Lessons Learned: Pore Pressure Underestimation
An influx occurred during the drilling of an offshore exploration gas well due to pore pressure underestimation, with mass flow meter detection limitations.
The pore pressure underestimation incident occurred on a Dynamic Positioned Rig while drilling 12 ¼’’ section – stand down – observed gas increasing, corresponding to a pump off event (previous connection).
A decision was made to circulate and evacuate connection gas prior to making the next connection. The gas trend kept on increasing – disabling Early Kick Detection mass flow meter readings – the pumps were stopped and a flow check was performed on the trip tank, with well flowing. Eventually, well was killed with the Driller’s Method and mud weight was increased.
What happened?
- While drilling 12 ¼’’ section with 1.34 sg NABM (1.38 sg ECD) – stand down – observed gas increasing at 5% corresponding to a pump off event (previous connection).
- Decision made to circulate and evacuate connection gas prior to make next connection.
- As gas trend kept on increasing (8% up to 16% peak) – disabling Early Kick Detection mass flow meter readings – pumps were stopped and flow check performed on trip tank, well flowing.
- Well was shut in on Upper Annular Preventor: SICP 7 bar, gain of 6 bbls was estimated.
- Initiated 1st circulation Driller’s Method to evacuate influx then 2nd circulation with 1.38 SG Kill Mud.
What went wrong?
- Entering pressurized formation with low mud density.
- Most likely case for Pore Pressure Prognosis underestimated.
Corrective actions and recommendations
- When connection gas events are observed, need to control drilling pace in order to maintain only one pump off gas event in annulus at the time.
- Early Kick Detection mass flow meter saturated giving erratic value with gas beyond 10%.
Content/image source/credit: International Association of Well & Oil Producers (IOGP), Well Control Incident 26-13, “Influx during drilling offshore exploration gas well due to pore pressure underestimation, with mass flow meter detection limitations.“
