Alternating and static magnetic field gradient measurements for distance and direction determination

Abstract

Magnetic field gradient sensing apparatus utilizing highly sensitive fluxgate magnetometers permit direct determination of the distance between two well bores and for determining the direction from one bore to the other. The gradient sensing apparatus may be responsive to alternating magnetic fields resulting from current flow in a steel casing within a target well and measured by two fluxgate magnetometers diametrically oppositely located in a sensor tool and oriented to measure the vector components of the alternating field at opposite sides of the tool to thereby obtain a measure of the gradient of the field across the diameter of the tool. Field gradient sensing apparatus may also be used to measure the gradient of a static magnetic field having a radial gradient, wherein the static field is produced by ferromagnetic materials in the target well. Again, a pair of magnetometers is utilized to measure the field gradient across the diameter of the sensor to determine distance and direction to the target. Both static and alternating magnetic fields may be simultaneously measured for improved accuracy and reliability.

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