基于基追踪-BI_Zoeppritz方程广义线性脆性指数直接反演方法

非常规油气资源的勘探开发在能源领域越来越受到重视.针对页岩油气、致密砂岩气储层的脆性指数预测,对突破"甜点区"、指导后续水力压裂等都具有重要的意义.针对常规叠前AVO反演技术预测脆性指数存在的问题:(1)稀疏脉冲反演垂向分辨率不高;(2)基于AVO近似公式的常规叠前反演,需要假设常背景纵横波速度比以及弱弹性参数反射率等条件,会影响到三参数反演的精度;(3)通常需要借助反演获取的纵横密三参数转换为杨氏模量和泊松比,再进一步转换才能获取脆性指数,在参数转换的过程中会将误差累积放大,影响最终的脆性指数预测精度.本文从精确Zoeppritz方程出发,通过对其进行重新推导,将其表达为脆性指数、P波速度和S波速度的函数(BI_Zoeppritz方程),并借助广义线性AVO反演,对基追踪反演获取的高分辨率角度反射系数进行迭代反演,直接提取高分辨率、高精度的脆性指数.通过模型和实际资料验证了该算法相对常规叠前反演获取的脆性指数有了进一步的改善.

A direct inversion for brittleness index based on GLI with basic-pursuit decomposition

Unconventional oil and gas are receiving more attention in the industry of energy. The prediction of the brittleness index of reservoirs in shale and tight sandstone has a great importance in predicting sweet spots and directing hydro fractures. However, the prediction of the brittleness index with the conventional pre-stack inversion faces some problems, such as the low vertical resolution of inversion results, the assumption of the constant background of V_P/V_S and the weak contrast of elastic parameters, and cumulative errors resulting from transformation among elastic parameters. To solve these problems, we rearrange the exact Zoeppritz equation into a function of the brittleness index, V_P and V_S(BI_Zoeppritz equation) instead of V_P, V_S and density. And the brittleness index can be inverted iteratively based on the theory of GLI from the high-resolution angle reflectivity obtained using basic-pursuit decomposition considering different incident angles. The tests of this algorithm on synthetic and real data show that this method can improve the resolution and the accuracy of the calculated brittleness index compared with the conventional approach.