Page 111 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 87

1.4.18
Casing Design [ ]

If the casing has been designed for the 1 psi/ft collapse gradient and
if the casing has a good cement sheath throughout the salt interval then
failure probability is minimal. It is important to note that for prevent-
ing distortion and shear, a complete and competent cement job is as
important as the casing strength. Refer to “Cementing against massive
salts” in Section 2.7.4.

1.4.18. Casing Properties and Other Considerations

Having calculated the minimum strength requirements and pre-
ferred weights and grades of the casing, you now have to check against
other considerations. These include:

1. Inside diameter for running completion tools. In production casing
the ID is important to ensure that all required completion compo-
nents can be run. For instance if a dual 3 1 /2 in completion was to
be run, 9 5 /8 in 57# casing may not give the required clearance to
run the completion accessories.
2. Seamless pipe vs. seamed, electric resistance welded (ERW) pipe. The
seamless method is most common for pipe production. Historically
seamed pipe was not used for casings below surface casing due to
considerations of quality of the pipe. Modern ERW pipe can now
be produced in quality equal to seamless pipe and because it is
cheaper, ERW pipe can save a lot on the cost of a well. Major oper-
ators such as Shell have decided that seamed pipe can be used as
casing for deeper strings where quality control is assured.
Whether or not you can use seamed pipe will probably be dictated
by company or government policy. It is certainly worth the effort
to consider seamed pipe.
3. Availability.
4. Cost. Of the casings that are both suitable and available in time, the
lowest cost string can be chosen.

1.4.19. Material Grades

API defines the characteristics of various steels and assigns letters
to identify those grades; refer to API Specification 5CT for complete def-

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