Wellbore imaging using magnetic permeability measurements

Abstract

A wellbore imaging tool is provided which is sensitive to magnetic permeability changes in the wellbore wall. In one embodiment, the measuring array comprises a transmitter coil, an array of receiver coils, and a signal processor. The transmitter coil generates an oscillating magnetic field in the wellbore wall, and each of the receiver coils generates a voltage indicative of a direct inductive coupling strength between the receiver coil and the transmitter coil. Variations in the magnetic permeability of the wellbore wall will cause the direct inductive coupling strength between the transmitter and receiver coils to vary accordingly. The signal processor is coupled to each of the receiver coils to determine the direct inductive coupling strength and configured to convert it into a pixel value for each of the receiver coils. The pixel value may represent the magnetic permeability of a wall region proximate to the receiver coil, or it may represent the direct inductive coupling strength. An image generator may take the pixel values and organize them into an image for display. Each of the receiver coils may comprise a first coil wound in a first direction and a second coil wound in a second direction and coupled in series with the first coil to balance the receiver coil with respect to the transmitter coil. Advantageously, direct inductive coupling measurements are not hindered by drilling mud conductivity or by the acoustic distortion of water droplets in the drilling mud. A high resolution image of wellbore walls may thus be obtained by this measurement technique when other techniques fail.