Severe Loss – Kick Situation in Reservoir Section
Situation:
In an offshore well, while drilling an 8-1/2” hole in a carbonate reservoir using 10.1 ppg water-based mud, 1,000 ft below the previous shoe and 600 ft from TD of the well, the bottom fell off, resulting in total losses. A gas zone was present above this point. The loss of fluid removed the overbalance, causing the (gas) well to kick, and soon the well started to flow. Closed the BOP and observed an increase in casing pressure. Soon, the casing pressure indicated almost a full column of gas in the annulus. It looked like the well entered an extremely porous and permeable formation or a cave.
Actions:
Prepared kill mud and requested support from base for additional chemicals and mud volume. Being offshore created logistical challenges where immediate supply was needed.
Some non-critical wells being drilled by nearby rigs were safely suspended to support the rig facing the emergency situation. These rigs virtually started working as offshore mud plants to build mud volume.
Bullheaded gas back into the formation by pumping mud with LCM material in the annulus.
Repeated the process multiple times as the rig received mud volume from other rigs and mud plants.
With every bullhead, the pressure would drop, and with pumps off, pressure would build back as mud was lost to the formation and gas filled the annulus.
It was evident that controlling the well by pumping small volumes of mud would be very difficult, as preparing mud with the required properties takes time and rig capacity was limited.
The decision was made to bullhead gas by continuously pumping large volumes of seawater. Over the next two days, nearly 20,000 bbl of seawater was bullheaded in the annulus.
There were signs that the formation was developing some resistance due to pumping a large volume of fluid and LCM pills. The hole could hold a column of seawater to a certain extent.
Each time gas was "pushed" back, the well would remain dead for a brief period before the imbalance set in again.
Opened BOP and pulled out a joint every time a brief stable period was achieved. Closed BOP and bullheaded again to gain a brief period of stability.
After pulling the string 200 ft above the loss zone, pumped a gunk plug, which allowed a stable seawater hydrostatic.
Circulated the hole with drilling fluid by gradually increasing the mud weight to the original weight of 10.1 ppg. This allowed pulling the string out of the hole.
Ran back in with a cement stinger and spotted a balanced cement plug above the gunk plug, which was set 200 ft off bottom.
Hard cement was cleaned to leave 100 ft of cement plug at the bottom. 7” liner was run to case off the well at this depth.
The rig was skidded off to other slots after securing the well.
Plan was prepared to slim the well by drilling a 6” hole using RBOP and completing the well with 4 ½” production liner.