Molten Metal Technology Sees 300 Downhole Deployments

A metal alloy that has water-like viscosity in its molten state and then rapidly solidifies into a gas-tight seal recently achieved its 300th downhole deployment. This is according to a new announcement from UK-based BiSN Oil Tools which develops bismuth plugs for a number of applications.

Founded in 2011 and backed by BP's venture capital unit, BiSN said oil and gas companies around the world have been using its innovation in places where they might otherwise use cement or elastomers. Established use cases for onshore and offshore wells include permanent well abandonment, casing/tubing repair, and to shut off water- or sand- production zones.

BiSN's technology hinges on a customizable alloy made primarily of bismuth and tin, whose respective elemental abbreviations of Bi and Sn form the company's name.

The firm's proprietary alloy is sent downhole in its solid state as a cast around the outside of a heating system. The heat element is made of thermite—a compound of iron oxide and aluminum powder—that needs only a small burst of energy delivered via an electric line to trigger a chemical reaction.

Once the heat rises to a predetermined melting point, the bismuth-based alloy begins to flow. BiSN said the liquid alloy will go anywhere in and around the well that water can flow. This includes perforations, screens, and the annulus.

BiSN described the solidification process as "almost instantaneous" as the alloy shifts from a liquid to a solid as soon as its temperature falls below the melting point. During this transition, the alloy expands similarly to how water does when it cools into ice.

All of this takes just a few minutes, and within a few hours, the well is ready for a pressure test to see if the seal holds.

BiSN said it controls the melting point and the heat of its thermite system using damping and binding chemicals. This prevents the exothermic reaction from creating temperatures that would damage the well's casing or its completions components. The upper limit BiSN uses for its plugs is about 300°C (572°F), whereas casing can start to degrade from excessive heat at above 800°C (1,472°F).

The biggest abandonment job BiSN has undertaken so far was the focus of a case study presented at this year's Offshore Technology Conference (OTC) in Houston. In OTC 31897, authors from BiSN and AkerBP shared details on how 35 bismuth plugs were used to plug and abandon (P&A) 30 wells offshore southern Norway.

The P&A campaign was performed at the Valhall oil field which was discovered in 1975 and has produced more than 1 billion BOE over the past 40 years. AkerBP wants to produce another billion BOE from the field over the next 40 years. To do that, it had to plug all of the original wells so it could drill new ones.

A highlight of the P&A project includes the use of the largest bismuth plug ever made. This is according to BiSN and AkerBP which noted in the paper that the record-setting plug measured 50 ft long and weighed almost 34,000 lb, about 20,000 lb of which comprised the bismuth alloy.

Over the course of the 30-well project, BiSN and AkerBP reported that 350 tons of bismuth alloy was sent downhole. The technology was credited with helping AkerBP trim its original P&A timeline from 10 years of operational time down to 4 years.

This followed the first P&A campaign done in 2014 that used conventional sealing technology. After that first attempt to seal the wellbores off, 14 showed no sign of annular gas pressure. However, the situation grew more concerning over time.

By 2019, the number of wells with zero pressure buildup shrank to just four.

It was around that point that AkerBP sought to qualify the BiSN technology with a 2-year test inside a well with one of the highest levels of sustained casing pressure.

Downhole gauges showed that the top section of the annulus where the first bismuth plug was used had a 0 psi reading for the duration of the 2-year monitoring effort. Downhole cameras were also used and provided visual confirmation that no gas bubbles were flowing past the plug location.

Because of its noncorrosive nature, BiSN noted that the alloy plugs should hold for many thousands of years. This is based on laboratory results cited in the companies’ technical paper which suggest the lifetime of the plugs may even exceed 100,000 years.

OTC 31897 Bismuth Plugs Used to Cap All Wells During the Final Phase of the Valhall DP Abandonment Campaign, Offshore Norway by Egil Thorstensen, Kjetil Vadset, Martin Knut Straume, and Laurent Delabroy, AkerBP; Gasser Abdelal, Queen'sUniversity, Belfast; Paul Carragher, Jeff Fulks, and David Leslie Mason, BiSN Oil Tools Ltd.