Modeling of electrical effects of borehole casing inhomogeneities

What is it about?

Measurement of the formation resistivity through a metal casing made it necessary a study of the electrical field distortions caused by casing inhomogeneities. A straightforward modeling of such effects is complicated due to high contrasts of the conductivity. Evaluation of the formation resistivity through a metal casing relies on high precision measurement demanding the corresponding accuracy from the modeling results. Such an accuracy is hardly achievable by a general SD modeling code. For low frequencies used to penetrate the casing, these demands can be met by the so-called thin sheet modeling approach, which was successfully applied to a number of problems in surface geophysics. An integral equation with a contraction kernel is obtained for the density of currents flowing along the casing. The equation is solved using the Iterative Dissipative Method. The main features of the modeling algorithm and an example of its application are considered.

Featured Image

Why is it important?

It is shown that while the distortions of the electric field dues to casing imperfection may easily exceed the measured signal, the system of the data acquisition used in the trough casing resistivity measurement allows for reducing the distortions to a fraction of the measured signal.