Tubular Materials Selection Challenges for Oil and Gas
Drilling/Completion/Production Applications
Ravi Krishnamurthy, Rudolf Hausler, Ming Gao
Materials selection requires integration of the strength requirements (defined by anticipated loads),
expected environments (drilling, completion, production) and relevant metallurgical properties. The
tubulars must be designed for exposure to drilling, completion and production environments. The
producing environments are the basis for corrosion design. Acid gas (H2S & CO2) concentrations and
water chemistry measurements are made on surface under ambient conditions. These concentrations
should be translated to equivalent partial pressures under downhole pressure and temperature. Partial
pressures at higher total pressure should be adjusted with fugacity coefficients to reflect acid gas
concentrations. Finally, separator acid gas measurements should be partitioned into various phases such
as water, oil and/or gas. Then the pH can be defined along the wellbore, and with other parameters
defines material response to the environment. There is a time dependent variation in the producing
environment that may be considered, depending on the available data and/or reservoir understanding.
In addition to produced fluids, high-pressure, high-temperature (HPHT) and ultra-deep HPHT wells
require a complete understanding of the completion and packer fluid interactions with the materials.
These interactions should address corrosion and cracking (SSC/SCC) as a function of time, temperature,
pressure, additives, acid gas and oxygen contamination.
Metallurgical options for tubulars and completion equipment range from conventional carbon steels to
martensitic/duplex and austenitic stainless steels and as well as precipitation hardened and cold worked
nickel based alloys. The SSC resistance of carbon steels decreases with increasing yield strength;
material chemistry and toughness are modified to mitigate SSC susceptibility. The brittle burst or SSC
resistance can be quantitatively established for load cases, such as cold-shut-in tubing leak.
Traditionally, all tubular material including production casing are carbon steel; however in the presence
of increasingly corrosive environments, alternatives to carbon steel are required for the liner and tubing.
The first option with increasing carbon dioxide concentration is the martensitic stainless steel. Further
increases in temperature, pressure, chloride, and acid gas concentrations, duplex/austenitic stainless
steels and nickel based alloys maybe required. However, changing alloys introduces a host of challenges
regarding galvanic effects, completion/packer fluids, stimulation options and others.
Tubulars and completion equipment are conventionally designed by ensuring that triaxial, axial, collapse
and hoop stresses are below nominal yield by a factor. Other properties such as fracture toughness and
ductility are not quantitatively used in design. As wells get deeper, and pressures and temperatures
continue to increase the yield strength requirements will continue to increase. As yield increases,
toughness and ductility that aren’t normally part of the design have to be considered in design.
Alternatively, the tubular design needs to incorporate limit states and reliability to minimize yield
strength requirements and mitigate other factors that could contribute to failure.
As the severity of downhole environments increase, all aspects of tubular design should be optimized.
Parameters that are not traditionally considered in the past could result in failures. Environmental
conditions should be comprehensively defined. All metallurgical properties must be considered, and
relevant qualification and QA/QC testing must be conducted. Tubular designs should incorporate
environment, metallurgy and loads to ensure that all possible failure modes are addressed in design.