Page 118 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 94

[ ] Well Design
1.4.21

The number of spring centralizers in the open hole may be reduced
if wear is likely in a subsea wellhead inner profile.

Liners 7 in or smaller (springs between stop collars due to the small
annular clearance):

Two springs on each of the first two joints
One spring per two joints in the open hole
One rigid inside the previous casing shoe

Float equipment and accessories. For a normal one-stage cement
job, the usual setup is a floatshoe with a float collar at two joints up the
string and bottom and top plugs. Other equipment will be needed for
multistage jobs, subsea operations, etc.
Correctly sized top and bottom plugs are both vital. If no bottom
plug is run, the top plug will wipe a film of mud ahead off the casing,
which may be enough to contaminate the cement around the shoe. For
deep, critical jobs two bottom plugs may be used to separate mud,
spacers, and slurry. This may take longer to drill out but an hour of
extra drilling is worth paying for a better cement job.
If a mixed casing string of more than one inside diameter is used,
special plugs may be needed. Generally the plugs will fit one casing OD
for a range of casing weights, but it is possible that use of extremes of
weights in the same size casing or different casing sizes cannot be cov-
ered with the standard plugs, and you may have to arrange special
order plugs. Do not accept standard plugs modified by sticking on rub-
ber strips around the top fins; the strips can become detached and plug
the floats.
Types of liner hangers. A liner hanger generally uses a set of
slip elements to grip the wall of the previous casing. Mechanical
and hydraulic setting systems are used, though hydraulics are used
more often.
In addition to the basic liner hanger, some systems contain integral
packers that may be set after cementing. These allow reverse circulating
out of excess cement without imposing circulating pressures on the for-
mations if they are set immediately after finishing cement displacement.

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