页岩气储层高角度裂缝指示因子地震反演方法

页岩气储层具有水平层理特征, 并且同时发育若干平行排列的高角度裂缝, 利用传统的VTI和HTI模型不能对这种裂缝型储层进行准确描述, 但利用正交各向异性(OA)介质能够更准确地对其进行描述。根据高角度裂缝在地震反射方程中的各向异性特征, 提出一种表征高角度裂缝发育强度的裂缝指示因子, 再利用正交各向异性介质反射系数近似方程以及等效正交各向异性参数, 推导出正交各向异性介质中含裂缝指示因子的方位弹性阻抗方程, 建立了裂缝指示因子及裂缝方位弹性阻抗反演的方法流程。为提高反演的稳定性, 将推导的方位弹性阻抗方程改写为双参数的形式, 并采用基于模型的迭代最小二乘反演来进行裂缝指示因子和裂缝方位的稳定估算。模型测试和实际地震资料处理结果表明, 该方法能够得到合理可靠的裂缝指示因子和裂缝方位预测结果, 有助于指导页岩气储层高角度裂缝发育区域的勘探开发。

It is difficult to use a vertical transverse isotropy (VTI) and a horizontal transverse isotropy (HTI) model to characterize fractured shale gas reservoirs with horizontal bedding and several groups of parallel high-angle fractures. However, an orthorhombic anisotropy (OA) model is a feasible alternative. Based on the anisotropic characteristics of high-angle fractures described using the reflectivity equation, an indicator that represents the intensity of high-angle fractures is proposed. We derive an azimuthal elastic impedance equation containing a fracture indicator for OA media from the approximate equation of OA reflection coefficient and an equivalent OA parameter. We also establish a method to invert fracture indicator and fracture azimuthal elastic impedance. To improve the stability of the inversion, the derived azimuthal elastic impedance equation is rewritten in a two-parameter form. A model-based iterative least-squares inversion is used to achieve the robust estimation of fracture indicator and fracture azimuth. A model test demonstrates the reliable estimation of fracture indicator and fracture azimuth even at the signal-to-noise ratio of 2. A practical application also yields reliable predictions. This method is a useful tool to predict shale gas reservoirs with high-angle fractures.