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Trapped Gas Released When Punching tubing – a Safety Alert




Incident Date: —   Time: —
Type of Activity: 
Type of Injury: 
P&A Operation on Gas Injector well

Semisubmersible in 318m water depth.

Course of events:

Annulus filled with 1.32 sg brine and tubing with approximately 1.2 sg. Performed flow check prior to punching tubing – well stable.

Observed increase in trip tank shortly after punching tubing. Some return was expected due to different fluid density.

Closed UAP and LAP after gain of 2 m3. Trip tank volume continued to increase and it was decided to close BSR and cut wireline.

Gas level increased to 82%LEL in rotary table on drill floor. This activated the rig ESD1 (20%) and ESD2 (60%).

Well was monitored with closed BSR and UAP. The pressure stabilized at 8 bar. Gas level in rotary decreased to 1-2% LEL.

After several hours with stable pressures, gas in BOP was circulated out. UAP and BSR were opened without observation of more gas. Performed flow check – well stable.


What Went Wrong?:

The tubing had been displaced (lube and bleed) to clear fluid using a light well intervention boat after they had set a mechanical barrier plug to isolate the reservoir. The insufficient displacement of the tubing did not remove all hydrocarbons. After the tubing was punched the fluid from the annulus u-tubed into the tubing and the remaining hydrocarbons escaped to surface. There was no landing string (HP riser) or WL BOP’s rigged up.

Gas released on drill floor.

UAP and LAP not able to seal around WL string.


Corrective Actions and Recommendations:

Evaluate to perform similar operations with workover riser/pressure control equipment.

Small amount of gas can be “hidden” in wells even if the well is lubricated and left for a long time (1 year in this case).


Figure 1 – Well bore schematic

safety alert number: 278
IOGP Safety Alerts https://safetyzone.iogp.org/


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