IOGP Well Control Incident Lesson Sharing
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Kick on subsea well following upper loss zone results in bullhead and sidetrack


Fracture gradient and formation integrity is critical in every well, particularly an exploration well, as offset data may be limited to non-existent. With pressure predictions, well monitoring and casing design over-pressured zones to weaker formations are isolated as well construction continues toward total depth.

The following incident describes a scenario where weaker zones, uncovered below the formation integrity test, can affect the ability to manage pressure on a well, but more importantly may represent a false sense of security with a recorded FIT that may be less than accurate. The event describes how losses were remediated with LCM and two balanced cement plugs until full returns were established with integrity verified through drilling rate alone, before continuing ahead.

The incident description helps detail the importance of reduced formation integrity versus pore pressure as the team attempted to circulate out an influx, and observed losses that resulted in the decision to bullhead, plug and sidetrack the hole section.


Whilst drilling 12 1/4'' phase of subsea exploration well, a kick was experienced at 4800m.

Well shut in and kick started to be circulated out with wait & weight method.

After heavy mud circulation completed still observed string and casing pressure.

Continued well control, increased mud weight further and resumed circulation with wait and weight method. Observed lost return while performing well control circulation with heavier mud.

Pumped barite pill followed by cement plug in bullheading. 12 1/4'' Section abandoned, performed side track.

What Went Wrong?:

  • Loss zone encountered before kick event and cured with 2 cement plugs.
  • Cement plugs not properly effective.
  • Cured loss zone tested only by drilling circulation.
  • LOT test not repeated after remedial job.
  • Well section fracture gradient derating not considered.

Corrective Actions and Recommendations:

  • Repeat LOT test after loss zone remedial job.
  • Derating of section fracture gradient should be considered.


safety alert number: 332
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/

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