When selecting steel type for pipes and connections it is important to consider the corrosive environment that the steel will be subjected to. There are several parameters in the well that affect the corrosion, like temperature, chloride ion concentration, partial pressure of CO2 and H2S, pH and presence or absence of Sulphur [Craig et al. 2011].
When selecting a material there are certain aspects that have to be taken into consideration [NORSOK M-001 2004]:
- Corrosivity;
- Design life;
- Availability;
- Failure possibility and the consequences related to failure;
- Resistance to brittle fracture;
API tubing steel grades are identified by letters and numbers which dictate various characteristics of the steel. For each grade, the number designates the minimum yield strength. Thus L-80 grade steel has a minimum yield strength of 80,000 psi. In other words, it can support a stress of 80,000 psi with an elongation of less than 0.5%. The letter in conjunction with the number designates parameters such as the maximum yield strength and the minimum ultimate strength which for L-80 pipe are 95,000 psi.
The following table shows the yield values for various API tubing grades:
In addition to API grades, there are many proprietary steel grades which may conform to API specifications, but which are used extensively for various applications requiring properties such as:
- Very high tensile strength;
- Disproportionately high collapse strength;
- Resistance to sulfide stress cracking.
Many tubing strings are run which contain these non-API Tubulars. This pipe is made to many but not all API specifications, with variations in steel grade, wall thickness, outside diameter, thread connections, and related upset. Due to these variations, the ratings of burst, collapse, and tensile specifications are non-API.
Guidelines applied to the selection/use of tubing grades:
The following table gives guidelines applied to the selection and use of tubing grades:
| Tubing Grade | Guidelines applied to the selection/use of tubing grades |
| H40 | · An API grade,
· The Yield strength is relatively low and the cost saving over J55 is minimal. This is why H40 is generally not used in tubing sizes, · Used only for non-critical, shallow wells, · Do not use for sour service applications, · H40 couplings don’t have an API toughness requirement. K55 couplings are used instead. |
| J55 | · A commonly used grade when it meets the design criteria and has been the “standard” grade for shallow tubing strings (< 9,000 ft) and low-pressure (< 4,000 psi) wells on land,
· Fit for H2S service at all temperatures. |
| K55 | · Fairly common for large diameter surface casing couplings, although line pipe grades X52 and X56 are becoming a popular replacement,
· Fit for H2S service al all temperatures, |
| C75 | · No longer an official API grade,
· It was developed as a higher-strength material for sour service but was replaced by L80 tubing. |
| N80 | · Acceptable for sweet oil and gas wells when it meets design conditions.
· Fit for H2S service at temperature greater than 150°F if quenched & tempered (Q&T), · Fit for H2S service at temperature greater than 175°F if Normalized (N) & tempered (N&T), · Normally less expensive than L80 grades. |
| L80 | · Very common tubular grade, and often used for sour service production casing, production liners, and tubing.
· Has a maximum hardness requirement. · Satisfactory for SSC (sulfide stress cracking) resistance in all conditions but may incur weight-loss corrosion. · Insist on hot rotary straightening when ordering this product. · Available in three different chemical requirements: Type 1, 9 Cr and 13 Cr, · Type 1 L80 is less expensive than 9 Cr and 13 Cr but more subject to weight-loss corrosion, · Type 9 Cr largely has been replaced by Type 13 Cr. L80 13 Cr tubing has gained popularity because it has good CO2 -induced weight-loss corrosion resistance properties; however, it is more costly, · Type 13 Cr may not be suitable in all hydrogen sulphide (sour) service environments. Typically, the H2S partial pressure should be less than 1.5 psi for safe use of L80 Type 13 Cr. For more details, refer to “ NACE MR 0175/ISO 15156: Materials for use in H2S-containing environments in oil and gas production ” issued by the National Association of Corrosion Engineers. |
| C90 | · API grade,
· Has a maximum hardness requirement. · Available in two different chemical requirements: Type 1 and Type 2. · Only Type 1 is recommended for use in sour service. · The use of this grade has been generally replaced by T95. |
| C95 | · Not a sour service grade.
· Fit for H2S service at temperature greater than 150°F. · Usually used for intermediate casing strings that may be exposed to H2Sduring a gas kick. |
| T95 | · A high-strength tubular grade,
· Has a maximum hardness requirement. · Often used for HPHT sour gas production casing strings when L80 and C90 don not have enough strength for the desired well thickness. · Available in two different chemical requirements: Type 1 and Type 2. · Only Type 1 is recommended for sour service. · Sulphide Stress Cracking (SSC) resistant but not weight-loss resistant. · Very expensive. |
| P110 | · The old P105 tubing grade,
· API has absolutely minimal chemistry requirements for this grade. · Insist on hot rotary straightening when ordering this product. · High-strength tubing typically is used in deep sweet oil and gas wells with high pressures. · Sensitive to Sulphide Stress Cracking (SSC) failures unless the temperatures are relatively high (> 175°F). |
| Q125 | · API tubing grade, often used for critical service wells.
· Fit for H2S service at temperature greater than 225°F. · Type 1 chemistry is preferred. |
Reference:
- http://petrowiki.org/International_standards_for_tubing
- Completion Design Review with Focus on Well Integrity and Productivity, Karianne Skårnes Vågenes, Norwegian University of Science and Technology https://brage.bibsys.no/xmlui/handle/11250/239878
- NACE-MR-0175-ISO-15156-2015: Materials for use in H2S-containing environments in oil and gas production https://www.nace.org/newly-revised-nace-mr0175-iso1516-standard-now-available.aspx
