Well Control Event while using Managed Pressure Drilling equipment

Drilling deep chalk section of a high-pressure, high-temperature well. Phase drilled with 1.95SG mud effective density exerted (ECD) required at 1.99SG due to increase of gas. Managed pressure drilling (MPD) system in place used for applying back pressure during drilling and at connections. Observed some dynamic losses – perform dynamic formation integrity test (FIT) indicated 2.01eSG maximum permissible. Drilling continued managing ECD ~1.99SG but gas behaviour increased. Dynamic losses resumed, attempted to cure same - observe influx, attempt to control with MPD back pressure then shut well in. 3m³ of gain / downhole crossflow developed between reservoir and thief zone.

What Went Wrong?:

• Significant reduction of the drilling window between P-Pore (pore pressure at bottom of hole) and P-Frac (fracture pressure at shoe) as a weak zone was encountered.
• Weak zone was not identified and therefore not covered by well architecture.
• Zone with flow potential not identified.
• Mud treatment & losses management plan not strong enough to consolidate “weak zones”.

Corrective Actions and Recommendations:

Use of MPD permits drilling phases with very narrow mud weight window, however:

• System relies on integrity of the wellbore, therefore periodic assessment of the true open hole strength is necessary (not just shoe FIT).
• Clear guidelines needed to define minimum mud weight window requirement for each section.
• Know when to stop!

safety alert number: 299
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