Orthorhombic Anisotropic Rock Physics Modeling for Fractured Marine Shale Reservoir in Sichuan Basin
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摘要: 含裂缝页岩具有复杂的各向异性，建立合理的岩石物理模型有助于裂缝参数的准确预测. 针对四川威荣含裂缝五峰-龙马溪组页岩，在准确表征其固有各向异性（VTI）的基础上，建立了具有裂缝倾角分布特征的正交各向异性岩石物理模型. 分析表明：页岩中高角度裂缝的倾角平均值、倾角标准差、倾角变化范围对方位各向异性有显著的影响；横波各向异性参数$ {\gamma }^{\left(2\right)} $和$ {\gamma }^{\left(3\right)} $仅对裂缝密度敏感，可用于裂缝密度的预测；纵波各向异性参数$ {\varepsilon }^{\left(2\right)} $、$ {\delta }^{\left(2\right)} $、$ {\delta }^{\left(3\right)} $、$ {\varepsilon }^{\left(3\right)} $受到裂缝密度和填充流体的影响，与横波各向异性参数相结合可用于裂缝流体的检测.将提出的岩石物理模型应用于实际测井数据，实现了井中各向异性参数的估测，为裂缝性五峰-龙马溪组页岩的地震裂缝预测提供了重要的理论依据.Abstract: Fractured shale often represents complex anisotropy characteristics. A proper rock physics model can be helpful for accurate prediction of fracture properties. In this paper it proposes an orthorhombic anisotropic rock physics model, considering the distribution characteristics of fracture dip and accurate characterization of VTI (Vertically Transverse Isotropy) characteristics, for the fractured shale reservoir of Wufeng-Longmaxi Formation in Weiyuan-Rongxian area, Sichuan Basin. The modeling results of the well analysis show that the average fracture dip, the standard deviation of fracture dip, and the variation range of fracture dip in shale have significant influence on azimuthal anisotropy. The anisotropy parameters related to S-wave velocity $ {\gamma }^{\left(2\right)} $ and $ {\gamma }^{\left(3\right)} $ are only sensitive to fracture density and insensitive to filled fluids, and can be used for prediction of fracture density in seismic exploration. The anisotropy parameters related to P-wave velocity $ {\varepsilon }^{\left(2\right)} $, $ {\delta }^{\left(2\right)} $, $ {\delta }^{\left(3\right)} $, $ {\varepsilon }^{\left(3\right)} $ are simultaneously affected by fracture density and filled fluid, and can be used to detect filled fluids of fracture by combining the anisotropy parameters related to S-wave velocity. In summary, the rock-physics modelling proposed in this paper is suitable for southern marine fractured shale reservoirs represented by Wufeng-Longmaxi Formation shale, and provides a theory for seismic prediction of fracture properties for shale reservoirs.
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Key words:
- shale reservoirs /
- fracture prediction /
- seismic rock physics /
- orthorhombic anisotropy /
- geophysics
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图 1 无裂缝VTI页岩（a）和含裂缝ORT页岩（b）的岩石物理建模流程
Fig. 1. Rock physics modeling of VTI shale (a) and ORT fractured shale (b)
图 2 含不同特征裂缝的页岩模型
a.具有垂直裂缝的页岩模型；b.具有考虑裂缝倾角分布特性的页岩模型，其中φ代表裂缝的方位角；c.裂缝倾角直方图. 裂缝倾角平均值$ {\theta }_{0}=\pi /2 $，蓝色条状及黄色实线表示裂缝倾角标准差为1的裂缝分布情况，红色条状及紫色实线表示裂缝倾角标准差为4的裂缝分布情况
Fig. 2. Shale models with different fractures
图 3 正交各向异性页岩的弹性刚度系数随裂缝参数（裂缝密度和裂缝填充流体）的变化
a.气饱和裂缝情况；b.水饱和裂缝情况
Fig. 3. Modulus of stiffness of orthorhombic shale varies with fracture parameters (crack density and filled fluid type)
图 4 正交各向异性页岩的各向异性参数随裂缝密度变化
Fig. 4. Anisotropy parameters of orthorhombic shale vary with fracture density
图 5 正交各向异性页岩的弹性刚度系数（a）和各向异性参数（b）随裂缝走向的变化
Fig. 5. Stiffness modulus (a) and anisotropy parameters (b) of orthorhombic shale variation with crack orientation
图 6 正交各向异性页岩的弹性刚度系数和各向异性参数随裂缝倾角平均值的变化
裂缝倾角标准差为1；裂缝倾角变化范围为$ \pi /2 $
Fig. 6. Variation of stiffness modulus and anisotropy parameters of orthorhombic shale with the average inclination angle
图 7 正交各向异性页岩的弹性刚度系数和各向异性参数随裂缝倾角标准差的变化
裂缝倾角平均值为$ 2\pi /9 $；裂缝倾角变化范围为$ \pi /2 $
Fig. 7. Variation of stiffness modulus and anisotropy parameters of orthorhombic shale with standard deviation
图 8 正交各向异性页岩的弹性刚度系数和各向异性参数随裂缝倾角变化范围的变化
裂缝倾角平均值为$ 2\pi /9 $；裂缝倾角标准差为1
Fig. 8. Variation of stiffness modulus and anisotropy parameters of orthorhombic shale with the integration interval of crack inclination
图 9 研究区地质概况及裂缝特征分析
a.研究区位置概况；b.五峰组‒龙马溪组页岩储层测井数据；c.页岩矿物三角图；d.根据井数据统计的裂缝倾角直方图
Fig. 9. Geological background of the study area and fracture properties
图 10 基于岩石物理模型的预测测井（灰色曲线）和真实测井（黑色曲线）对比
Fig. 10. Comparison between predicted log data (grey curve) based on rock physics modeling and real log data (black curve)
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