1. vb. [Drilling, Well Workover and Intervention]

To use a mill or similar downhole tool to cut and remove metal downhole. A mill is usually used to remove junk in the hole or to grind away all or part of a casing string. When milling out casing, the intent is to cut a window through the side of the casing or to remove a continuous section of the casing so that the wellbore may be deviated from the original well through the window or section removed. Successful milling operations require appropriate selection of milling tools, fluids and techniques. The mills, or similar cutting tools, must be compatible with the fish or casing materials and wellbore conditions. The circulated fluids should be capable of removing the milled material from the wellbore. Finally, the techniques employed should be appropriate to the anticipated conditions and the likely time required to reach the operation objectives.

See: casing string,  junk,  milling,  packer