Page 63 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 39

Casing Design [ ]
1.4.1

the conductor in, it is not likely to support the extra hydrostatic pres-
sure imposed by closing the circulation system. No diverter is attached;
if shallow gas is encountered the rig will drop the drillstring and move
off location.
If cemented, the conductor is always cemented to the surface or to
the mudline. Sometimes losses may occur during cementing, and if this
causes the cement to drop below the surface, a top-up job is needed
where tubing is run outside the conductor from the surface and cement
is pumped into the annulus once the primary cement has set. This is
only possible where the annulus outside the conductor is accessible.
The conductor will have to resist the compressive loads imposed by
subsequent casing strings, completion strings, wellhead, and BOP
weights. Buckling may be a consideration if the conductor extends a
significant height above the soil and is not supported. On a bottom-sup-
ported offshore facility it will also be subjected to waves and currents as
well as severe corrosion conditions in the splash zone. If driven it must
handle the driving loads. It may have to resist collapse pressures if loss-
es are encountered or if a diverted gas kick evacuates the conductor.
Surface casing. This is normally the first pipe that can take a
blowout preventer on top. The shoe must be set deep enough so that the
formation fracture pressure is high enough for the well to be closed in
on a kick while drilling for the next casing string. Any gas encountered
before a BOP can be nippled up is termed “shallow gas.”
As surface casing in some development areas is set quite deep
(sometimes deeper than 3000 ft), shallow gas can be encountered fair-
ly deep. It is not correct to refer to gas as shallow gas if it occurs after a
BOP is nippled up on surface casing.
During the well life, surface casing may be subject to burst pressure
from well kicks, bad cement jobs (fluids migrating up outside subse-
quent casings), or to collapse pressure if the fluid level inside drops due
to losses or if bad cement jobs allow migration of gas outside the casing.
Surface casing is normally cemented to the surface or to the mudline.
Intermediate casing. Intermediate casing is run in deeper wells
where kick tolerances or troublesome formations make it unsafe or unde-
sirable to drill from surface casing all the way to the production casing
setting depth in one hole section. Therefore, its primary drilling purpose
is to resist the forces imposed by kicks, losses, and mobile formations.
The top of cement may be planned below the previous casing shoe.
This gives you the option to cut and pull casing and sidetrack out with-
out losing a hole size, as long as the wellbore remains stable enough for

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