含中大尺度离散裂缝的饱和孔隙介质地震波衰减特征研究

Understanding the seismic wave propagation properties in fluid-filled fractured porous media is of great importance in the fields such as hydrocarbon exploration and geological engineering. Existing effective medium theories are invalid for explaining the strong scattering phenomenon in the complex fractured reservoir. Furthermore, because the lack of thorough understanding on the dispersion and attenuation of seismic wave through fractured porous media, the use of dynamic information still meets difficulties such as the dependence of prediction results on the frequency range. In this proposal, we employ the discontinuous Galerkin finite element method (DGFEM) to solve the strain discontinuity problem for simulating wave propagation in fractured porous media. We simultaneously consider the intrinsic attenuation associated with wave-induced fluid flow (WIFF) between fractures and pores and scattering attenuation due the heterogeneous structures. The coupling between intrinsic and scattering attenuation are considered during the simulation process. Laboratory experiments are performed on synthetic fractured rock specimens, the results of which are compared with that of DGFEM for the medium with different pore structure and fracture parameters. By doing so, the effects of parameters of fractures and pores on both intrinsic and scattering attenuation at different frequency can be characterized quantitatively, which could help interpret data obtained from seismic exploration and laboratory experiments.

流体充填裂缝孔隙介质的地震波传播特征研究在油气勘探、地质工程等行业有着重要的理论及应用价值。现有基于等效介质理论的裂缝模型无法解释裂缝储层介质强散射等现象，同时由于对地震波衰减和频散认识的不足，在利用振幅及频谱等动力学信息时出现如预测结果依赖数据频段区间等问题。本项目拟利用间断Galerkin有限元方法处理裂缝界面的优势，研究孔隙介质中地震波穿过裂缝时衰减的完整过程。数值模拟时同时考虑与裂缝孔隙间在波扰动导致的流体运移（wave-induced fluid flow, WIFF）相关的内耗衰减和裂缝非均匀性导致的散射衰减，重点分析WIFF过程与散射过程的耦合关系。开展裂缝孔隙人造岩石的波衰减实验，其结果与数值模拟结果进行对比分析，对地震波衰减原因进一步深入研究。最终得到裂缝参数及孔渗条件与两类衰减的定量关系，实现对裂缝储层地震资料与储层岩石实验室声波数据动力学信息利用的有效理论指导。

分析地震波在复杂介质中的衰减过程具有重要的理论与实际意义。本研究基于数值模拟与实验测试方法，还原了地震波在孔隙裂缝介质中的演变过程。通过处理裂缝边界条件，把裂缝位移不连续信息与孔隙局部WIFF过程进行了关联，给出了柔度的流体运移量依赖解析方程式，实现了含散射衰减和内耗衰减的数值模拟算法。同时，通过胶结及3D打印技术制作具有不同裂缝参数及孔渗特征的物理模型，与数值模拟结果进行对比分析。最后通过衰减因子提取方法，分析了裂缝尺度及孔渗特征对两类衰减机制的影响，重点分析了两种机制之间的相互影响及其本质。重点分析了横波在裂缝孔隙介质中的传播机理，揭示了横波导致WIFF的物理本质，给出了各类横波频率依赖特征与裂缝参数之间的关系，得出慢横波具有一定流体指示作用等结论。