Case Studies

Sealing Multiple Annuli During Well Abandonment in Offshore Australia October 2019

Well Abandonment with Wel-lok™ MXD

Identification of problem

BiSN had been contracted for a 20 well P&A campaign to provide gas tight seals for eliminating gas migration in the annulus between 9 5/8” production casing and 13 3/8” surface casing. One well in this campaign had a different casing program but the same issue. In addition to the 9 5/8” and 13 3/8” casings, this well also had a 7” casing which was cemented in place. This required a tool that could be run inside the 7” casing and seal through a section milled window through the 9 5/8” casing and out to the 13 3/8” casing.

With the 7” casing cemented inside the 9 5/8”, the challenge was to provide a tool with a small enough OD to pass through the 7” casing and provide sufficient alloy to create a gas tight seal out to 13 3/8” casing. The client also required a solid bismuth alloy plug across the wellbore so the modified thermite heater would need to be removed, not left in the center of the plug.

For this well, a window was section milled through 2 strings of casing (7” and 9 5/8”) out to 13 3/8” casing. BiSN deployed a 5.75” OD Wel-lok™ MXD that was set across this window. The MXD utilizes bailers to deploy the bismuth alloy in pellet form rather than casting the alloy onto the outside of the heater as other BiSN tools do. This allows for the maximum amount of alloy to be deployed while providing a smaller OD tool.


The Wel-lok™ MXD was successfully set and gas migration was eliminated. This well is currently being monitored until all wells on the platform are plugged. This is the largest MXD BiSN has deployed and the first multiple annuli plug.