Identification of ferrous changes in piping deployed in a wellbore

摘要

A method and apparatus for detecting ferrous changes (90, 95) passing axially through a cylindrical space comprises surrounding the space with a non-magnetic cylinder (110) and creating an alternating magnetic field in the cylindrical space by means of a permanent magnet (20) rotatable by a motor (40). Sensors (30) detect magnetic field changes as the ferrous matter passes axially through the cylindrical space. The apparatus may be used for detecting tool joints (90) that connect a tubing string (95) as the tubing string is moved in or out of a wellbore, or the presence or absence of a coiled tubing string in the wellbore. The permanent magnet provides a safe field generating means within a hostile well environment.