A ball seat placed deeply in one of the wells in the North Continental Shelf (NCS) had to be removed as part of a reconfiguration work-over. To achieve this, a milling operation scheme was devised. As this was the first time a ball seat of this type had to be removed by milling in the NCS the operation was rehearsed and evaluated onshore.
An identical setup with two sizes of seats and balls was established in an almost horizontal pipe setup onshore and the tool-string consisting of a conveyance tractor mounted with a milling device fitted with a tungsten carbide bit was driven into the pipe. The milling was carried out with different levels of weight on bit by adding back-tension on the cable as well as using different numbers of drive sections on the tractor. Then the results were evaluated. Additional tests were carried out with parts of a milled ball dropped down hole on the ball seat to ensure that milling could be done. The tests showed that the tool-string as configured could mill out the ball seats in one operation. This was later confirmed as the actual seat situated at 12.647 ft. measured depth was removed in one run.
The paper provides a detailed account of the preparations and actual milling operation, including the lessons learned and how the objectives were achieved.