Page 112 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 88

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1.4.19 Well Design

initions. Some manufacturers also define non-API grades; refer to the
manufacturer’s literature for details.
The different grades may vary by chemical composition and/or heat
treatment. The grade of steel needs to be suitable for the application. If
a range of suitable grades is available, then the cheapest can be used.
Minimum yield stress as defined by API is the stress required to
elongate the specimen by 0.5% in normal strength steel, 0.6% in Grade
G and 0.7% in Grade S.
Exposure of high-tensile grades of steel to partial pressures of
hydrogen sulfide (H S) greater than 0.05 psi at less than a threshold
2
temperature (which varies by steel grade) can lead to catastrophic
failure. Refer to NACE Standard MR-01-75 (current version) for spe-
cific information on suitable steels for use in an H S environment.
2
API Specification 6A also covers selection of wellhead equipment for
H S exposure.
2
×
Pp = ppm psia
,
1 000 000
,
where P is partial pressure, ppm is the concentration of H S (or
p
2
other gas) in parts per million and psia is the absolute pressure of gas
at the depth of interest. For instance at 1000 psi, 50 ppm gives a par-
tial pressure of 0.05 psi.
Resistance to CO corrosion (alone or combined with H S) is
2
2
improved by using chromium alloy steels such as L80 13% Cr. Cr-Mo
alloys improve performance in low pH conditions.
Where the application is for severe service (combinations of wet gas,
combined CO /H S, higher pressures, high flow velocities, presence of
2
2
chlorides in brines, etc.) then a thorough investigation will be needed
once the application parameters have been defined. Expensive alloys
with high levels of chromium, molybdenum, and nickel may be needed.
These alloys are very costly, require special handling to and on the
rig, and tend to gall more than carbon steels.
Following is a brief summary of some currently available common
API grades and their characteristics. Further reference should be made
to the literature (see references in Section 1.4.24, “References for
Casing Design”) and to manufacturer’s data to obtain specific and up-
to-date information.

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