Sealing Success: Overcoming Complex P&A Challenges - North West Shelf, Australia

Gas-Tight Annular Plug: The operator has a poor cement bond between the 10-3/4” and 13-3/8” casing and needed to section mill and set a plug that would permanently seal the annulus. Traditional cement plugs require long section mill windows to allow for sufficient plug lengths to meet regulations. The operator needed a solution that would create a reliable gas-tight seal across the entire well-bore without requiring a long section mill window.

Deployment from Floating Platform: The operator needed to deploy the P&A plug from a semi-submersible rig. This introduced operational challenges, as the tidal heave introduced a need for new operating equipment and procedures when compared with a fixed platform. For deployments using bismuth beads, this is especially challenging, as the beads deploy from surface around a heater that needs to stay at the setting depth throughout the operation.

The BiSN Seal Through Casing tool is the premier option for creating gas-tight, permanent plugs in annular applications. To prepare the well, a 10-meter window was section milled in the 10-3/4” casing, providing access to the 13-3/8” in which the plug would be set. The unique properties of a bismuth seal allow for much shorter plugs to achieve higher pressure differentials when compared to traditional cement plugs, allowing for a shorter section milling operations.

Once the setting zone was prepared, an inflatable bridge plug was deployed to the bottom of the window to act as a base upon which the bismuth beads would be dropped. The use of a thermite heater with bismuth beads allows for operational flexibility, as the same tool can be used with varying lengths of windows or plugs. Once the inflatable bridge plug was set, a layer of beads was deployed to fill the gap around the inflatable fishing neck.

A thermite heater with a timer activation running tool and a ported sub was then deployed to depth and slacked off to rest on the inflatable plug. At surface, a special side entry sub which had been designed for the application was to be used for deploying beads. On a fixed platform, the pipe string is typically held in slips while beads are deployed from surface, but because of the tidal heave, this was not an option on the semi-submersible platform.

Instead, the operation was planned such that the thermite heater remained stationary, and the beads were deployed using a flexible hose into the side entry sub. This allowed the platform to move relative to the tool string without creating a safety or operational risk at surface. Additionally, the side entry sub was equipped with an air/fluid inlet to allow for flushing of the beads.

Once the bismuth beads reached the setting depth and the timer activation expired, the thermite reaction initiated, melting the bismuth alloy. The alloy flowed out to the surface of the 13-3/8” casing, filling the cross-section of the well. Once the bismuth alloy was melted, the thermite heater was removed, allowing for a full cross-section plug to form. When the bismuth alloy solidifies, it expands and presses against the surface of the casing. This contact pressure forms a gas-tight seal capable of holding significant pressure differentials.