Slide drilling through a closed annular

We are drilling offshore deepwater (1000m WD) carbonate structures in West Africa. The risk here is massive / total losses in the vugular reservoirs.

Using a RCD is the conventional strategy of dealing with this issue and I've read through the carbonate section of this website. However I was wondering if anyone has performed slide drilling with a straight motor through a closed subsea annular BOP. It's recommended by Exxon for drilling carbonates with a subsea BOP (IPTC-14423-MS, Kick Mechanisms and Unique Well Control Practices in Deepwater Vugular Carbonates, DuPriest 2011). Seawater is pumped down the drillstring which turns the drillbit, the drill pipe is kept stationary, mud pumped down the kill line and any gas / cuttings is pushed into the loss zone.

We've worked up LCM strategy and a Floating Mud Cap programme. However these strategies only work for minor or total losses (not the large grey area in-between).

If there's anyone who has experience (both good and bad) of slide drilling through a closed annular, and can make any advice / recommendations it'll be much appreciated.

From my standpoint the Concerns:

1. Damage to the Upper Annular caused by stripping - need to minimise the closing pressure. Will suspend and trip the LMRP if the Upper Annular fails
2. Liability discussions with the Rig Contractor for using Well Control Equipment for drilling operations ("preventative well control")
3. How far can you practically drill using this technique?
4. What strategy do you use for stabilising the hole and  tripping the BHA? POOH to shoe and pumping gunk?

The Advantages are:

1. No significant requirement for Long Lead Item equipment (12 month lead on RCD and rig up)
2. Prevents HC into the riser (whereas a RCD is set ontop of the riser)
3. Allows drilling to a safe casing seat to be performed with total /partial returns while monitoring returns (this is more difficult with Floating Mud cap).

Any thoughts / suggestions would be much appreciated.

Ryan Heng

4 Answer(s)

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