Study On Propagation Mechanism Of The Elastic Waves In Periodic Double-porosity Media

In addition to different kinds of minerals and rock particles,the underground rocks were filled with various fluids(oil,gas and water).When elastic waves propagated in porous media saturated with fluid,the mechanical and elastic characteristics of minerals,rock particles and fluid influenced the propagation of elastic waves.Geophysicists paid more attention to the influence of fluid in pores on the propagation of elastic waves.Because elastic waves carried a lot of information related to fluid properties in the process of propagation,it was very important to master the propagation law of elastic waves in rock saturated with fluid in the process of reservoir prediction and fluid identification.In this thesis,the influence of fluid flow in pores on elastic waves propagation was studied.Due to heterogeneity in underground rocks varied in scale,when elastic waves propagated in porous rocks,the heterogeneity in different scales could cause fluid flow in different scales.When the scale of heterogeneity was much smaller than the wavelength scale and much larger than the single pore scale,it was called mesoscale heterogeneity.The causes of mesoscale heterogeneity in rocks were complex,which may be the difference of local porosity,the patchy saturation of fluid and the existence of multi-scale cracks in rocks.Because the mesoscale heterogeneity could cause the attenuation and velocity dispersion of elastic waves in the seismic frequency band(1Hz ? 100Hz),it had become the focus of the study of the theory of two-phase media.In order to study the influence of mesoscale fluid flow caused by local heterogeneity on elastic waves propagation,three periodic double-porosity models were established based on White periodic layered partially-saturated model and the double-porosity model.Based on Hamilton's principle,the wave equations of the three models were derived by calculating the expressions of potential energy,kinetic energy and dissipation function.The phase velocity and quality factor of three kinds of P wave and one kind of S wave were calculated by plane wave analysis.The attenuation and velocity dispersion characteristics of P wave and S wave were studied.At the same time,the effects of physical parameters such as fluid viscosity coefficient,permeability and layer thickness on the attenuation and velocity dispersion characteristics of P wave and S wave were analyzed.When elastic waves propagated in porous media saturated with fluid,fluid flow of various scales often occurred simultaneously,and affected the propagation of elastic waves in different frequency bands.In this thesis,based on the periodic double-porosity model established earlier,the influence of transverse squirt flow was introduced,and a partially-saturated model with transverse squirt flow was established.By solving the average fluid pressure in the media,the wave equations of partially-saturated model with transverse squirt flow were derived.The phase velocity and quality factor of fast P wave and two kinds of slow P wave were obtained by plane wave analysis.The characteristics of P wave attenuation and velocity dispersion in the presence of fluid flow at three scales were studied.The effects of rock physical parameters such as gas saturation,permeability,layer thickness,porosity and characteristic squirt flow length on P wave attenuation and velocity dispersion were also analyzed.In the process of reservoir prediction,the reflection and transmission coefficients of elastic waves were very important parameters.The fluid flow in the pores not only caused the attenuation and velocity dispersion of elastic wave,but also led to the dispersion of reflection and transmission coefficients of elastic waves.In this thesis,the periodic double-porosity media was considered on both sides of the interface.According to the characteristics of the periodic double-porosity model,eight boundary conditions at the interface were given.Based on the plane wave solutions of reflection and transmission P and S waves on both sides of the interface,the characteristic equations of reflection and transmission coefficients of P and S waves were derived.The dispersion characteristics of reflection and transmission coefficients of P and S waves were studied.The effects of physical parameters such as fluid viscosity coefficient,permeability and layer thickness on the dispersion characteristics of reflection and transmission coefficients of P and S waves were also analyzed.