Well Architecture and Casing Design
In GATE PE, 'architecture' refers to wellbore construction: casing programs, cementing, and well trajectory. The exam tests casing seat selection, burst/collapse/tension ratings, and directional drilling geometry.
Core rules and concepts
- Casing design must satisfy burst (internal pressure), collapse (external pressure), and tension (axial load). Typical API design factors: burst 1.1, collapse 1.0, tension 1.8.
- Casing seat selection uses the pore-pressure and fracture-gradient window: the mud weight must be above pore pressure but below fracture pressure.
- API burst rating: (P_{burst} = 0.875 \times \frac{2 Y_p t}{D}), where (Y_p) is minimum yield strength (psi), t is wall thickness (in), and D is outside diameter (in).
- Directional drilling: dogleg severity (DLS = \frac{100 , \Delta \theta}{\Delta MD}) in degrees per 100 ft; TVD = MD × cos(inclination) for a constant build section.
Worked example
Calculate burst rating for casing with (Y_p) = 80,000 psi, t = 0.5 in, D = 9.625 in. (P_{burst} = 0.875 \times (2 \times 80000 \times 0.5) / 9.625). The numerator: 2 × 80000 × 0.5 = 80,000. Then 80,000 / 9.625 = 8311.7 psi. Multiply by 0.875: 7272 psi. Apply a design factor of 1.1: allowable internal pressure = 7272 / 1.1 ≈ 6611 psi.
Traps that cost marks
- Using nominal wall thickness instead of minimum wall thickness (API tolerance).
- Forgetting the 0.875 factor, which accounts for manufacturing tolerance.
- Mixing burst and collapse formulas: collapse uses a different API equation.
- Ignoring temperature derating for yield strength in deep, hot wells.