The fire started at the Strongman 2 underground mine in the Greymouth coalfield.
Christchurch Press said part of the fire had been extinguished by cutting off the air supply, however one area is still smoldering.
The smoldering area is being targeted by Solid Energy with the high-expansion foam trials. The sealant will be used to stop the fire from spreading before a clay slurry is applied to smaller cracks.