Materials and Design Basis of Oil Well Casing
Oil well casing is the large-diameter steel pipe assembled and run into a drilled borehole and secured by cementing. Unlike production tubing, which carries reservoir fluids to the surface, casing is permanently installed to isolate downhole formations and provide a durable conduit for the entire well life. All oil casings are made from low-carbon steel, and the American Petroleum Institute classifies them into grades according to the mechanical properties of that steel.
Primary Functions and Critical Performance Metrics
Casing performs five indispensable functions, each governed by a specific mechanical property defined in API 5CT:
| Function | Critical Metric |
|---|---|
| Borehole stability | Collapse resistance against external overburden pressure |
| Formation isolation | Internal yield pressure for zonal integrity |
| Pressure control | Tensile strength for hanging weight and running loads |
| Production support | Combined mechanical performance for a uniform conduit |
| Corrosion resistance | Alloy-dependent resistance to H2S, CO2 and chemicals |
API 5CT Grades and Mechanical Properties
API 5CT defines grades covering the full range of well conditions: H40, J55, K55, M65, N80, R95, L80, C90, T95, P110 and Q125. Representative yield strength bands are shown below:
| Grade | Yield Strength Min (MPa) | Yield Strength Max (MPa) | Tensile Strength Min (MPa) |
|---|---|---|---|
| H40 | 276 | 552 | 414 |
| J55 | 379 | 552 | 517 |
| K55 | 379 | 552 | 655 |
| N80 | 552 | 758 | 689 |
| R95 | 655 | 758 | 724 |
| M65 | 448 | 586 | 586 |
| L80 | 552 | 655 | 655 |
| C90 | 621 | 724 | 689 |
| T95 | 655 | 758 | 724 |
| C110 | 758 | 828 | 793 |
| P110 | 758 | 965 | 862 |
| Q125 | 862 | 1034 | 931 |
Key specifications also cover thread connection types - STC (short round thread), LTC (long round thread) and BC (buttress thread) - outer diameter, wall thickness, length ranges (R1, R2, R3) and chemical composition limits that define hardenability and corrosion behavior.
Types of Casing Strings
Oil wells typically require several concentric strings, each serving a distinct purpose:
Conductor casing:
the first string set, protecting loose near-surface formations and enabling drilling-fluid circulation
Surface casing:
blowout protection, water-sand isolation and lost-circulation prevention
Intermediate casing:
isolates unstable hole sections and abnormal-pressure transition zones
Production casing:
contains formation pressure and supports tubing and completion loads
Liner:
a string hung from another casing instead of returning to the wellhead, reducing cost and improving hydraulics
Tieback string:
restores pressure integrity from the liner top back to the wellhead
Selection Variables
Casing grade selection must be right before the pipe is ordered and depends on four primary variables: well depth (collapse pressure), downhole pressure (burst rating), temperature (derating and connection sealing) and fluid chemistry (CO2, H2S and chlorides).
Frequently Asked Questions (FAQ)
What material is oil well casing made of?
Low-carbon steel processed to API 5CT, divided into grades by mechanical properties rather than by alloy content alone.
Which casing grade is the most common?
J55 and K55 are widely used for shallow to medium wells; N80, L80 and P110 dominate deeper, higher-pressure completions.
What is the difference between casing and tubing?
Casing is the larger, cemented structural string that lines the borehole; tubing is the smaller string that carries reservoir fluids to the surface.
What do STC, LTC and BC threads mean?
They are API connection types: short round thread, long round thread and buttress thread, chosen by tensile efficiency and sealing requirements.
Why does sour service need special grades?
Grades such as L80, C90 and T95 are available with restricted hardness to resist sulfide stress cracking in H2S environments.
How is collapse resistance determined?
It is the maximum external pressure the casing can withstand without permanent deformation, calculated from dimensions, yield strength and ovality per API 5C3 methods.
