A premium bismuth-based alloy plug system has been developed to achieve a gas tight seal in annuli that cannot be achieved with traditional cement plugs, providing a new methodology for abandoning wells. To date, 35 plugs in total have been run in 30 wells, with this technique consented to by the offshore Norway regulator for Valhall conditions.
No surface pumping equipment is needed for this type of plug. The alloy – when molten – has a viscosity like water, but a very high density. It can therefore flow through perforations or section milled windows, with gravity, into the annulus or into multiple annuli. When the alloy solidifies, it expands to create an impermeable seal. The entire process, from melting to solidification, takes place in a matter of minutes and the seal is ready to test within a few hours.
The first at scale programme for abandonment applications has been done in the Valhall field offshore Norway. This was following an extensive plug and alloy development and qualification programme over a 4-year period, working in partnership with the local regulator from the outset. The testing programme demonstrated that the bismuth-based material is, in many ways, superior to cement. Following successful development of a system, a single well in Valhall was selected for a trial and tested for 2 years. Following success in the trial well, the operator extended the application to a total of 30 wells.
Three different tool sizes were required for the campaign. Section milling of inner casings of 13-3/8", 18-5/8" and 20" had to be performed. Significantly shorter lengths for section milling were required as the bismuth plug – set inside the window – contributed to the efficacy of the system. All plugs were set by running on pipe. The tools run in these wells resulted in the world's largest bismuth alloy plug ever deployed with the largest tool (20"x30") weighing in at nearly 16,000 kg (34,000 lbs) and over 9,000 kg (20,000 lbs) of alloy. After the tools were melted downhole, the heater was extracted resulting in a solid bismuth alloy barrier across the wellbore.
Extensive work was done to assess corrosion risk. Numerous tests were done according to industry standards and the bismuth-based plug substantially outperformed Inc 718 and 13Cr L80, thus a substantially longer lifetime is projected than had conventional materials been used.
The authors believe that this technique can – when coupled with appropriate engineering work and in partnership with local regulators – be more widely applied, leading to simpler operations, lower costs and better long term well integrity.