关键词: 电阻率, 各向异性, 感应测井, 梯度测井, 反演

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