Determining the depth and orientation of a feature in a wellbore

A method of determining the depth, shape, and orientation of a window in a wellbore containing a ferrous tubing, the method comprising the steps of:

• running a downhole device comprising at least one magnetic field sensor, at least one orientation sensor, and a processor through the ferrous tubing in a wellbore;
• monitoring an inherent magnetic field of the ferrous tubing using the at least one magnetic field sensor;
• observing a window of the ferrous tubing, the window having an inherent magnetic field different than the inherent magnetic field of the ferrous tubing;
• determining an orientation of the downhole device within the wellbore using the at least one orientation sensor;
• determining an orientation of the window by correlating an output from the at least one magnetic field sensor with an output from the at least one orientation sensor using the processor;
• and
• determining a shape of the window by determining changes in the circumferential width of the window based on changes in the monitored inherent magnetic field across the window.

A downhole device comprising:

• a plurality of magnetic field sensors spaced around a periphery of the downhole device that detect an inherent magnetic field of a ferrous tubing contained in a wellbore;
• at least one orientation sensor that determines an orientation of the device within the wellbore;
• a processor that receives and correlates an output from the at least one magnetic field sensor and an output from the at least one orientation sensor to determine an orientation and a shape of a window of the ferrous tubing, the window having an inherent magnetic field different than the inherent magnetic field of the ferrous tubing, wherein the inherent magnetic field of the window changes along a length of the wellbore due to changes of circumferential width of the window along the length of the wellbore, the shape of the window being determined based on detection, by the plurality of magnetic field sensors, of the changes in the inherent magnetic field of the window along the length of the wellbore;
• and
• a communication arrangement for transmitting data to surface relating to the orientation of the window in real-time.