Severe Loss – Kick Situation in Reservoir Section

Situation: While drilling an 8-1/2” hole in a carbonate reservoir using 10.1 ppg water base mud 1,000 ft below the previous shoe and 600 ft from TD of the well, the bottom fell off resulting in total losses. There was a gas zone above this point. The loss of fluid removed the overbalance making the (gas) well kick and soon the well started to flow. Closed BOP and observed an increase in casing pressure. Soon the casing pressure was indicating 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 other offshore rigs to build mud volume for the rig.

·       Bullhead 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 will build back as mud is lost to the formation and gas filled the annulus.

It was evident that the well it will be very difficult to control the well by pumping small volumes of mud as preparing mud with required properties takes time and rig capacity was limited.

·        The decision was made to bullhead gas by continuously pumping sea water in large volumes. Over the next two days, 20,000 bbl of seawater was bullhead in the annulus.

·        There were signs that formation was developing some resistance due to pumping of a large volume of fluid and LCM pills. The hole was able to 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 sets in again.

·       Opened BOP and pulled out a joint every time a brief stable period was achieved. Closed BOP and bullhead again to gain a brief period of stability.

·       After pulling the string 200 ft above the loss zone, pumped a gunk plug, which made it possible to achieve a stable hydrostatic of seawater.

·       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 cement stinger and spot 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.