Using diamond-impregnated bits and a standard rotary percussion rig, deep drilling expert Torque Africa Exploration recently drilled its way into the record books with a hole drilled in the Springs area reaching 1 148m.
This is an African record and one of the deepest rotary percussion holes drilled worldwide in mixed and largely unchartered geologies and the contract calls for yet more and deeper holes in future. Nardus Bezuidenhout, Torque Africa director, explains that the company is part-way through its contract to bore several deep-level holes into disused mine stopes for the treatment of acid water mine drainage.
These range from a few hundred metres below the surface to over 1 200 m for the deepest hole and require all the ingenuity of the company’s technical experts, as well as its owner, and renowned drilling expert, Bezuidenhout.
According to Bezuidenhout, Torque Africa took on the job at the expense of other experts who said the risks of failure were too high. “We were never in doubt though and in conjunction with Pieter Coetzee and his team from Sandvik Mining and Rock Technology’s rock tools division, we planned the technical details of the operation.”
Sandvik’s Product Manager – DTH Bits, Matthew Shoffner, also worked with the local team of experts to find the right technical solutions and it soon became apparent that the right tooling would be pivotal and that Sandvik bits and hammers were going to be key to getting the job done efficiently. “Considering the unchartered nature of the geology of the area we chose to use standard bits and then switch to diamond impregnated bits as soon as formations became too hard or abrasive for them.”
“This is exactly what we encountered after the initial depth of about 300 metres of dolomite and shale and from there on it was mostly hard quartzite with some khaki shale, black slate shale, mixed dolomite and quartzite, diabase and more quartzite. This made for some tough drilling, but was made considerably easier with the use of the diamond impregnated bits,” says Bezuidenhout.
He explains that considering the forces at these depths only the highest quality bits could be used and even despite the higher cost of diamond impregnated bits, the durability was extended beyond 378 m per bit. This far outweighed the standard bit alternative which would have been 70 m and required constant pullouts.
Bit changes can be complicated and time-consuming as pull back of the rods would take up to 24 hours to retrieve the 6m lengths to replace the tooling. A shanked bit at these depths would be even more catastrophic and could require drilling of a new hole if the bit could not be retrieved.
“We had to deal with dolomite wetlands, dykes, voids and water ingress at all levels which made it tough. We were also required to drill the hole telescopically from its original diameter of 558.9mm at the top through various layers including a 310m deep grouted section, casing and eventual intersection of quartzite just beyond 300m at which point the drill diameter was down to a 172 mm bit.
Thereafter we continued through various phases until prematurely intersecting the stope at 1 148 m instead of the predicted depth of 1 200 m. At this point the bit size was 124 mm and the hole ready to be used by our clients to receive pumped products that are a by-product of the treatment of millions of litres of acid-mine drainage.
