三维各向异性裂缝探测的纵波地震响应方法研究

This project addresses a few key specific scientific issues in the use of wide azimuth P-wave seismic data for 3D fracture detection based on 3D anisotropic seismic forward numerical modelling in 3D anisotropic fracture media. These issues include three dimensional forward numerical modelling the Discrete Fracture Model (DFM), the seismic response of 3D DFM, the influences of the fracture spacing and azimuthal Amplitude Versus Offset (AVO) variation and the effects of the anticline, finite dip and other complex structures, and uncertainties in field data analysis. For this, we implemente the new finite difference scheme for modelling the seismic response in 3D fractured media; appropriate approaches are then selected to study 3D DFM and the effect of the anticline, finite dip and other complex structures with 3D seismic modelling, followed by an integrate real case study from an oilfield in China. Finite difference (FD) is widely used in seismic modelling. There are three FD schemes that will be studied in this project, the standard staggered grid (SSG), the rotated staggered grid (RSG), and the diamond staggered grid (DSG). Both qualitative and quantitative comparison has been made to reveal their capability in modelling 3D anisotropic fractured media. The SSG has shown best performance for anisotropic media with orthorhombic symmetry or higher symmetry system. For lower anisotropy symmetry, the DSG is preferred than the RSG in terms of computation efficiency. A new solution to the diamond grid issue is developed (the two different diamond grids can be merged to form a regular rectangular one in order to handle all wavefield components and easily calculate the derivatives in the DSG scheme) which can simplify the 3D DSG implementation of the finite difference method in general anisotropic media, and an optimized workflow (by separating model parameters into model medium parameters and model structure parameters，this is a parameter index method; this is suitable for any of the three FD schemes) is proposed to simulate large 3D fractured models. With the above tool, a number of modelling studies have been carried out, on the effects of the discrete fractures and of the presence of anticline, finite dip and other complex structures: the 3D DFM study provides many insights into seismic response of discrete fracture and the link between the discrete fractures, the fracture spacing, as well as the features in scattering waves. The study of the anticline, finite dip and other complex structures, which is built with the parameters from a real case, is to assess the anticline, finite dip and other complex structures effects on fracture parameter inversion based on the Singular Value Decomposition (SVD) method. P-wave azimuthal attributes are commonly employed to invert fracture density and orientation. Many factors may affect the accuracy of the inversion results. The integrated study of both geological observation and pre-stack and post-stack seismic attributes can reduce the uncertainties for 3D anisotropic fracture detection.

项目以三维各向异性裂缝介质正演数值模拟为基础，讨论宽方位角纵波地震资料在裂缝探测中的关键科学问题：三维离散裂缝模型（DFM）的正演、裂缝间距的影响、方位AVO响应及背斜、有限倾角等复杂构造的影响。研究三个有限差分方案：标准交错网格法（SSG）、旋转交错网格法（RSG）、菱形交错网格法（DSG）。通过定性和定量比较，揭示它们正演三维裂缝介质的能力。SSG至多能够正演正交介质，且性能最好。对极端各向异性介质，DSG优于RSG。针对DSG的特点提出将两个不同的菱形网格合并成一个规则的矩形网格，简化三维DSG的数值实现新方法。提出优化的计算流程（将模型参数分离为模型介质参数和构造参数，参数索引法），实现大尺度的三维DSG正演。揭示三维DFM地震响应、裂缝间距和散射波之间的规律。使用奇异值分解评价复杂构造对裂缝参数反演的影响。综合研究地质观测、叠前和叠后不同方位地震属性，减少裂缝探测的不确定性。

利用地震各向异性进行裂缝储层表征是当前地震勘探的一个热门课题。利用地震波的振幅、速度、走时、衰减等地震属性的方位角变化，来推断地下裂缝储层的性质。.(1)主要研究内容.研究三维各向异性裂缝介质的地震正演数值模拟，将正演数值模拟方法应用到三维纵波实际地震资料中，评价裂缝储层表征中的关键科学问题。本项目主要研究以下内容：(1)发现了三维裂隙介质地震正演数值模拟中存在的问题，研发了解决这类问题的实用方法和技术，提出了一种新的正演模拟计算方法；针对DSG的特点，提出将两个不同的菱形网格合并成一个规则的矩形网格，简化三维DSG的数值实现的新方法。提出优化的计算流程（将模型参数分离为模型介质参数和构造参数），实现大尺度的三维DSG正演。(2)使用新的正演方法，模拟离散裂缝的三维各向异性介质，分析其地震响应，揭示三维DFM地震响应、裂缝间距和散射波之间的变化规律。(3)背斜、有限倾角等复杂地质构造对裂缝性质反演的影响；(4)利用三维纵波实际地震资料进行裂缝检测的综合研究，使用奇异值分解评价复杂构造对裂缝参数反演的影响。研究裂缝参数反演、裂缝预测的影响因素。综合研究地质岩心观测、叠前和叠后不同方位地震属性，减少裂缝探测的不确定性。.(2)项目的重要研究结果：.(a)提出三维复杂各向异性介质的纵波正演数值模拟的新方法；(b)揭示出与离散裂缝模型的性质直接相关的地震属性的独特的波现象和方位变化规律。证实利用本方法，使用实际三维地震资料反演地下离散裂缝介质的可行性。最终达到使用地震各向异性表征裂缝储层的目的；(c)已发表13篇论文(10篇SCI源刊，含3篇行业国际顶级SCI期刊；1篇EI；1篇CSCD核心，1篇中文会议论文)，录用待发表5篇论文(2篇本行业国内顶级SCI期刊；3篇CSCD核心)。6个中国发明专利、1个实用新型；研究成果获得2021年(排名第1)湖南省自然科学三等奖。锻炼一支从事油气的精细地震勘探理论研究队伍，培养8名硕士生及一些本科生。.(3)科学意义.本项目取得的新方法新理论促进了三维裂缝储层表征、裂缝预测方法理论的进步。