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Drilling 8 1/2" hole out of 20" shoe
08 July 2005
At a recent DWOP, a suggestion was made to drill 8 1/2" section out
of 20" shoe (Reason : to appraise a nearby shallow horizon, before
plugging back and drilling the main bore to TD.

The idea was rejected due to concerns about cleaning the casing ID
at the low rates that would prevail through the BHA.

Does anyone have any experience of this and, if so :
> Was any special equipment used ?
> How did it go ?
> What would you recommend ?

Many thanks

Dave Taylor
4 answer(s)
Drilling Supervisor
Oil Search Limited
Total Posts: 6
Join Date: 08/12/05
Woodside used to (as regular practice) drill fairly long 8.1/2" hole
sections from 13.3/8" casing, using SB mud. I know it's an order of
magnitude less than out of a 20", but it worked well. Issues we had
were (as has been mentioned) cement falling from around shoe and
causing stuck pipe - don't recall losing a string, but sometimes
cost time. One other issue can be logging tools not keen to find the
wellbore at the shoe - need some inovative hole-finders. The
suggestion to drill out 17.1/2" is good, but of course costs you a

Jet subs placed in the string are a suggestion to improve annular
velocities. These can be nozzled to balance hydraulics.
Managing Director -Relentless Pursuit of Perfection Ltd.
Relentless Pursuit Of Perfection Ltd.
Total Posts: 26
Join Date: 10/01/05
Thanks for replies to date. For clarification, this is NOT a
shallow gas horizon that is being probed. Dave
Drilling Specialist/Well Engineer/Training Consultant
Total Posts: 339
Join Date: 10/01/05
Quick off the cuff response as follows;

Assuming drilling riserless here? I hope we are also drilling this
from a MODU and not a fixed rig e.g. platform

Drilling shoe and hole cleaning? For me based on my experiences I
view negligible risk of drilling out a 20" shoe and perhaps 40ft of
cement with an 8 1/2" bit. e.g.

Higher flowrates could be applied until on shoe?
Sure transport efficiencies would be less, this in no big deal, one
perhaps just has to pump more and if cannot control ROP to stay out
of minor troubles that may or most likely not occur!
Cutting volujmes would be very low (8 1/2" hole!) and readily
cleaned, aided if required by hi-viscous sweeps?
Once on shoe?
Pump another sweep?
Then normal 8 12" flow for drilling pilot hole?

The most likely result is a perfect 8 1/2" cylinder in the 20" shoe.
(Because cement, even in as a sheath, is much harder to de-stabilise
that the formation you will likely to be drilling? e.g. I pulled a
conductor on a land rig in 2004 that had a perfect 26" cement sheath
in the bottom 5 ft of it, and we banged the hell out of it for hours
trying to drill a tricky 26" wellbore.

Worst risk? is cement sheath falling in (a low probability event)
where one would challenge anyone to find historic data of a
consequential stcuk string? Perhaps 15-20mins to clean up the cement
blocks would be worst consequential effect collated from drilling
reports. For me cement block falling into wellbore from shoe annulus
is a much higher risk that we have at times lost an hor or two
cleaning up before getting drilling ahead. Never getting stuck
(permenantly) however

Drilling an 8 1/2" pilot hole into a shallow horizon with a 20"
casing annuli is however very risky?
Not to be recommended as a best practise at all.

Because one would loose any dynamic kill effect from the pilot hole
in the 20" shoe annulus? Unless wellflow dyanmics modelling supports

Therefore 'best practice' is not to attempt unless you have
conducted wellflow dynamics modelling and have engineered
capability to pump higher killing dynamic flowrates required? This
often take note results in a 12 1/4" pilot hole being the least risk

All these issues are of course the usual can of worms one can
have 'to drill or not to drill shallow horizons' via casing shoes,
what pilot hole size etc.

Best practise from a MODU is 'from lessons learned' to drill the
pilot (investigation?) hole from seabed all way through horizon
before any casings are set at all!.

Information gained from the pilot (investigation?) wellbore then
enables 'data for decision' e.g. optimal casing setting, if horizon
is a problem and if casing can now be set shallower or deeper. If
well flows all you have commited to is a pilot hole that you may be
able to dynamically kill, but more importantly can get the rig,
equipment and people safe from at least risks! and then readily move
rig and re-spud at minimal risk and operational cost/loss. Before
commiting capital equipment, time and money to the well?

Drilling Supervisor
Total Posts: 5
Join Date: 22/06/05
Cannot see any problem with this practice given that: -

1) 20" shoe is drilled-out to its nominal size

2) Rat-hole is also 'cleaned out' with a 17 1/2" bit & BHA before
drilling 8 1/2" commences.

This practice is no worse (probably better) than drilling through a
marine riser!

Thanks Ian Frizell
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