Page 106 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 82

[ ] Well Design
1.4.14

Fs = A P – A P
o o
i i

where

A = Area of a circle of tubing OD size = 0.7854D 2
o
P = Pressure outside tubing
o
A = Area of a circle of tubing ID size = 0.7854d 2
i
P = Pressure inside tubing
i
Buckling will occur in unsupported tubing when the compressive
force exceeds the stabilizing force. The point where F = F is the neu-
b
s
tral point for buckling (N ). It is not the same as the neutral point for
b
axial force (i.e., where tensile stress = 0).
An open tube suspended in a fluid will not buckle, as long as the
fluid density does not exceed the density of the tube material. This is
not going to happen for casing unless you are using mercury instead of
mud! N is at the bottom of the tube. At the bottom end, F = tube
b b
cross-sectional area x hydrostatic pressure (buoyancy force) and F =
s
(area of outside diameter - area of inside diameter) x hydrostatic pres-
sure. A - A = cross-sectional area, F clearly equals F , which is the
o
i
b
s
neutral point for buckling, N .
b
A suspended, unsupported tube will buckle if the bottom end is
closed and internal pressure is increased. This will not occur during a
cement job when the cement is inside the casing because the bottom
end is not closed, fluid is exiting the bottom, the weight of the slurry
will increase tension, and the casing will almost always be supported
by centralizers.
In practical terms then, we do not need to consider buckling of cas-
ing during cementing. Once cemented, however, the picture changes.
F will increase with increased temperature (as the bottom end is fixed
b
and the casing expands). F will decrease with increased internal pres-
s
sure—say as heavier mud is used while drilling the next hole section.
N will move up and if it moves above the point where casing is well
b
supported, buckling will occur.
For a deeper cemented casing string in a vertical well, carry out the
following calculations.

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