A treatment directly performed in the active pit caused an unexpected mud density reduction in the well followed by a formation influx
IOGP Well Control Incident Lesson Sharing #23-2
After reaching section TD, the well was circulated clean and mud conditioning was initiated to prepare the casing run & cement. A first cycle at a low dilution rate was performed without reaching the objective. It was decided to change for a stronger pump and a bigger hose inducing a significantly higher dilution rate. This sudden incorporation of additives - on the fly - in the active pit had the unexpected effect to lower the density of the mud system, causing the ECD to drop slightly below formation pressure. A gain was then detected following a decision to flowcheck the well.
This lesson sharing reminds us of the importance to restrain any direct treatment in the active pits and keep a full control of the active mud properties – this being the primary well barrier.
The Wells Expert Committee/Well Control Incident Subcommittee believes that this incident description contains sufficient lessons to be shared with the industry. We further encourage the recipients of this lesson sharing to share it further within their organization.
Following the under-reaming of the 22" pilot hole to 26", the well was circulated clean at TD and mud conditioning commenced to reduce YP from 36 to ~26 in preparation for running and cementing the 20" casing. Mud weight was expected to be maintained at 1.35+sg throughout the operation due to minimal overbalance. While circulating the ECD dropped from 1.365sg to 1.334sg which was lower than the predicted pore pressure. A flow check was performed and a gain of 2.4bbl was observed over a period of 1.75 hours. Circulation was re-established and conditioning continued while increasing the mud weight to maintain ECD above 1.35sg. A second flow check showed a loss of 1bbl over the flow check (+/-1hour).
What Went Wrong?:
- Initially the active system was treated by adding 1.15sg premix at 0.5bbls/min with a 2” hose. However, after one full system circulating volume was completed, mud conditioning objectives had not been met due to the low dilution rate. The operation was changed to use a mix pump and 6” mixing line, the subsequent higher dilution rate led to a reduction in the mud weight which was pumped downhole.
- Lack of availability of appropriate equipment (pumps/agitators) resulted in a reduction of mud weight in the pits and consequently the mud pumped into the well.
- PWD data alarm point in Well Monitoring Matrix set too high and resulted in no alarm when the mud weight was lower than required to maintain an overbalanced condition.
- Lack of timely detection and response when the ECD reduced to a value lower than the surface MW value.
- Procedures in place did not indicate the minimum allowable ECD.
Corrective Actions and Recommendations:
- A clear and approved procedure must be in place for any mud treatment where there is a risk of reduction of fluid weight.
- Confirm appropriate equipment and procedures are in place to conduct mud treatments in a controlled manner.
- Elimination of Mud Conditioning on future Top-Hole Sections where possible. Removes the risk associated with mud dilution and Well Control where there is a narrow overbalance window.
- Conduct regular verification of the pit management plans / pit room tasks related to fluid density control.
- Well programme shall include the minimum and maximum mud weights and ECD’s for each hole section.
safety alert number: 348
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/
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