1. n. [Drilling]
A storage place for the kelly, consisting of an opening in the rig floor fitted with a piece of casing with an internal diameter larger than the outside diameter of the kelly, but less than that of the upper kelly valve so that the kelly may be lowered into the rathole until the upper kelly valve rests on the top of the piece of casing.
2. n. [Drilling]
Extra hole drilled at the end of the well (beyond the last zone of interest) to ensure that the zone of interest can be fully evaluated. The logging tool string may be as much as 120 ft [36.5 m] in length, so the rathole allows tools at the top of the logging string to reach and measure the deepest zone of interest. In addition, there is usually a small amount of extra hole drilled to allow for junk, hole fill-in and other conditions that may reduce the effective depth of the well prior to running logging tools.