Numerical Simulation Of Seismic Wave Fields And Fluid Pressure In Mesoscopic Porous Medium

The upper crust rock is generally a complex pore structure with a certain porosity,and the pore filling material can be composed of clay,oil,gas and water.This heterogeneous heterogeneity makes the study of seismic wave propagation in underground media more complicated.Under the action of force source,the stress and strain state of the rock particles will be changed,which can indirectly reflect the multiparameter physical characteristics of the transmission medium,so the study on the propagation mechanism of elastic waves within the seismic frequency can be more effective for the reservoir geophysics,engineering and environment et al.The research on the structure and composition of complex medium is the basis for the discussion of wave propagation.For porous media,fractures and cracks are usually representative pore structures.In this paper,a two-layer porous model is established.In order to be more practical,we also take into account the viscoelastic and anisotropic properties of the medium.By changing the physical parameters of the two layers,such as the density and the modulus of elasticity of the cracks,comparing of waves propagation in different physical properties,the influence of the variable parameters on the experimental results are also obtained.The propagation law of seismic waves in different types of media has a certain indicative function,so it is used as the main means of seismic exploration.Due to the absorption of the formation,the waves will be scattered and energy will be lost to a certain extent,thus this can be used for the analysis of rock saturation and the quality control of the reservoir.In recent years,the study of wave attenuation shows that the wave induced fluid flow(WIFF)at the mesoscopic scale(between the wavelength and the particle diameter,far less than the wavelength)is one of the main reasons for the loss of seismic waves in the complex heterogeneous medium.As to the study on the propagation mechanism of seismic waves and the influence of related parameters have become more meaningful at the mesoscopic scale.In this study,we apply an improved high-order staggered-grid finite difference method to model seismic wave propagation in complex heterogeneous porous media with cracks.Upon changing the mesh step and applying a broadband wavelet,we obtain numerical simulation results at the mesoscopic scale.As a fundamental result of applying Biot’s theory,three types of waves are also found.We discuss the relationship between the shear wave splitting parameter and the crack density of an equivalent medium in detail.Moreover,the formulas derived from a Fourier transform are used to calculate the inverse attenuation quality factors and phase velocities of the fast compressional wave,slow compressional wave and shear wave.The effects of the porosity and elastic modulus of an equivalent medium on wave attenuation are also taken into account in this study.Based on Darcy’s law,the finite element method is used to investigate the distribution of the fluid pressure field in porous media at the mesoscopic scale.A flat ellipse is treated as an approximation of a crack.According to the simulation results,we further study the influence of variations in the crack angle on the distribution of the fluid pressure.In this paper,the numerical simulation of wave-fields at different scales is realized and the attenuation characteristics of three types of typical waves are obtained.In view of the influence of the fracture parameters in the porous media,we discuss the shear wave splitting coefficients and the fluid pressure respectively.Especially for the fracture density,the fracture rotation angle and the porosity of the medium,we make a detailed discussion,which also provide a basis for further research on the propagation of seismic waves in porous media with fractures.