Oil & Gas
Aler BP Uses Bismuth Alloy to Plug Old Wells at Valhall Field
Aker BP was the first operator worldwide to use bismuth alloy to plug the top section of old oil wells. The technology is now used on 30 wells on the Valhall
Aker BP was the first operator worldwide to use bismuth alloy to plug the top section of old oil wells. The technology is now used on 30 wells on the Valhall field. That means safer, permanent well plugging. Cement, which is commonly used as a barrier material to plug wells, can fail when subjected to wellbore or casing stresses resulting from subsidence and compaction events. In the worst case, hydrocarbons in old wells could migrate upwards and potentially leak into the sea. Aker BP installed a trial plug over two years ago, and was the first operator worldwide to use bismuth alloy in the top section of the well.
Bismuth is a metal with unique properties that make it particularly well-suited for applications in plug and abandonment operations. As a solid metal, it is completely impermeable. It is also as heavy as lead, making it less prone to contamination during its placement into the well. When melted, liquid bismuth flows like water, giving it the ability to flow into the smallest interstices in the well. When bismuth solidifies, it expands, which helps provide permanent sealing capability inside a wellbore.
Unlike cement plugs, which need to be several dozens of metres in length in order to qualify as barrier, a 2.5-metre-long bismuth plug suffices to provide long term isolation in the well.
The gigantic Valhall field in the southern part of the North Sea has produced over a billion barrels of oil equivalent. Old oil wells must be plugged to make room for new wells that will ensure 40 more years of production from Valhall. In fact, the ambition is to produce a total of two billion barrels from the area. Aker BP has already started removing the old field centre on Valhall. The living quarters platform was removed in 2019. Another two installations will disappear over the next five years. All wells connected to the old drilling platform will be permanently plugged over the course of 2021.