Surface Plug for Well Abandonment in Norway North Sea

Identification of problem

Traditional well abandonment includes placing a surface cement plug through a section milled window or by perforating and washing out existing cement. While cement is widely recognized as the standard abandonment material throughout the world, its ability to block gas in a well is lacking because it is a porous material that shrinks when it solidifies. To reduce environmental risk, and operator’s long-term liability, a new abandonment material is needed to ensure a well is properly capped during well abandonment operations.

Issues to overcome

Surface plugs are set in the outermost casings which are relatively large, requiring a large volume of material to seal. This was the largest tool BiSN had ever run with over 3,500 kg of bismuth alloy to seal out to 18 5/8” casing. Due to abandonment regulations, a single material plug was required across the wellbore. To achieve this, the modified thermite heater used to melt the alloy would need to be removed while the alloy was still liquid. This was also a first for BiSN.

Application

BiSN deployed the largest ever bismuth alloy plug, a Wel-lok™ STC to seal through a section milled window in 13 3/8’ casing out to 18 5/8” casing.

Outcomes/Conclusion

Prior to running the Wel-lok™ STC, a window was section milled in the 13 3/8’ casing out to the 18 5/8” casing and the client perforated a shallow gas zone to ensure the plug would have gas and differential pressure from below in order to verify the seal. Live and memory gauges were hung in the well for long term monitoring below the STC. After running to the desired depth on electric line, the modified thermite heater was activated melting the bismuth alloy. After 35 minutes, a time pre-determined during the 1 ½” year qualification testing program, the heater was pulled from the alloy, leaving a single material barrier in across the wellbore out to the 18 5/8” casing. This seal is now under a 2-year monitoring plan and no bubbles or pressure have been observed above the plug since it was set.