Where exactly are these 6500 gallons of drilling fluid and how would they have been retrieved if the project was succesfull? They are not boring a tunnel... So is there just a quite thin pipe in the ground? is it filled with the fluid? Or is there some storage for the fluid?
The drilling fluid(mud) is both inside the drill pipe and in the annulus between the pipe and the rock.
After the drilling is complete, cement grout is pumped into the annular space, replacing the mud. then the mud in the pipe will be replaced with sea water. Then they will pull the fiber optic cable through the pipe.
Drill pipe comes in various sizes, probably about 4-8 inch diameter for this job.