December 28, 2020 | Susan Napier-Sewell

Incorrect Determination Leads to Increased Personnel Risk at Wellsite

During drilling operations on a fixed offshore rig, an oil & gas kick was encountered drilling in the production hole section.

The well was successfully shut-in by the driller. The shut-in well pressures were interpreted as an indication of Ballooning and, based on this interpretation, the choke was opened, allowing additional influx into the well. This was repeated a few times. The well was finally shut-in and successfully killed using the driller’s method. An incorrect determination of Ballooning by well site and office operations personnel, then opening of the well, led to increased personnel risk exposure at the wellsite.

Kick taken while drilling in the production hole section resulting in well control incident

While drilling the 6 1/8” production hole section, an increase in flow – with pumps on – was observed (positive kick indicator). The rig crew spaced out the drill string, shutdown the pumps, and shut-in the well with the BOP’s annular preventer. After three hours, the Casing Pressure (CP) stabilized at 130 psi with no appreciable measurable surface pit volume increase. The rig team bumped the float and obtained a Shut-in Drill Pipe Pressure (SIDPP) of 190 psi.

After discussion with the office, the rig team suspected trapped pressure from ballooning and bled off casing pressure from 130 psi to 100 psi

Despite the CP coming back to 130 psi in 33 seconds and without discussing with the office, the rig team bled pressure over three more cycles. Following each bleed cycle, the CP increased back to the starting pressure of 130psi. Still believing the pressure was from ballooning, the rig team decided to fully open the choke and monitor flow on the trip tank. After bleeding back 14 bbls they decided to shut-in again. The wellsite team re-bumped the float and calculated a different SIDPP of 250psi. The rig team killed the well with the Driller’s method using the Kill Weight Mud determined from the “second” SIDPP; however they did not achieve the planned total depth due to lost circulation issues.

What went wrong?

  • Well had limited offset well data, the actual pore pressure was greater than the pre-drill prediction.
  • Insufficient mud weight then led to an underbalanced well state while drilling.
  • A confirmation bias existed with the well site operations and office team’s decision characterizing the first kick indication as ballooning.
  • The well site team did not consult with the office team when they decided to open the well three more times.

Corrective actions and recommendations

  • More stringent review of pore-pressure prediction for wells with limited off-set data must be done by office technical staff.
  • Enhanced training and drills for well site operations staff regarding ballooning guidelines and kick detection and response.
  • Improve clarity of communication between office and rig well site personnel.
  • Follow up verbal agreements with written documentation describing the exact steps agreed upon by both the well site and office staff.
  • Recognize and reinforce the understanding of what SIDPP represents and that a properly measured value (assuming the bit remains on bottom) will not change unless the mud weight or formation pressure changes.

This Safety Zone Lesson Sharing report is an IOGP (International Association of Oil & Gas Producers) Well Control Incident. 

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Categories
Accidents, Human Performance, Investigations
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