Fiber Optic Current Monitoring for Electromagnetic Ranging

A wellbore ranging system for surveying a target wellbore from a second wellbore, said system comprising:

• a bottom hole assembly carried at the distal end of a drill string disposed in the second wellbore, the bottom hole assembly comprising an electromagnetic field sensing instrument and a drill bit;
• an elongated conductive member disposed in the target wellbore, the conductive member oriented along an axis that is substantially parallel with the target wellbore;
• an electric current source exciting current flow in the conductive member of the target wellbore;
• and
• a fiber optic sensor system, the fiber optic sensor system comprising:
• a plurality of spaced apart fiber optic sensors, each sensor having a core formed of material selected from the group consisting of magnetically responsive material and electrically responsive material;
• a fiber optic interrogation system;
• and
• an optical fiber extending from the fiber optic interrogation system to the cores, wherein a portion of the optical fiber is disposed adjacent to each core.

A method for electromagnetic ranging comprising:

• positioning an optical fiber in a target wellbore having a conductive member disposed therein;
• applying a current flow to the conductive member;
• and
• utilizing the optical fiber to measure the current flow on the conductive member.