Drilling 8 1/2" hole vs 6" hole

Dear Members,
can you forward me your views for the following issue:
if you are planning to drill from 1800 m (casing depth)to 3000 m (TD of the well) and you have a choice to drill this section with a 8 1/2" or a 6" bit what would you select???
A few ad's and disad's I could think of are:

8 1/2"

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1) ROP better > than 6" bit
2) torque limit better than 6" hole
3) fishing more room than 6" hole
4) csg cost more
5) log more room for MDT
6) contin. csg 6" is contingency hole
7) ECD moderate
8 ) WOB avail. good with 6 1/4" DC

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6"

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1) less ROP than 8 1/2" bit
2) torque: limited due to 3 1/2" DP
3) fishing: limited room
4) csg cost less than 8 1/2" cheaper(csg design)
5) MDT (OD>5") small room
6) non standard contingency
7) ECD high
8 ) WOB avail. limited with 4 3/4" DC

6 Answer(s)

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