Well Control Incident: Limitations of MPD Tests
Influx during MPD operations: Well Control Incident
Following drilling of a 17” hole section to 3851mMD with MPD, the pore pressure was assessed by bleeding off the surface pressure and flow checking the well. The well was assessed as overbalanced with the static MW. However, subsequently it was concluded that an influx of 153bbl had been taken over a number of days while tripping out of hole, rigging up, and running the casing. Incorrect interpretation of the static bleed down test and thermal effects led to gains being incorrectly interpreted as ballooning.
What happened?
While running 14” casing, a consistent positive discrepancy between actual and theoretical metal displacement was observed. Numerous flow checks were performed at various depths, the results of which were concluded as static due to a decreasing trend in flow out. On several occasions, the well was shut in, but no pressure build up was observed. With the casing at 3110m, the well was shut in following a pit gain and positive casing pressure build up to 80 psi was observed. The well was controlled by displacing a heavy mud pill (2.11sg) to the riser, the well was opened, and the casing was run to depth managing losses and gains by bullheading the annulus and maintaining the riser cap. It was not possible to circulate without losses. The seal assembly was set and the casing cemented by bullheading to the loss zone. A retrospective analysis showed that a cumulative influx of153bbl had been taken over a number of days while tripping out of hole, rigging up, and running the casing.
What went wrong involving MPD tests?
• Prior to the trip out of hole, pore pressure was estimated by bleeding off the MPD surface back pressure and flow checking the well. The well was incorrectly assessed as overbalanced. Opportunities to update the prediction based on LWD pressure measurements closer to section TD were missed.
• A consistent procedure for static bleed down tests using the MPD system was not applied, leading to incorrect interpretation of results.
• Mud contraction due to thermal effects on the trip out was interpreted as losses. Subsequent gains were then interpreted as ballooning and the potential for an influx was not investigated.
• Fingerprinting of the thermal effects on static & dynamic bleed downs and flow checks was not performed.
• Trends during static bleed down tests and flow checks were not investigated fully and the well was concluded static on multiple occasions.
• Partially full casing due to partially plugged auto-fill float equipment compromised flow checks.
Corrective Actions and Recommendations:
• Consider the thermal effects on MPD static bleed down tests and flow checks. Understand the expected response to the well fluid volume when circulation is stopped and when tripping.
• Be aware of the limitations of MPD static and dynamic bleed down tests in pore pressure estimation.
• Apply consistent procedures for MPD static bleed down tests and interpretation of results.
• A well with indications of flow but does not show pressure build up when shut in could be a kick/loss situation and not ballooning.
• Consider the effect of partially full casing on flow checks when running autofill float equipment.
Content credit: IOGP (The International Organization of Oil & Gas Producers) Well Control Incident Lesson Sharing 23-1, January 2023, https://safetyzone.iogp.org/.
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