Casing is steel pipe cemented into the wellbore to provide structural integrity, isolate formations, contain wellbore pressure, and enable controlled production. Multiple casing strings of decreasing diameter are set at progressively greater depths as the well is drilled.
Casing Types and Functions
Conductor casing is the largest diameter (20-30"), set shallow to support subsequent operations. Surface casing protects freshwater aquifers and provides the BOP mounting foundation. Intermediate casing isolates problematic zones including abnormal pressures and unstable shales. Production casing is the final string reaching the target zone, designed for well life conditions.
Design Standards
API design safety factors govern casing selection: Collapse SF 1.125, Burst SF 1.1, Tension SF 1.6-1.8. Maximum surface test pressure is set at 80% of API internal yield rating per API TR 5C3. Casing grades range from H40 to Q125, with minimum yield strengths from 40,000 to 125,000 psi (API 5CT/ISO 11960).
Cost and Reliability
Casing represents 30-40% of tangible well costs in onshore operations. For offshore wells, drilling costs including casing and cement comprise 60% of total D&C expenditure (EIA, 2016). Failure rate data shows 6.3% of 8,030 Marcellus wells experienced cement/casing impairment (Davies et al., PNAS 2014).
Why It Matters
Casing design directly impacts well integrity throughout the asset lifecycle. Undersized casing limits future intervention options; over-designed casing wastes capital. The setting depth of each string determines the pressure containment envelope for subsequent drilling.