SCSSV Control-Line Decay Before Installation
You are preparing to run a 4½-in, 12.6 lb/ft completion with a surface-controlled subsurface safety valve (SCSSV) at 610 m. The flapper-type valve is rated 10,000 psi working pressure, and the control line is ¼-in 316L. The well has 3,600 psi shut-in tubing pressure and is filled with 9.2 ppg brine. On surface, the valve opens at 4,000 psi control-line pressure and closes when the pressure is bled to 1,800 psi. During a function test with the valve open, the control-line pressure decays from 4,000 psi to 2,500 psi in 15 minutes. Tubing pressure remains steady at 3,600 psi, and no fluid is seen at the control-line fittings. The completion is still on the rig floor, and the valve has not been run. The manufacturer requires a 15-minute control-line integrity test before installation.
1. Is the control-line pressure decay acceptable? Explain what the decay indicates and why 2,500 psi may be a problem if the valve closes.
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No. A 1,500 psi fall in 15 minutes is a leak, not normal thermal or gauge behaviour. The control line is a safety-critical circuit: it must hold pressure to keep the flapper open and to reopen it after closure.
With 3,600 psi tubing pressure, the flapper is held closed by tubing pressure plus the spring. To reopen, control-line pressure must overcome that closing force. At 2,500 psi, the line pressure is below the 3,600 psi tubing pressure, so the valve may not reopen if it closes. That could leave the well isolated below an inoperable safety valve. The leak may also worsen with temperature and pressure after installation.
2. How would you locate the leak and what test would you perform before running?
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Break the circuit down systematically:
- Close the surface needle valve at the control-line outlet and monitor pressure. If decay stops, the leak is in the surface fittings or pump; if decay continues, it is in the control line or valve actuator.
- Check the ferrules, fittings, and control-line connections with leak-detection fluid.
- Pressure-test the isolated control line and valve to the manufacturer's specified test pressure, typically at least 1.5 times the maximum anticipated operating pressure, and hold for the required 15 minutes.
Acceptance should be no visible leak and essentially no pressure decay over the hold period. A decay from 4,000 to 2,500 psi is far outside any acceptable criterion. If the leak is downhole in the valve, do not run it; repair or replace the valve.
3. If the valve is run as-is, what operating risk does the leaking control line create during a well-control event?
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The immediate risk is loss of reopening capability. If the valve closes during a kick or emergency, the leaking control line may not build enough pressure to open the flapper against 3,600 psi tubing pressure. You could be unable to re-establish communication with the well below the valve.
A secondary risk is unstable valve position. A decaying control line can allow the flapper to partially close or flutter, damaging the seat and seals and giving misleading surface indications. Because the SCSSV is a primary barrier, a known control-line leak should be treated as a failed test: do not run the valve until the leak is repaired and the circuit holds pressure.