复杂裂缝储层地震频散衰减与频变各向异性研究-理论分析与数值模拟

Seismic detection and characterization of fractures is of great importance in the exploration and development of carbonate and unconventional reservoirs. Due to effects of seismic waves, the oscillatory fluid flow occurs between fractures and porous background medium and also between fractures, which result in energy dissipation and hence seismic dispersion and attenuation, as well as frequency-dependent anisotropy. These seismic attributes in turn can be used for fracture detections. Hence, based on the generalized Biot’s theory for multi-porosity and multi-permeability media, this work will study these two fluid flow mechanisms and their couplings. Firstly, we will derive the analytical solutions for the fluid pressure and flux within the porous rock containing two orthogonal sets of fractures. In addition, the rock effective elastic properties will also be given. The two fluid flow mechanisms will hence be studied and the corresponding seismic dispersion, attenuation, and frequency-dependent anisotropy will be calculated. Then, we will modify the constitutive equation and fluid flow equations to re-derive the solutions, based on which we can quantify the effects of fracture intersection angle, fracture connectivity degree, and multi-phase fluid saturation on the fluid flow mechanisms. Furthermore, to validate the theoretical results, we will also carry out the numerical simulation studies. The theoretical model built in this work and the corresponding numerical simulations will give the quantitative relation between fluid flow related parameters (such as fracture parameters, formation effective permeability) with seismic dispersion, attenuation, and frequency-dependent anisotropy. This will provide the basis for seismic inversion of such parameters.

裂缝的地震探测与描述是碳酸盐岩与非常规油气藏勘探开发领域的重要内容。在地震波激励下，裂缝与背景孔隙介质之间以及裂缝之间发生振荡式的流体流动，产生能量损耗进而引起地震波的频散衰减与频变各向异性，故可利用这些地震属性探测裂缝。为此，基于多孔Biot理论，本研究将考察上述两种流体流动机制及两者之间的耦合。首先，我们将推导含正交裂缝饱和孔隙岩石中流体压力、流量及岩石等效弹性性质的解析解，从而可考察两种流体流动特征并计算相应的地震频散衰减及频变各向异性。其次，通过修正本构方程及流体流动方程等重新建模可定量研究裂缝交角、裂缝连通性、多相流体饱和等因素对流体流动机制的影响。此外，为了分析验证理论结果，还将开展相应的数值模拟研究。本研究所建理论模型及相应数值模拟将给出裂缝参数、储层有效渗透率等流体流动相关参数与地震频散衰减及频变各向异性之间的定量关系，从而为地震反演该类参数提供基础。

地震波频散、衰减与频变各向异性与裂缝性质具有密切关系，其主要机制为波致流体流动（Wave-induced Fluid Flow，简称WIFF）与弹性散射。为研究两种机制共同作用下的地震波频散、衰减与频变各向异性，本项目建立了相应定量模型。首先，研究了P波垂直于平行裂缝入射时的频散与衰减。基于Biot动态孔弹性理论，建立了裂缝与岩石背景基质间波致流体流动（Fracture-Background WIFF，简称FB-WIFF）与弹性散射的耦合统一模型。基于该模型研究发现，背景基质渗透率与流体粘度对耦合作用影响最大且两者具有显著的非线性耦合特征。其次，进一步将模型扩展为任意角度入射P波的情况。通过分别研究裂缝表面的法向振动与切向振动，计算任意角度下的P波频散与衰减（即频变各向异性）。结果表明，由FB-WIFF与弹性散射引起的P波频变各向异性具有显著差异。当FB-WIFF与弹性散射发生耦合时，P波频变各向异性发生明显变化。此外，在平行缝模型基础上，进一步发展了含交叉裂缝流体饱和孔隙岩石中的P波模型。通过在边界条件中考虑裂缝间的波致流体流动（Fracture-Fracture WIFF，简称FF-WIFF），建立了FB-WIFF、FF-WIFF及弹性散射的统一模型。利用模型研究发现，裂缝连通性对P波频散与衰减具有显著影响且P波频变各向异性对裂缝交角具有高度敏感性。最后，通过将P波入射场改为SV波入射场，进一步发展了SV波模型。利用该模型，研究了SV波对交叉裂缝的响应并与P波情况进行了对比。结果表明，SV波对裂缝连通性与裂缝交角同样具有高度敏感性，但其特征与P波存在显著差异。. 通过理论、数值及实验等方法，本项目所建立模型均得到了验证，保证了模型的正确性。本项目通过循序渐进的方法，逐步建立了一套用于描述复杂裂缝储层中地震波频散、衰减与频变各向异性的理论体系，在裂缝储层勘探与开发中具有重要的潜在应用价值。