,the original DEMA formula is improved to BA-DEMA.The influence of spatial correlation function and correlation length on dispersion curve and attenuation curve is analyzed.When the correlation length is small,the peak value of attenuation will appear at the higher frequency and the dispersion curve will move towards the lower frequency when the correlation length is large.When pore-filling fluid have largely different bulk moduli,the variable characteristic length model is used instead of the correlation length in the original BA-DEMA formula,which is called the variable characteristic length model.In partially saturated rocks,the characteristic length is closely related to water content in rocks,which can be roughly divided into drainage process and imbibition process.Random medium modeling is carried out by means of random medium modeling.As a result,the correlation length is the distance between one material and another in the medium.The larger the correlation length,the more similar the relevant properties of the medium in this direction.The dispersion and attenuation of waves when they propagate in layered media are analyzed.Interlayer wave-induced fluid flow is a major cause of wave attenuation.At the same time,permeability and effective viscosity of pore-filling fluid will influence attenuation of wave.At low frequency,rock permeability is close to that calculated by means of harmonic mean value,and at high frequency,rock permeability is close to that calculated by means of arithmetic mean value.Because permeability varies with frequency,the effective viscosity of multiphase fluids also varies with frequency.When waves propagate in layered media,the scattering effect can not be ignored.When passing through the layer boundary,not only will the energy of the primary transmitted P-wave be weakened by the wave transformation,but also one wave will enhance the energy of the primary transmitted P-wave and make the theoretical predictions consistent with the field observations.