The Shell/Esso Brent Field contains many small pockets of oil; 0.5 to 1.5 million barrels that are uneconomic to develop using conventional sidetrack techniques. The sidetrack of Brent Bravo well BB-08 takes advantage of the latest slim- hole through tubing technology reducing the well costs by 50% over a conventional sidetrack. The operation was carried out with a conventional rig utilising a slim-hole drillstring to exploit an expected 1.5 million barrel target and also provide a technical comparison with a coiled tubing sidetrack. The entire operation was carried out through the existing xmas tree and completion string.
The original perforations were isolated then a through tubing whipstock was used to exit through the existing 7" liner and 9-5/8" casing strings. A 1500 ft 3-7/8" hole section was drilled with a steerable BHA, which gave an average dogleg of 9 deg/100 ft in the build section. The mud weight required for hole stability meant high over-balance pressures through the reservoir section. The hole was logged with a wireline conveyed resistivity tool. A 2-7/8" liner was run and cemented in place with a liner top production packer to provide well integrity. Wireline access into the 2-7/8" liner was not possible due to debris and therefore the well had to be cleaned out with coiled tubing before perforating.
This paper describes the planning process, the operations (from abandonment to perforations), lessons learned and the teamwork which made it all possible.