Coiled Tubing Operations
Coiled Tubing Operations tests pressure control around a continuous pipe string, BOP stack configuration, check-valve philosophy, and the limits that make CT different from jointed pipe.
Core rules
- CT is not a standalone barrier. The barrier envelope is the CT string plus the stripper, BOP rams, and downhole check valves.
- The CT BOP stack normally includes blind/shear rams, pipe rams, slip rams, and an annular or stripper. Function-test and pressure-test before the job and after BHA changes.
- Fit at least two back-pressure check valves in the BHA; the CT string itself can be plugged by solids or damaged.
- Monitor stripper leakage, CT weight, circulation pressure, and fatigue. Do not exceed the CT operator's fatigue or ovality limits.
- Keep the lubricator or riser length compatible with the BHA.
Worked example
A 1.75 in CT string has a 1.5 in ID and is 3,500 m long. Internal area = π/4 × (1.5 × 0.0254)² ≈ 0.00114 m². Volume = 0.00114 × 3,500 ≈ 3.99 m³, or about 25 bbl. This volume is used to size a displacement pill. If pump output is 0.1 m³/min, displacement takes about 40 min, plus correction for CT stretch and shoe position.
Traps
- Treating CT as the only barrier.
- Forgetting to test the BOP after changing the BHA.
- Using a single check valve as the only backflow barrier.
- Ignoring CT stretch when correlating depth.
- Shearing CT without confirming the shear ram can cut that size and grade.