Well Control Incident: Managing gas breakout in SOBM
While drilling at a depth of 4747m, the well was shut-in due to an increase in returns with a total gain of 17bbls recorded. The well kill needed an increase in density from 1.40sg to 1.61sg to achieve a stable situation. With the well open the BHA was pumped out to the shoe and tripped 400m to pick up a BOP test tool to perform the post-kill BOP test.
The BOP and choke manifold test were performed as well as some rig maintenance. The BHA was then tripped into the hole and the last 2 stands were washed to bottom. Total pumps-off time without circulation was 44 hours.
Gas levels during the bottoms-up initially peaked at around 14% and then dropped steadily to around 5%. HPHT procedures were being followed and this operation required circulation through the choke for the last 1/3 of the bottoms up. This corresponds to taking returns through the choke after 162m3 is circulated.
After 124 cubic metres of the bottoms-up had been pumped the gas detector at the bell nipple was triggered. Simultaneously, mud started to be pushed up out of the hole, reaching a height of around 1 joint above the drill floor. The flow continued for around 30 seconds corresponding to a bubble of gas exiting the riser. The pumps and rotation were shut down, followed by closure of the diverter, annular and upper pipe rams. Approximately 2bbls of SBM were lost over-board through the diverter line. The flow stopped by itself after just a few seconds and casing pressure was recorded as zero. No-one was on the drill floor at the time and no movement, damage or displacement of equipment occurred.
After verifying that there was no flow (monitored on the stripping tank) the diverter was opened and 10 cubic metres of mud used to refill the riser, equal to a drop in height of 56m.
The riser was circulated to fresh mud with maximum gas levels recorded at 54%. This was followed by a full bottoms up through the choke.
A full muster of POB was conducted due to the gas alarms being triggered.
What Went Wrong?:
Conclusion - An undetected influx was swabbed into the well during the BOP test which was then circulated up inadvertently though a non-closed system breaking out in the riser.
- Stroke counter was reset to zero after washing 3 stands to bottom (this resulted in 136 cubic metres of circulation not being accounted for in the bottoms up monitoring).
- Review of Monitoring While Drilling Annular Pressure memory logs identified several swabbing events identified – main event was when the BOP test tool was POOH from the wellhead – ESD as measured by APWD dropped to 1.59sg on 10 or 11 occasions.
- Swabbing was exacerbated by Kill Weight Mud not having sufficient margin above PP.
Corrective Actions and Recommendations:
- Take into account all washing to bottom for any circulation where bottoms up is to be via choke.
- Tool Pushers shall cross check the bottoms up calculation and joint agreement on reset of the stroke counter.
- All BHA tripping speeds to be modelled so that potential swabbing operations are identified and so that tripping speed limits can be specified.
- Verify, when possible, actual swabbing magnitude using PWD memory logs (ie after a trip out of the hole).
- Pumping out (even inside liner/casing) shall be considered in tight tolerance liner/drilling BHA. Modelling shall be used to underpin the decision.
safety alert number: 258
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