Break of formation in a narrow mud weight window drilling. Well controlled using the modified wait and weight method

While drilling the 18 1/8” x 22” hole at 2721 mMD, (2717 mTVD) using pressure ramp drilling procedures with 11.5ppg mud weight (MW). The driller observed a 1% increase in flow, followed by pit volume totaliser (PVT) increase. After picking up off bottom with pumps running, a 7-8 barrel gain was observed. The well was then shut in with total pit gain of 14 bbls. Choke pressure increased to 109 psi over 5 min and subsequently broke down to 69 psi. The drill pipe pressure (DPP) remained steady at 60 psi (Non-ported float in string, did not bump float during shut in period to obtain actual shut in DPP due to casing pressure drop during initial shut in). The shut in casing pressure (SICP) then slowly built back up to 109 psi over approx 90 mins, and then suddenly dropped from 109 psi to 36 psi indicating formation break-down for second time. SICP stabilised at 36 psi for several hours while developing plan forward and consulting well control team in office.

Started kill operations using modified weight and wait method: zero choke safety margin, calculated kill mud to be 11.75 ppg. Initiate well kill while pumping 11.75 ppg surface mud weight (SMW) kill mud including first 200 bbls with 50 ppb loss circulation material (LCM) at 20 spm. Initially encounter 75% losses at 20 spm then observe partial returns, reduce rate to 10 spm and observe improving trend in return flow. As circulation continued with LCM entering open hole, able to resume full circulation at 10 spm and continue displacing annulus to kill weight mud.

There were no injuries during this incident and the well was successfully brought under control. Severe subsequent losses led to plug back and sidetrack.

What Went Wrong?:

Critical factors:

1. Incorrect casing setting depth - TD criteria called to set 17 7/8'' max of 7mMD inside the Messinian formation. Messinian top to be detected on ROP drop based on previous wells. TD criteria aim to achieve isolation of weak zones above Messinian top. The TD criteria did not consider the risk to penetrate the Messinian formation with limestone/anhydrite lithology in this hole section.
2. Failure to identify Messinian top - Drilled 8m into Messinian formation because top was not identified. Top detection was based on likely ROP drop as experienced with prior wells, however different drill bit was used which could have masked ROP behaviour. Team did not respond to real time update from Geophysicist as their focus was on ROP drop.
3. Loss of hydrostatic overbalance - Surface MW was 11.5ppg compared to 11.6ppg in prior well. 0.4ppg difference between equivalent static density (ESD) and Surface MW suggest annular pressure while drilling (APWD) calibration issues. Historical data from previous wells shows between 0.1 and 0.2 ppg difference between surface mud density (SMD) and ESD.
4. Well control response - Initial SICP drop was interpreted as formation break down. SICP rose above MAASSP and fractured the formation. During initial well kill, with zero choke margin, overbalance was lost allowing more influx without timely response. The team could have been better resourced to deal with a long term well control event.

Corrective Actions and Recommendations:

1. Incorrect casing setting depth criteria - casing criteria should consider setting the casing deep enough inside the Pressure Ramp to cover any potential weak zone with no penetration of Messinian Top.
2. Failure to identify Messinian Top - ensure good coverage and communication from new well delivery team, operations and engineering during critical operations and that all geological information is communicated to well team in good time. Include biostratigraphy as part of data acquisition program to support real time data decision making. Treat Messinian casing points with an appraisal mind set.
3. Loss of hydrostatic overbalance - Wells team should ensure the surface MW and PWD reading is in alignment of offset wells and virtual hydraulic models. In case of deviation, risk assess balancing kick vs loss scenarios.
4. Well Control Response - Procedures updated to ensure that:
   a) Resources are adequately allocated to allow 24hr during well control events.
   b) Engagement and communication between teams
   c) Discussion of all likely scenarios before start of operations
   d) Inclusion of complexity drills
   e) WC modelling is performed prior to start of operations

Findings were shared with the rig contractor and the offshore team.

safety alert number: 285
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/

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