Sour service is not a grade — it is an environment. Once a well's produced fluids contain enough hydrogen sulfide to cross the threshold in NACE MR0175 / ISO 15156, the casing and tubing in that well must resist sulfide stress cracking (SSC). Choosing the wrong grade does not show up on day one. It shows up as a parted string.
Start with the partial pressure
The governing number is the H₂S partial pressure, not the concentration alone. Multiply the mole fraction of H₂S in the gas by the total system pressure. Under ISO 15156-2, when the H₂S partial pressure is at or above 0.05 psia (0.3 kPa), the well is in the sour domain and material must be qualified for it.
If you do not have the gas analysis yet, tell your supplier what you know: field, formation, expected bottom-hole pressure. A conservative assumption is cheaper than a workover.
Why P110 is usually excluded
High-strength grades are more susceptible to SSC because hardness and yield strength go up together. P110 has a minimum yield of 110 ksi with no maximum hardness limit in API 5CT, which is why it is generally not acceptable for sour service at ambient temperature. Some operators accept P110 in deeper sections where temperature is high enough to reduce SSC risk — that is an engineering decision, documented in the well design, not a procurement shortcut.
The sour-service grades
- L80 Type 1 — 80 ksi minimum yield, hardness limited to 23 HRC maximum. The workhorse for sour production casing and tubing.
- C90 — 90 ksi minimum yield with tighter hardness control and mandatory SSC testing. Used where L80 strength is not enough.
- T95 — 95 ksi minimum yield, also hardness-controlled and SSC-tested. The highest-strength standard sour grade in API 5CT.
Note that L80 9Cr and 13Cr are corrosion-resistant variants aimed at CO₂ (sweet) corrosion; they are not interchangeable with L80 Type 1 for SSC resistance.
What to check on the MTR
Before the string is run, the mill test report should show:
- Grade and type exactly as ordered — "L80" alone is not enough; look for L80-1.
- Hardness test results for each heat and lot, within the grade maximum.
- Heat treatment (quench and temper) and tempering temperature.
- SSC test results where the grade requires them (C90, T95).
- Heat numbers that match the pipe stencils.
Connection matters too
The coupling and any accessories — pup joints, crossovers, float equipment — must be the same sour-service grade as the pipe. A T95 string with a P110 crossover is a P110 string.
If you are unsure which grade fits your well, send us the casing design and gas analysis. We will confirm the grade options and quote them side by side.