Abstract: Real geological formations usually exhibit resistivity macro anisotropy in two ways: the mean result of formation microstructures and the structures such as fracture and strike. Three-component induction well logging may be the best method to determine the resistivity anisotropy. However, this technique has not been used in China so far. The data available in China at present are normally lateral and dual induction logs. Neither lateral well logging nor induction well logging methods alone can resolve the anisotropy of a formation. However, a joint inversion of lateral and inductive data makes it possible to resolve the coefficient of anisotropy. In this paper, an analysis of the importance of taking anisotropy into account in inversion modeling is presented. It is shown how the combined use of lateral and inductive logs can resolve the coefficient of anisotropy of a formation. Synthetic 2D modeling shows that inductive methods will only sensitive to the horizontal resistivity of a layer, while the thickness is undistorted. That is to say, the horizontal resistivity and formation thickness can be determined by inductive methods, but the vertical resistivity cannot be defined, and hence the coefficient of anisotropy cannot be defined. As the apparent resistivity deduced from lateral methods can be approximately considered as the geometric mean of horizontal and vertical resistivities, then the apparent thickness deduced from lateral methods is the product of anisotropy coefficient and real thickness. Therefore, we cannot determine any parameters by lateral logs alone. However, a joint inversion of data from lateral and inductive logs may determine three parameters: the coefficient of anisotropy, horizontal resistivity and formation thickness. Synthetic data show that the joint inversion method is feasible.

Key words: anisotropy, resistivity, lateral logging, induction logging, modeling, inversion

摘要： 实际地层可能出现2种电阻率的宏观各向异性:微观的统计平均效应和裂缝走向等因素的构造效应。三分量感应测井方法可能是确定地层电阻率各向异性的最好方法,但是,该仪器目前在中国还没有应用。已有的数据一般是梯度和双感应曲线,梯度和双感应曲线单独使用都难以揭示各向异性,但是,将二者联合使用则有可能。在反演模型中考虑了电阻率各向异性的存在,给出了梯度和双感应曲线联合求地层电阻率各向异性的方法。通过直井水平层的二维人工模型表明,用感应方法可确定地层的水平电阻率和厚度。而梯度方法的视电阻率可以近似看作水平电阻率和纵向电阻率的几何平均值,梯度方法的视厚度是各向异性参数和真实厚度的乘积。因而不能用梯度方法单独确定各向异性参数和真实厚度2个参数中的任何1个。然而,联合梯度和双感应的方法则可能确定3个参数:各向异性参数,水平电阻率和地层厚度。人工数据的例子表明了这种联合的可行性。

关键词: 电阻率, 各向异性, 感应测井, 梯度测井, 反演