页岩储层叠前密度稳定反演方法

Robust pre-stack density inversion method for shale reservoir

密度参数是页岩储层评价的重要敏感参数,但通常认为准确获得密度信息难度较大.本文探索提出了一种稳定的基于弹性阻抗的叠前密度反演方法,以提高优质页岩的识别精度.首先,基于不同的参数化弹性阻抗方程开展密度敏感性分析,优选了敏感性最高的Aki-Richards近似弹性阻抗方程作为反演方程.然后,在弹性阻抗方程线性化的基础上,假设反演方程系数矩阵的逆存在,将密度(取对数)表示为多个角度弹性阻抗(取对数)的加权和,通过井旁道弹性阻抗反演结果与测井数据的回归求取加权系数,避免了常规反演方法对大型系数矩阵的求逆计算,从而提高了密度反演的稳定性,同时由于利用测井资料作为匹配目标,密度反演的精度也得到了提高.模型试算和涪陵页岩气田的实际资料应用实践均表明了该方法能够有效地提高密度反演的精度与稳定性.

Density is an important sensitive parameter in shale reservoir evaluation, but it is generally considered difficult to determine it accurately. This study proposes a robust pre-stack density inversion method based on elastic impedance to improve the accuracy of high-quality shale identification. Firstly, the density sensitivity of different parameterized elastic impedance equations are analyzed using a shale model, and the Aki-Richards approximate elastic impedance equation with the highest density sensitivity is chosen as the inversion equation. Then, the elastic impedance equation is linearized, and density parameter (logarithm) is expressed as a weighted sum of multiple angular elastic impedances (logarithm) assuming the inverse of the coefficient matrix exists. The weight coefficients are obtained by the regression of the inverted elastic impedance at wells location and density log data, which avoids calculating the inverse of the large-scale coefficient matrix, hence the stability of the density inversion is improved. Meanwhile, density well log is utilized as the matching target, so the accuracy of density inversion is also enhanced. The model test and the application to the Fuling shale gas field show that the proposed method can effectively improve the accuracy and stability of density inversion.