Oil & Gas Glossary 1.0

A special mill that has a heavy tubular extension below it called a pilot or stinger. The pilot, smaller in diameter than the mill, is designed to go inside drill pipe or tubing that is lost in the hole. It guides the mill to the top of the pipe and centers it, thus preventing the mill from bypassing the pipe. Also called a piloted mill.

To use a mill on the end of a workstring to remove a permanent tool or fish.

To use a mill to remove the outside area of a permanent downhole tool.

A fishing accessory run above a bit or a mill to recover small, nondrillable pieces of metal or junk.

A device employed to protect seals, seating nipples, etc., during mill-out operations.

A tool used to mill over permanent bridge plugs/cement retainers while retrieving the milled-out debris

A pinned-end pup joint used to provide additional length and inside diameter necessary to accommodate a standard milling tool.

A length of pipe whose bottom edge is serrated or dressed with a hard cutting material and that is run into the wellbore around the outside of stuck casing, pipe, or tubing to mill away the obstruction.

A rod-like or tube-like extension below a downhole tool, such as a mill, that serves to guide the tool into or over another downhole tool or fish.

The tool used in the operation of milling. See mill.

A downhole tool with rough, sharp, extremely hard cutting surfaces for removing metal by grinding or cutting. Mills are run on drill pipe or tubing to grind up debris in the hole, remove stuck portions of drill stem or sections of casing for sidetracking, and ream out tight spots in the casing. They are also called junk mills, reaming mills, and so forth, depending on what use they have.

A device made up in the drill stem above the mill to collect bits of junk ground away during a milling operation. During milling, drilling mud under high pressure forces bits of junk up the narrow space between the boot sub and the hole wall. When the junk reaches the wider annulus above the boot sub and pressure drops slightly, the junk falls into the boot sub. A boot sub also can be run above the bit during routine drilling to collect small pieces of junk that may damage the bit or interfere with its operation.