Ultra deep 17 1/2 in sections WBM vs OBM?

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Ultra deep 17 1/2 in sections WBM vs OBM?

This discussion sharing pertains to UDW >2500m WD

Normally pressure regimes where pp does not increase until 12 1/4" sections.

Example shown 2502m RTR-mud line
20" at 3462m
17 1/2" 4209m
Mud weight at TD 9.2ppg.
Max EMW 9.6 @ 20m/hr.

Last Casing & Depth	LOT
20" at 3461m	9.8ppg

07:45	15:00	7.25	Drilled 17 1/2" hole from 4073m to section TD at 4209m. 15-25K, 180rpm, 1090/3890psi, 5-20kftlbs torque. Max ECD 9.49ppg. Worked single prior to and performed surveys after connections. Mud weight increased to 9.2ppg from 4175m at 13.00hrs. (Note: Av ROP for bit run 20m/hr)
15:00	18:45	3.75	Circulated bottoms up to obtain TD formation sample at 1050gpm on the hole and 525gpm on the boost.
18:45	21:00	2.25	Pulled out from 4209m through shoe to 3461m on elevators. Wiped intermittent drag as required. Good wellbore condition observed.
21:00	21:30	0.50	Flow-check well - static. Meanwhile held trip drill with crew.
21:30	23:00	1.50	Ran in and performed pre-logging wiper trip from 3461m to 4171m, normal drag observed. (Note: NOC insisted on wiper trip despite good hole conditions)
23:00	23:30	0.50	Precautionary washed and reamed last stand from 4171 to 4209m, no fill observed.
23:30	00:00	0.50	Circulated at 1050gpm on the hole and 525gpm on the boost.
00:00	02:30	2.50	Completed bottoms up with 1050gpm on the hole and 525gpm on the boost.
02:30	02:45	0.25	Flow checked well - static.
02:45	03:30	0.75	Pulled out of hole from 4209m to 3991m on elevator and trip tank, normal 5-10klbs drag observed.
03:30	04:00	0.50	Pumped 45bbls of 11.7ppg slug.
04:00	05:15	1.25	Pulled out of hole from 3991m back into shoe to 3436m. Normal 5-10klbs drag observed.
05:15	05:45	0.50	Flow checked well - Static. Meanwhile, held trip drill with crew and installed drill pipe wipers.

History / Lessons learned: using WBM this section was almost undrillable.
Well often failed due to self induced fractures.
To mitigate this ROP and all operations had to be managed at extremes i.e. slow and controlled.

Shifting to flat rheology mud benefits can be realised in reports and derived worked example shown.

Case study example attached and as per DDR above.
Sharing to conclude if anyone has anything of further value to share when drilling these intermediate sections? Particularly experience with WBM and drilling similar pressure regimes and stratgraphy with OBM in UDW.

Cheers.