Shallow Water Flow in Land Well
On a remote land operation, 20" conductor was set at 27m and the diverter system nippled up. The 20" conductor was cleaned out with a 17.5" drilling assembly and drilling progressed. A carbonate formation was penetrated at 316m with 10.6 ppg mud. At 433m total losses were expereienced in the carbonate formation. An LCM pill was pumped with no impact. The drill string was pulled to 178m to mitigate hole instability and to mix more mud. Additional LCM pills were pumped with minor impact on loss rate. While at this depth, the well began flowing and the diverter was closed. While diverting, a broach occurred around the conductor pipe. Two off-bottom dynamic kill were attempted but unsuccessful at stopping water flow into the wellbore. The pipe was eventually stripped back to bottom and the well was dynamically killed with 14.5 ppg mud at 35+ BPM.
What Went Wrong?:
The shallow carbonate zone was overpressured from a water aquifer higher in elevation.
During the mud loss event, fluid hydrostatic at the top of the flow zone dropped below pore pressure and allowed the well to flow.
With the bit significantly off bottom for the first two kill attempts, building fluid hydrostatic pressure to overcome flow zone pressure was not possible. In addition, very little annulus friction pressure could be built during the dynamic kill because of the large hole size.
Corrective Actions and Recommendations:
When drilling with only a diverter installed and dynamic kill as the contingency plan for well control, always keep the bit near bottom during a mud loss event. Having the bit near bottom will maximize the amount of friction and hydrostatic pressure that can be applied to the well during a dynamic kill attempt.
If it is suspected that overpressure could exist in a shallow zone, consideration should be given to setting surface casing above the zone and installing a BOP.
safety alert number: 289
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