Page 116 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 92

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1.4.21 Well Design

recommendations should be examined closely to ensure that they meet
your requirements.
In between centralizers in a deviated well, the casing will sag down
towards the low side. The amount of this sag can be calculated as follows:

Dmp =
Dmp . 3 0558 × 10 − 6 wL 4 sin f
D −
D 4 d d 4

where

Dmp = Deflection at midpoint, in inches
w = Weight of casing, in lbs/ft
L = Length between centralizers, in feet
f = Inclination, in degrees
D = Casing OD, in inches
d = Casing ID, in inches

Of course, the amount of sag will have to be added to the distance
from the wellbore center to the casing center at the centralizers if the
total amount of eccentricity is needed where the centralizer OD is less
than hole diameter, such as in a washed out hole.
In a mudline suspension system the manufacturer sometimes sug-
gests that centralizers not be used, to avoid wear on the inner profile.
An uncentralized casing will give you little chance of successful
cementing. Minimize the number to those run over critical sections
(especially the shoetrack), but not running any at all will compromise
the cement job.
Placing a centralizer over a stop collar will reduce wear on the
wellhead, casing, and centralizer, and will also reduce drags. Some
small diameter centralizers do not have enough clearance for this and
so must be run between two collars. Rigid centralizers are usually
placed between two collars.
Centralizer placement. Rigid centralizers must not be run in open
hole. Following is a general guide to use when the centralization pro-
gram is not calculated. It may be modified as necessary. Where a sub-
sea wellhead or suspension system is in use and the manufacturer rec-

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