In deep wells, the most common casing failure mode is not burst or tension — it is collapse. The external pressure from heavy drilling fluid columns, cement hydrostatic head, and formation pore pressure can exceed the casing’s ability to resist inward deformation, causing the pipe to buckle, deform, and ultimately restrict or block the wellbore. Understanding collapse resistance is essential for safe and economical casing design in wells deeper than 2000 meters.
What Drives Collapse Loading
Collapse pressure on casing is the difference between external pressure (formation pore pressure plus cement column pressure) and internal pressure (fluid column inside the casing). The worst-case collapse load typically occurs during the cementing operation, when the heavy cement slurry outside the casing creates maximum external pressure while the lighter displacement fluid inside provides minimum internal support.
In production service, collapse loading can also occur when the casing is evacuated for workover operations (internal pressure approaches zero while external pressure remains), during production from a depleting reservoir where pore pressure changes alter the external loading, or in salt zones where creep loading applies sustained external pressure that increases over the well’s life.
How API Calculates Collapse Resistance
API Technical Report 5C3 defines four collapse regimes based on the D/t ratio (outside diameter divided by wall thickness) of the casing: yield strength collapse (thick wall, D/t < 15), plastic collapse (moderate D/t), transition collapse, and elastic collapse (thin wall, high D/t). Each regime uses a different formula, reflecting the different physical mechanisms that govern collapse in thick versus thin cylinders.
For typical production casing (7″ OD, 29–32 lb/ft, P110 grade), the D/t ratio falls in the plastic collapse regime where the collapse resistance is a function of both the pipe geometry and the steel’s yield strength. Increasing the yield strength (e.g., from N80 to P110) provides a proportional increase in collapse resistance within this regime, making grade selection a primary tool for managing collapse loading.
Grade Selection for Collapse Resistance
The collapse resistance of a given casing size increases roughly linearly with yield strength across the standard grade range:
• J55 at 379 MPa yield: baseline collapse resistance
• N80 at 552 MPa yield: approximately 45% improvement over J55
• P110 at 758 MPa yield: approximately 100% improvement over J55
• Q125 at 862 MPa yield: approximately 125% improvement over J55
However, upgrading from N80 to P110 also increases material cost by 15–25%. The casing designer must evaluate whether the collapse resistance improvement justifies the cost premium, or whether the same collapse resistance can be achieved more economically by increasing wall thickness (weight) in the lower grade.
Wall Thickness as an Alternative
For a given grade, increasing wall thickness (reducing D/t ratio) increases collapse resistance. In the plastic collapse regime, a 10% increase in wall thickness provides approximately 20–25% increase in collapse resistance. This means that heavier-wall casing in a lower grade (e.g., N80 at 35 lb/ft versus P110 at 29 lb/ft) can sometimes provide equivalent collapse resistance at lower material cost — though the heavier casing increases running weight and may require a larger rig.
Practical Design Considerations
Modern casing design uses computer-based triaxial analysis that evaluates burst, collapse, and tension simultaneously at every depth point, accounting for temperature effects on yield strength, biaxial loading reductions on collapse resistance, and the beneficial effect of internal pressure on collapse capacity. This approach replaces the older single-load-case methodology and produces more accurate — and often more economical — casing designs.
Designing casing for a deep well? Send us your well schematic and load cases and we will recommend the optimal grade-weight combinations.