API 5CT defines the casing grades that protect wellbores from formation collapse, groundwater contamination, and uncontrolled fluid migration. Selecting the right grade for each casing string — surface, intermediate, and production — requires matching the grade’s mechanical properties to the specific downhole loading conditions at each depth interval.
Surface Casing: J55 and K55
Surface casing is the first structural string set in a well, cemented from the shoe to surface. Its primary functions are to protect shallow freshwater aquifers from drilling fluid contamination, provide structural support for the wellhead and BOP equipment, and serve as the foundation for all subsequent casing strings. Because surface casing is set at relatively shallow depths (typically 50–500 meters), the external pressure from formation fluids and the internal pressure from well control operations are both moderate.
J55 (minimum yield 379 MPa, minimum tensile 517 MPa) and K55 (minimum yield 379 MPa, minimum tensile 655 MPa) are the standard surface casing grades. K55’s higher tensile strength provides a modest improvement in burst resistance over J55 at the same wall thickness, making it preferred for wells where the surface casing will carry well-control pressures during deeper drilling.
Intermediate Casing: N80 and L80
Intermediate casing isolates troublesome formations — lost-circulation zones, over-pressured shales, and salt sections — from the deeper wellbore. It operates at higher external pressures and temperatures than surface casing, requiring grades with greater yield strength and collapse resistance.
N80 (minimum yield 552 MPa) is the workhorse intermediate casing grade for sweet-service wells. Its combination of yield strength, availability, and cost-effectiveness makes it the default choice for conventional onshore and offshore drilling in non-corrosive environments. L80 (minimum yield 552 MPa, maximum yield 655 MPa with hardness limit of 23 HRC) is the sour-service equivalent, specified whenever H₂S exposure is anticipated. The “L” designation indicates controlled yield strength and hardness per NACE MR0175 to prevent SSC in sour environments.
Production Casing: P110 and Q125
Production casing is the final structural string through which the well produces. It must withstand the full reservoir pressure, the collapse pressure from the packer fluid column, and any stimulation or workover pressures applied during the well’s producing life. For deep wells (3000+ meters) and HPHT reservoirs, the combined burst, collapse, and tensile loading demands grades with yield strengths exceeding 100,000 PSI.
P110 (minimum yield 758 MPa) is the standard high-strength production casing grade. Its high collapse resistance makes it suitable for deep wells where the external pressure differential across the casing wall is significant. Q125 (minimum yield 862 MPa) provides additional capacity for the deepest and highest-pressure wells, though its limited availability and higher cost restrict its use to applications where P110’s collapse or burst ratings are genuinely insufficient.
The Complete Casing Design
A typical onshore well uses three or four casing strings: conductor (large diameter, structural only, grade unspecified or J55), surface casing (J55/K55), intermediate casing (N80/L80), and production casing (N80/L80/P110). Each string is designed using the triaxial casing design methodology that evaluates burst, collapse, and tension simultaneously at every depth point in the well.
Grade selection interacts with wall thickness: a heavier wall in a lower grade can sometimes provide the same collapse resistance as a thinner wall in a higher grade. The casing designer optimises the grade-weight combination to minimise total casing cost while satisfying all load cases with adequate design factors.
Need casing for your well program? Send us your well design — we will confirm grade, weight, connection, and delivery for each string.