Page 108 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 84

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1.4.15 Well Design

2
The cross-sectional area of this casing is 26.44 in , therefore, a
stress of 7440 psi represents a load of 7440 x 26.44 = 196,700 lbs due
to the effects of thermal expansion.
Applying extra tension of 196,700 lbs (which would require
stretching the pipe by 2.48 ft as calculated above) before setting the
slips would cause the “heated” neutral point to be back to the original
position before the “unheated” casing was stretched.
Possible courses of action. If the neutral point is above the
planned top of cement, then several actions can be taken to prevent
buckling taking place after cement has set.

Change the cement program. If using an extended lead slurry, then
reduce the depth of the planned TOC and/or further extend the
slurry to reduce the buoyant force. Casing supported by set cement
can be below N but will not buckle.
b
Calculate the effects of temperature increases after setting cement;
land the casing under extra tension once cement has set such that
at the top of the supported casing interval, F < F .
s
b
Place centralizers higher up to cover the length from the neutral
point down to the TOC, calculating where the new neutral point
will be after accounting for temperature effects any extra tension
applied when landing the casing.

1.4.15. Calculating Torsional Loads

The limiting factor for torque is the casing connection. Makeup
torque will always be less than the casing strength in torsion. This
only becomes a factor to consider when the casing (or more com-
monly liner) will be rotated during cement displacement to improve
mud removal.
On the last trip out of the hole, stop with the bit at the liner hang-
er depth. Record rotating torque. Limiting torque for the liner is then
50% of liner makeup torque + string rotating torque.
Torque can be estimated, given the assumed coefficient of friction,
the radius of the casing, the force acting to press the pipe into the well-
bore wall and the length. Generally the assumed coefficient of friction
in open hole is 0.35 and in casing it is 0.20. These can be used unless

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