Pure-Bore helps a North Sea operator overcome difficult top-hole conditions on sub-sea wells
Drilling through a mixed sequence which contains potentially mobile sand and consolidated claystone provides a unique challenge to any riser-less drilling program. When potential for washed out hole is combined with the requirements for hole cleaning, the usual Deep-Water solution is to use large volumes of pre-hydrated bentonite fluids applied in a “Pump and Dump” method. With no returns, a limited pit system and adverse weather conditions, such volumes cannot be sustained in a typical North Sea autumn or winter. Even with good weather and a supply boat on hand, any reduction in ROP from the hard clay sediments would make the volume requirements unmanageable within a short time.
The top-hole section was successfully drilled using the optimised Pure-Bore® based fluid, with a high frequency sweep regime implemented throughout the section. After drilling to casing point, a weighted Pure-Bore® displacement fluid stabilised the open hole prior to conductor running. The conductor casing was run and cemented successfully with cement returns to the seabed. A cement top-up job was not required, and the subsequent shoe FIT reached its designed value.
By using Pure-Bore® the operator successfully completed the top-hole section within the planned time and budget and without the NPT that had been experienced on previous wells.