Summary
Carbon steel CT (CT) strings have been used in sour wellbore environments
for many years. The use of CT in sour service has increased by job number, job
complexity, pipe size, and the stresses to which the pipe is subjected. A
number of papers have been written on the steel chemistry and low-cycle fatigue
behavior. These papers have been based on both theoretical and laboratory work.
Great progress has been made in understanding the chemical and physical
interaction of CT and H2S, but little information is available from
“real-world” situations in which the pipe has been worked in a variety of job
types (e.g., acidizing, gas lifting, and drilling) and in which a large number
of other factors may come into play (e.g., injector damage, well-stimulation
chemicals, sour inhibitor application techniques, erosion of inhibitors, CO2,
and high-chloride water production).
This paper discusses the application of a CT technical specification (based
on theoretical and laboratory work) in the operational world. Comparisons will
be made between theoretical expectation and practical observation for 70- and
80-grade CTs with regard to low-cycle fatigue, pipe life, and damage in sour
environments.
Introduction
CT has been used successfully for more than 20 years in sour well
environments in Canada and elsewhere. The nature of the wells, however,
has changed, which, in turn, has changed the demands placed upon
CT. Accessing sour wells under live conditions, to perform the usual range
of interventions offered by CT, is an increasing requirement. To meet these
requirement changes, the CT manufacturers—for their part—have responded by
offering a greater range of yield strengths, diameters, and wall
thicknesses.
Coincident with these changes to CT have been changes in the requirements
placed upon the industry in Canada by the regulatory authorities. Parts of
these requirements are outlined by MacArthur et al.1
The Alberta Energy and Utilities Board (AEUB) began to extend the concepts
noted in the above paper when considering approvals for the underbalanced
drilling of critical sour wells. Work was jointly undertaken by several
industry experts to develop an industry recommended practice (IRP) under the
auspices of the Drilling and Completions Committee (DACC). Part of the
difficulty with developing the IRP with respect to CT is that the material is
used in such a way that continually subjects it to plastic-strain
reversals. This type of mechanical-strain situation is not examined by
the Natl. Assn. of Corrosion Engineers (NACE) when preparing its list of
recommended materials for sour serviceability.2 These issues are discussed
by H.B. Luft and G. Wilde.3
Additionally, sour-environment testing of CT by use of the bent-beam
methodology, as specified by NACE Standard TMO177-96,4 was performed by a Joint
Industry Project (JIP) on several grades of CT,5 as well as low-cycle fatigue
of samples exposed to a sour environment. Part of the outcome of the JIP’s
testing would suggest that there is no difference in susceptibility to a sour
environment for the differing grades of CT, but all are equally affected when
subsequently subjected to low-cycle fatigue.
Further testing work continued to determine the suitability of CT for use in
critical sour underbalanced drilling for the IRP, and under what operating
conditions.6
These previous bodies of work can not be directly translated for use in
general workover applications because the work contained in both the JIP study
and in paper SPE 81723 relates specifically to CT that is exposed for extended
periods, solely to sour environments.
However, some information was extracted out of the testing that had been
performed to help formulate a purchase specification for CT that would be used
in general workover well interventions.
This paper presents a review of how strings of CT used in interventions have
performed after the introduction of the purchase specification and how a
complete string-management system has been developed. It should be noted that
none of the strings specified have been used in drilling critical sour wells
because there is a moratorium on this activity. The data presented, and the CT
strings reviewed, are from a single service company.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
4 February 2004
- Revised manuscript received:
4 January 2005
- Manuscript approved:
19 February 2005
- Version of record:
15 March 2005
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