PAPER C

USING INDUCTION TOOLS TO IDENTIFY MAGNETIC FORMATIONS AND TO DETERMINE MAGNETIC SUSCEPTIBILITY AND DIELECTRIC CONSTANT

Thomas Barber, Barbara Anderson, and Gordon Mowat

Schlurnberger

ABSTRACT

Modern induction tools measure the quadrature, or X-signals, from the formation as an aid in skin-effect correction. The tool response models and formation models on which skin-effect corrections are based assume negligible formation magnetic permeability. However, many sedimentary rocks contain magnetic minerals that produce weak param­agnetism or diamagnetism. Since both R-signal and X-signal measurements are a product of the magnetic permeability and the conductivity, the effect of very thick magnetic for­mation layers should be negligible. However, thinly-layered magnetic formations can cause significant perturbations on the X-signal, as has been observed on the X-signal logs from modern induction tools.

These X-signal logs contain significant information about the formation, which can be extracted through a detailed analysis of the cause of these X-signal perturbations. In addition to indicating skin effect, the X-signal is also a measure of the mutual balance of the tool. Commercial induction arrays are arranged so that the direct coupling from transmitters to receivers is cancelled, with the magnitude of cancellation being on the order of 10,000 mS/m. With a mutual-balanced array, magnetic layers can produce mutual imbalances on the order of tens of mS/rn. This imbalance is manifested as high-spatial-frequency excursions on X-signal logs as the tool crosses bed boundaries between layers containing magnetic materials.

This magnetic effect on the X-signal has been studied through modeled formations with susceptibility based on logs, cores and tabulated data on rock magnetic susceptibility. A simple technique to derive the relative magnetic susceptibility of formation layers has been developed which gives values for susceptibility that correlate well with core measure­ments. Uses of this information include detection of iron-containing layers and use as an additional correlational tool.

Another electromagnetic parameter that can affect the induction X-signal is dielectric constant. Although this is normally negligible at induction frequencies, some minerals can produce very large dielectric effects. The effects of high dielectric constant on the X-signal logs are distinguishable from magnetic effects, and modeling can be used to estimate the dielectric constant.