论文详情
含倾斜裂缝的地层在地下广泛分布，可等效为倾斜横向各向同性（TTI）介质。TTI型富有机质页岩气储层即使具有较高的脆性程度，但其杨氏模量可能依旧较低，从而降低了杨氏模量作为储层脆性指数的适用性。为此，将杨氏模量与拉梅第一常数的组合作为脆性指数，推导了由脆性指数、裂缝密度和倾角直接表达的TTI介质线性化纵波反射系数方程及其傅里叶级数形式，有效分离了裂缝信息和背景基质信息。然后，在贝叶斯反演框架下，结合离散傅里叶级数，提出了一种直接预测TTI型富有机质页岩气储层脆性特征和裂缝参数的分级地震反演方法，该方法分别利用地震数据的二阶分量和零阶分量直接预测裂缝参数和脆性指数，以解决裂缝参数和脆性指数对地震数据贡献度不同而导致的裂缝参数预测稳定性差的问题，同时避免了裂缝方位估计误差对裂缝密度和裂缝倾角预测精度的影响。模型测试和实际应用结果表明，该方法估计的裂缝参数和脆性指数合理地圈定了优质脆性储层，提高了裂缝参数的预测稳定性。
Subsurface formations with aligned dipping fractures can be equivalent to the media with tilted transverse isotropy (TTI). For TTI-type organic-rich shale gas reservoirs, the Young's modulus is not a sensitive brittleness indicator as a low value may emerge at a high degree of brittleness. Therefore, a hierarchical seismic inversion method is proposed to directly predict the brittleness and fracture parameters of TTI-type organic-rich shale gas reservoirs. We derive a linearized P-wave reflection coefficient equation for TTI media in terms of a new brittleness indicator formulated as the function of the Young's modulus and Lamé constant (λ), fracture density and fracture dip. The reflection coefficient equation is rewritten in the form of the Fourier series to separate fracture information from background matrix information. Based on the Bayesian framework, the hierarchical inversion method is established to predict brittleness indicator and fracture parameters. The process is composed of three steps. Firstly, we utilize the discrete Fourier series to decouple the isotropic (zero-order) and anisotropic (second-order) components of azimuthal seismic data. Secondly, we invert fracture density and fracture dip from the second-order component of azimuthal data. Thirdly, the brittleness indicator can be estimated using the zero-order component. The model test and practical application indicate the robustness of the method and credible prediction of high-quality brittle reservoirs.