Page 88 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 64

[ ] Design
1.4.9Well

indicates that using thicker wall sections is preferable to using higher
grades of steel.
Temperature correction factors for steel. Correction factors have
an engineering basis, unlike safety factors that are arbitrary. Correction
factors are applied as well as the relevant safety factors.
The yield strength of steel usually decreases with increasing tem-
perature. A temperature correction factor can be applied to the mini-
mum yield strength before applying the safety factor as mentioned.
This correction factor should be obtained from the casing manufactur-
er. Apply the temperature correction factor as noted here.
Tension. Tensile load will decrease with depth so that as the casing
gets hotter, it is also subjected to less tension. If the top joint of an
unmixed casing string is strong enough in tension, it should be fine
lower down. If a mixed string is used (different sections of the overall
string that have different weights and/or grades) apply the factor when
evaluating the tension applied to the top component of each section.
Compression. Compression is unlikely to be relevant. Helical buck-
ling is more likely to occur than failure in compression in the hot part
of the string.
Burst. Burst could be very relevant at depth, especially in a high-
pressure, high-temperature well. This is not likely to be a problem
while drilling but may be a problem later in the life of the well. If a frac
treatment or other procedure can be used where significant surface
pressure may be applied, this burst pressure will be imposed down the
exposed casing string (i.e., above any packers set).
Multiply the burst strength by the temperature correction factor
and apply the safety factor before comparing the amended burst
strength to the calculated burst pressure.
Collapse. Collapse could be a problem while drilling if severe loss-
es are taken, high drawdowns are used during production, and the
reservoir becomes depleted. Also if massive salts are covered, we gen-
erally assume that the salt transmits the full overburden pressure
against the casing in collapse (1 psi/ft).
Multiply the collapse strength by the temperature correction factor
and apply the safety factor before comparing the amended collapse
strength to the calculated collapse pressure.
The following table is supplied by Nippon Steel. Figures for other

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