Delayed Well Control Shut-in during auto-choke Managed Pressure Drilling
Drilling 8 1/2 hole - high pressure caprock - with 1.83 SG mud weight (MW) at 5459m. It is drilled on a managed pressure drilling mode (MPD) with a MW of 1.83SG, a permanent surface back pressure is applied to get an equivalent mud weight (EMW) of 1.94 SG on Bottom. MPD system is set up in detection mode with auto-choke enabled (choke will adjust itself if a discrepancy flow in/out is observed). Background gas 1-2% as per prognosis (pore pressure (PP) prognosed 1.93 –1.98 SG EMW).
While drilling at 5459m MDBRT, Driller flushed choke and kill lines with fresh 1.83 SG non-aqueous base mud. Coincidentally, a higher pressure layer was crossed at the same moment. MPD coriolis flow meter detected that flow in and flow out were not equal (i.e. increase in flow out). MPD automatically started closing choke to increase BHP and bring back flow out equal to flow in. Surface back pressure increased up to 77 bar (1120 psi). Flow in and flow out still not equal. With MPD limit reached – decision taken to close the well on the BOP Annular preventer.
Commenced monitoring shut in drill pipe pressure (SIDPP) and Shut in casing pressure (SICP).
Total gain in active of 4.8m3
SIDP = 109 bar SICP = 152 bar
What Went Wrong?:
Confusion on gain diagnostic - the increase in the flow out was explained away by the action to flush choke and kill lines - whereas it was caused by a formation gain. It had the consequence to delay the well shut-in.
Kick taken because drilling ECD 1.94 SG EMW no longer balanced when entering unexpected 2.06 SG EMW PP
Corrective Actions and Recommendations:
Flushing choke and kill must not be performed as drilling progresses.
Precautions must be taken when drilling on MPD automatic choke adjustment mode. In case of a significant kick intensity, the reaction of the system will tend to adjust the choke to reduce the flow out and will delay the shut-in action. Resulting in a significant gain volume.
Figure 1: Sequence of events
safety alert number: 302
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/
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