I have traveled to about 3.5 kilometers beneath the surface at the Western Deeps mine in South Africa. The problems of going deeper were obvious.
We had to descend three separate shafts to reach the face, which took over an hour. Shafts are limited to about 2,500 meters depth because the weight of the wire rope holding the man-carrying cage exceeds the strength of the rope. Even to reach that depth it is necessary to have a tapered rope.
The rock temperature was over 60 degrees C (140 degrees F). The mine pumped thousands of tons of slurried ice down the mine every day in order to maintain the air temperatures at workable levels.
Such depths are only possible because the rock surrounding the gold reefs is very strong. However, the rock is subject to immense stress. Sometimes it breaks explosively in a ‘rockburst’. A month earlier a rockburst had killed three miners.
The workings at the Mponeng shaft at that mine have now reached 4.2 kilometers below the surface.
The Russians drilled (not dug) a deeper hole at the Kola Superdeep hole. They had planned to go to 15 kilometers but stopped at 12.26 kilometers because the rock at 180 degrees C was hotter than expected. In a 23 cm (9 inch) wide hole, it is not possible to pump down enough drilling fluid to keep the drill head cool.
Drilling so deep is tedious and expensive. Pulling out the 600 or so drill pipes in the string is very time-consuming every time the drill head needs replacing. Maintaining as straight a hole as possible when it is 12,000 meters long but only 23 centimeters wide is difficult. Too many turns and kinks in the hole cause unsustainable friction and forces on the drill pipe.
The temperature of the rock causes difficulties with drill fluid chemistry and degrades the strength of the drilling head and the drill pipe.
The same problem of supporting the weight of the pipe occurs and the hole must be tapered.
In short a hole to a depth of 12 kilometers pushes drilling technology to the limit.
In 1994 the Germans drilled a hole to 9100 meters using this rig:
As Share By Chris Seymour he wrote: