Looking at an old bit selection chart.
Cretaceous is more akin to Northern North Sea but with no chalk that often sits in lower Palaeocene or into Maastrichtian.
Hello Mark, thanks for the comments.
Cretaceous has been uplifted so coming in at about 600m below seabed at about 1900-2000m brt Not sure exactly what this does to drillability. A tad easier perhaps as less overburden compaction forces acting on rocks I would think!
No chalk is prognosed or seen in offset. No under-reaming conducted on offsets but we have some regional experience. I am hopeful of some good supporting data from vendors that I expect in next few weeks.
We therefore expect large upper cretacous sandstone packages with claystone interbeds and occasional thin hard stringers perhaps that would need to be carefully managed. How well cemented sandstones are?, could be a genuine risk!
So perhaps more drillable than we would expect in Central North Sea once we hit chalk and then the harder sandstones below this.
Deeper in well in 8 1/2 to 9 1/2in options, then perhaps getting into lower Cretaceous equivalent of Cromer knoll group in CNS!
In worst case scenarios, e.g. we hit shallow water flow and have to set an extra surface string. At some point we would have to under-ream from 12 1/4in to perhaps 13 1/2 to accommodate 10 3/4in casing planned.
Then 8 1/2in hole would be under-reamed to optimal size with few contingent options around this required.
In shallower sections it is likely we would try (if data supports this) to drill and under-ream in one run.
Depending on geological results and uncertainties we may also drill and then under ream later.
So I am planning to have bit/bha, designs and engineering scrutinised for plans a,b and c as usual!
Anything else springing to mind, all comments welcomed in review and assessment currently on-going.
We have many runs now utilising the new anti-vibration tool (HI Tool) from Franks Drilling, this tool allows you to decouple the cutting structure of the under-reamer from that of the bit which in most cases can be 30-50m away at the bottom of an RSS being run in the pilot section of the hole.
I have a few suggestions in order to optimise you UR BHA design:-