Effect of multicoil electromagnetic tool eccentricity on measured signals

Abstract

The article discusses the effect on the measured signals of a multicoil electromagnetic tool offset with respect to the borehole axis. The tool is a non-conductive cylindrical body with several coaxial coils. It is assumed that the borehole is vertical and the medium is axisymmetric with respect to the borehole axis. A number of computations based on the finite element method were carried out. Simulations took into account the tool body and finite sizes of coils. A wide range of transmitter-receiver spacings (0.18–1.0 m) was studied. The operating frequency was 1.75 MHz. Different drilling muds and values of the tool offset were examined. It was shown that, for highly conductive drilling muds, the type of dependence between apparent resistivity and eccentricity varies greatly with the array length. It was discovered that increasing the contrast between invaded zone resistivity and mud resistivity also increases the eccentricity effect. The eccentricity effect increases with the decrease in mud resistivity and decreases as the array length increases. Also analyzed was the effect of the tool offset on electromagnetic field pattern.