本征各向异性对地震波频散、衰减与频变各向异性的影响

Effects of intrinsic anisotropy on seismic dispersion,attenuation and frequency-dependent anisotropy

层间波致流体流动是饱和流体孔隙层状介质中地震波频散、衰减与频变各向异性的重要机制,目前已提出较多模型对其进行定量表征.然而,现有模型忽略了层状介质中各层本身可具有的各向异性特征(即本征各向异性).对此,本文基于Biot孔弹性理论推导了由横向各向同性饱和流体孔隙地层组成的层状介质中地震波频散、衰减与频变各向异性的近似理论模型.同时,为验证理论模型,开展了数值模拟与理论模型的对比,两者吻合良好.利用理论模型,分析了4种情况(流体性质不同、骨架性质不同及两者之间的两种不同组合)下本征各向异性的影响.结果表明,对于气层与水层交互形成的层状介质,含水层的本征各向异性对地震波的频散、衰减与频变各向异性影响最大,而对于含高孔薄层的层状介质,背景基质本征各向异性与流体分布起主要影响.本文构建的模型简洁易用,在页岩或致密砂岩等具有明显层理特征的储层中应用前景广阔.

Interlayer wave-induced fluid flow is an important mechanism for seismic dispersion,attenuation,and frequencydependent anisotropy in fluid-saturated porous layered media.The previous models assume that the layered medium is composed of isotropic layers,whereas the layer itself can exhibit anisotropic properties(i.e.,intrinsic anisotropy).To study the effects of intrinsic anisotropy,based on the Biot’s theory of poroelasticity,we propose an approximate theoretical model for seismic dispersion,attenuation,and frequency-dependent anisotropy in a layered medium which is composed of transversely isotropic fluid-saturated porous layers.To validate the approximate theoretical model,we compare the theoretical model with the numerical simulations,which show good agreement between each other.Using the theoretical model,we analyze the effects of intrinsic anisotropy in four cases(different fluid properties,different matrix properties,and their two different combinations).The results show that,for the layered medium composed of alternating gas-saturated and brine-saturated layers,the intrinsic anisotropy of the brine-saturated layers have the largest effects on the seismic dispersion and attenuation,whereas the influence of the gas-saturated layers is much smaller.When the brine-saturated layers have the property of transversely isotropy,the seismic dispersion and attenuation are more notable.Contrarily,for the layered medium containing highly porous thin layers that is saturated with a single phase of fluid,the seismic dispersion and attenuation are more notable when the background layer is isotropic.In such a layered medium,if the background layer and the highly porous thin layer are saturated with different phases of fluids,the effects of intrinsic anisotropy depend on the fluid distribution.Different fluid distribution can enhance or diminish the interlayer wave-induced fluid flow and the corresponding seismic dispersion and attenuation.The effects of intrinsic anisotropy on seismic dispersion and attenuation thus vary with the fluid distribution.In terms of the frequency-dependent anisotropy,the effects of the intrinsic anisotropy of the brine-saturated layers are also the largest in the layered medium composed of alternating gas-saturated and brinesaturated layers.For the layered medium containing highly porous thin layers,the intrinsic anisotropy primarily affectsεandεQ,but has little effects onδandδQ.This means that the difference between the P-wave velocities(attenuation)in the directions perpendicular and parallel to the layers is greatly affected by the intrinsic anisotropy,whereas the variations of Pwave velocities(attenuation)with wave incident angles in the vicinity of the layer normal are little affected by the intrinsic anisotropy.The model proposed in this paper is concise and easy to use,which has a great potential to be applied in the shale and tight sandstone that have distinct layering features.