Several contributing factors add up to a NAF discharge overboard
Gas entrained mud in riser. 20" casing was set at 1188m MD/TVD (measured depth / true vertical depth) in 576m of water, and an 11.2 ppge leakoff test was established. 17 1/2" intermediate hole drilling progressed with 9.9 ppg NAF to 1279m MD where 75 bbls were lost, but ballooned back. Drilling resumed and increased gas was observed at 1900m to 2337m MD hole section total depth (TD). The mud weight was raised to 10.3 ppg (integrity limit) due to continued high gas readings. 13-5/8" casing running operations were commenced. Flow checks were negative (drilling, conditioning and running casing) over a 44 hour period. Flow was observed with the hanger three stands below the rotary, and 72 bbls of NAF was safely diverted while the well was shut in. No drill pipe pressure observed.
What Went Wrong?:
This was an exploration well structurally updip from the nearest offset and assumed sands from offset well were pinched out (hydrocarbon buoyancy from offset well was not included in the pore pressure prediction).
Siltstone encountered at 1900m was over pressured due to hydrocarbon bouyancy effect.
Drilling was not stopped to establish pore pressure.
A sand at 1220m MD limited formation integrity to ~ 10.4 ppge preventing raising mud weight high enough to prevent feed in.
Dissolution of gas in NAF at the silstone was not saturated enough over the 44 hour period of no circulation to show a gain, but when it reached its bubble point above the BOP the well began to flow.
Corrective Actions and Recommendations:
Figure 1: Wellbore Sketch at Time of the Event
The illustration shows the wellbore sketch at the time of the event.
safety alert number: 292
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