Running Casing Drilling Bit in Extreme-Lean-Profile Wells: A Possibility that Becomes Reality

Abstract A major oil company drilled an offshore well in Libya where extreme-lean-profile (X-LP) casing was programmed. The X- LP well, a further development of the lean-profile, requires casing strings to be run in openhole sections with very little clearance between the borehole wall and the casing or liner. These include 16-in. casing through 17 1/2-in. open hole, 13 3/8- in. casing through 14 3/4-in. open hole, 11 3/4-in. casing through 12 7/8-in. open hole, and finally, 9 5/8-in. liner through 10 5/8-in. open hole. This well was programmed and drilled using the X-LP concept with vertical sections to solve particular drilling problems while minimizing drilling costs. It was also drilled using an automated vertical drilling system and managed pressure drilling (MPD) technique in some sections because of the difficulty to drill these formations. The major difficulty consisted in drilling a very thick shale section, where formation is highly unstable with a high tendency to swell and slough and with several fractures that worsen the scenario. This formation also contains some gas that can produce a kick if not controlled with MPD techniques while drilling. Other drilling hazards included thief zones where mud losses occurred. Because of these difficulties and because of the very tight clearance between liner and borehole wall, running the liner to bottom and properly isolating these formations was a real challenge and could determine that fine line between the success or the failure of the well. For this reason, the operator and the bit manufacturer selected to run the liner with a casing drilling bit (CDB) instead of a reamer shoe. The operator believed a standard aluminum reamer shoe would have failed to reach the bottom. Two liner strings were run with the specially designed CDB, which is made of real PDC cutters and a special steel alloy body very similar to that of a PDC bit, is more robust, and drills more effectively than a reamer shoe. These runs enabled the operator to run and set the liner strings through sections of unstable swelling shales, using the CDB to do hard reaming of long sections. The 9 5/8-in. liner string was set higher than openhole total depth (TD) because the hole was filled with cavings and the liner reached its torque limit. The 7-in. liner string was reamed down to setting depth. This paper describes the application of these specially designed CDBs in the X-LP well, using MPD techniques, which enabled the operator to run and set liners through very difficult formations with very tight borehole/casing clearances.