What is the
benefit to drill a Build Up Section from KOP to Landing point at 89° with
8-3/8in instead of 8-1/2in.
8-1/2in and 8-3/8in Bits have more or less the same Diameter but some companies
prefer to run with 8-3/8in instead of 8-1/2in why ?
But for vertical wells they run 8-1/2inCan any one clarifies?
The difference is the perfection in making casing which is measured by the amount of rejects when using "ALTERNATE" drift versus the "STANDARD" drift.
Obviously, we should try to use the largest hole possible to allow not only the passage of stiffer bottom hole assemblies but also for more cement thickness in case of cementing.
I recall at least one situation where we forgot to mention alternate drift. The mill, being a good mill, provided casing which drifted - without rejects - in spite of being drifted with an alternate drift ie a slightly longer drift.
N. B: During my tenure, we were always encouraged to use the API correct "alternate drift" expression and not the misleading "special drift" which could attract a higher cost per foot
Since you are talking about drilling the whole section from KOP to landing point, I don't see any real gain in drilling 8 3/8" compared to 8 1/2". The bit OD on the other projects is most likely being driven by the casing ID, nothing else.
Davidwhitepeng has hit on a possibility for running one towards the end of the section - but then when do you decide it's OK to pick up a slightly under-gauge bit. What happens if the bit doesn't make TD and it has gone under-gauge? Do you run an 8 1/4" bit?
What happens if you have issues and you have to go fishing? You're only slightly under-gauge, but every little bit of OD helps.
If the casing can fit an 8 1/2" bit, run an 8 1/2" bit - keep it simple, standard and avoid going down the special drift size if you don't have to. You will have more readily available, "off the shelf" drill bit options.
I suspect that what you're noticing is companies finishing their angle build section with a slightly smaller diameter bit to avoid having to ream the hole section to get the last bit into the well before running their intermediate casing. With the side loading on the bits the bit will sometimes go under gage resulting in some hole section that will be somewhat smaller diameter. Going with a smaller bit makes it easier to get the bit into the well.
Your bit size would be dependent on the id / weight of your last casing string run ei 47 ppf 9 5/8 casing has an id of 8.681 and a drift of 8.525, 53 ppf 9 5/8" casing would have an id of 8.535 and a drift of 8.379".
You can get special drift 53 ppf to allow the passage of a 8 1/2 bit and bha.
Any csg heavier than 53 ppf would require a 8 3/8" bit and bha.