Definition - What does Rathole mean?
Rathole is a hole on the rig floor in which the Kelly and swivel are placed. The hole has a scabbard into which Kelly is inserted. The internal diameter of the Rathole is smaller than the Kelly’s outside diameter. It can be 30 to 35 feet deep in the rig floor and lined up with casing which comes above the floor.
Petropedia explains Rathole
Once the drill site is selected and surveyed, the drilling equipment is brought by an operator and the ground is cleared and leveled. After that, a large pit is constructed to hold the water for drilling operations. A small diameter Rathole is also drilled along with the primary borehole. The Rathole is a part of the well drilling process and it is drilled as per standard drilling guidelines. It allows the entry of electric logging equipment. It can also be used to drop the perforating guns ingot thereby helping in keeping the perforated zone free from any type of obstructions.Mobile auger rigs are used to drill Ratholes of 20 to 35 inch diameter.