Active ranging-while-drilling with magnetic gradiometry

A method for ranging while drilling comprising:

• employing a rotating magnet assembly, wherein the rotating magnet assembly induces a changing magnetic moment and/or electric current in a conductive members disposed in a target wellbore, wherein the rotating magnet assembly is employed on a bottom-hole-assembly (BHA) disposed in a second wellbore, the rotating magnet assembly comprising a plurality of magnet arrays configured to provide a magnetization direction, and wherein the magnetization direction is oriented at an angle between thirty degrees and sixty degrees with respect to a central axis of the BHA;
• measuring one or more components of a magnetic gradient tensor using rotating magnetic sensors at two different positions along the second wellbore;
• calculating a gradient field from the one or more components of the magnetic gradient tensor;
• and
• calculating a relative location of the conductive members with respect to the BHA based at least in part on the at least two measurements of the magnetic sensors.