Ultra deepwater drilling is challenging. When easily fractured carbonates are encountered, the challenge increases further still. High hydrostatic pressures, as well as the narrow gap between pore and fracture pressures, add to the complexity. Performing successful cementing operations is equally challenging.
This paper describes the drilling of a well at 2180m in the eastern GoM. The formations were carbonates with potential for high-to-total losses. The rig was a new Sixth Generation DP MDU. To manage the BHP and prevent losses, the rig had been equipped with a new type of DGD/MPD system; a pumped riser system, with the pump mounted around 400m below the bell nipple. The pump reduces the riser level in a precisely controlled manner to manipulate the BHP as required.
Due to a very weak 20"shoe the 17 ½" section had a drilling window of only 0,3ppg. The pumped riser system was used to drill the section with nearly constant BHP, compensating for the ECD by manipulating the hydrostatic column level while drilling and bringing the level back up for connections. No losses were seen. The same applies for cementing of the 13 3/8 casing.
In the 12,25" section the system was rested due to robust formations and a generous drilling window but was used again successfully to ensure a good 9 5/8" cement job.
In the 8,5"section the pumped riser system was again necessary due to potential severe losses in weak reef-type carbonates. It soon became evident that this formation could not have been drilled conventionally as weak zones were penetrated at the start of the section. Loss pressure was 8,4 ppg. Drilling with returns was still required in order to get the required geological information from the well, which was achieved. As drilling proceeded a normal-pressured zone was encountered. The pumped riser system was used to balance the resulting pressures downhole with required returns to the surface.
The system was also used to perform a good Plug and Abandonment operation after TD was called.
This paper will describe the planning and drilling operation of the mentioned ultra deepwater well and present the operational results.
