Working in hydrogen sulfide environments requires specifically designed metal compounds.

The manufacturing technology for critical service drill pipe has evolved significantly in the last several years. Major advances relating to drill pipe specifically developed for use in areas with significant H2S content have been realized. Traditionally, sulfide stress cracking (SSC) of drill pipe was prevented by controlling the environment. As the severity of sour drilling applications has increased, the need for drill stem materials resistant to SSC has accelerated.

Material selection for drill pipe is more complex than material selection for production casing and tubing in sour applications. If the production casing or tubing designer determines that materials resistant to SSC are required, NACE MR0175 guidelines and tables provide clear guidance for selecting materials that can be used with a high level of confidence.

The situation is somewhat different for the drill string designer. Per NACE MR0175, standard American Petroleum Institute (API) drill pipe material grades are all acceptable if SSC is avoided by controlling the drilling environment. Situations where the H2S concentration and partial pressure are at levels above where drilling fluid control is reliable and underbalanced drilling (UBD) situations where the drill string can have prolonged, direct H2S exposure are not addressed.

In addition, a drill string assembly incorporates a tool joint and a friction-weld that attaches the tool joint to the pipe body upset. These areas of the drill pipe assembly must be considered to provide reliable operation in critical sour drilling applications.

Evolution

The process of developing drill pipe for critical sour drilling has been evolutionary rather than revolutionary. The first string of sour service (SS) drill pipe, manufactured in 1993, consisted of tubes with minimum yield strength of 95,000 psi that were resistant to SSC combined with standard API tool joints.
This pipe was initially developed for use in the Rocky Mountain region of Wyoming where operators were experiencing SSC related failures with standard API drill pipe. The pipe was later used extensively in the Middle East and some parts of Canada. Over 5 million feet of this drill pipe have been manufactured since 1993.

No documented SSC failures have occurred in the pipe bodies or tool joints of this product, even though the tool joints were manufactured to standard API specifications. The peak stress in a drill pipe assembly occurs in the pipe body upset transition zone. SSC is a function of several factors, including total tensile stress in the component exposed to H2S. Since the stress levels are relatively low in the tool joints due to their larger cross-sectional areas, the product has performed well in moderate to high intensity H2S environments when combined with proper control of drilling fluid parameters.

The next evolution to occur in sour service drill pipe was the development of a higher strength material grade resistant to SSC for the pipe body, and controlled yield tool joints that provide better cracking resistance. A major drill pipe manufacturer and operator worked together to develop a 105,000 psi minimum yield strength grade, for use in the Tengiz oil field in Kazakhstan. Standard API G-105 drill pipe had experienced SSC failures in this high H2S concentration area.

The first string of 5-in. 105,000 psi SS drill pipe went to work in the Tengiz area in early 1998. Four strings of 5-in. and two strings of 27/8-in., 105,000 psi SS were ultimately used to drill in this area. These strings are still in use with no recorded SSC failures in spite of water drilling operations, underbalanced mud columns and prolonged direct H2S exposure. Additional 105,000 psi SS strings are being used in the Gulf of Mexico and in Canada. A total of approximately 600,000 ft (183,000 m) of 105,000 psi SS has been manufactured.

More recent sour service drill pipe development has focused on higher strength material grades, tool joint metallurgy and welding technology with improved SSC resistance for high H2S concentration environments, including applications where environmental control may be difficult such as during well control or UBD operations.

Specifications

More recently, tool joints resistant to SSC have been developed and used to manufacture pipe for critical sour drilling. These tool joints are manufactured using specialized chemical compositions and highly controlled heat treatment processes.

Since SS tool joints have reduced yield strength relative to standard API tool joints (110,000 psi vs. 120,000 psi), special consideration must be given to the connection torsional strength. One method to address torsional capacity concerns is to use proprietary double-shoulder tool joints (Figure 1) with increased torsional strength compared to API designs.

Over 45 drill strings with SSC resistant tool joints comprising approximately 450,000 ft (137,250 m) of pipe have been manufactured since 1999.

Conclusion

Major strides in metallurgy and manufacturing technology have resulted in drill pipe systems with enhanced SSC resistance. Moving forward, operators, contractors, manufacturers, rental companies and regulatory boards must work together to develop guidelines and requirements to enable our industry to safely exploit hydrocarbon reservoirs in more severe H2S bearing formations.

Comments