Case study: Torque Africa exploration drilling, South Africa
Published by John Williams,
Global Mining Review,
Between 10 - 15 February 2020, work began on setting up a site for exploration drilling at a remote greenfield site in the Northern Cape province of South Africa. Nardus Bezuidenhout, director of Torque Africa Exploration, moved in on site with one of the company’s Thor 8000 RC truck-mount drill rigs. Joining the main rig on site were a fleet of support vehicles, including rod carrier trucks, compressors, water and fuel tankers, and medical service vehicles – as well as a full team of staff to support the operation.
With the nearest town more than 120 km away – along drifting dirt roads – it is no small undertaking to set up camp in the middle of nowhere. Even with extensive planning and logistics, fuel and water are precious resources that cannot be wasted. As the old adage goes, time is money, and the longer these resources are on site, the higher the total cost of the project would be. When drilling exploration holes to 1000 m or more, it is not uncommon to be on site for up to 3 months.
This estimated drilling timeline is based on using diamond coring systems – known for their reliability in deep-hole drilling, albeit at a great cost. Additionally, diamond coring requires the use of expensive muds for stabilising the hole during drilling. When encountering underground cavities these muds are then wasted, and when encountering aquifers the muds can potentially contaminate a supply of drinking water.
However, for this project, Torque Africa elected to use the Mincon MR range of exploration tools. These reverse-circulation (RC) tools use a variation of the familiar air-powered down-the-hole (DTH) hammer, but designed to flush cuttings through an inner tube. Until recently, such air-powered RC systems were only used for exploration drilling up to 500 or 600 m – but changes to Mincon’s RC hammers meant that the team could expect deeper drilling. Drilling in remote locations requires careful management of scarce resources.
Drilling commenced on the afternoon of 17 February 2020, using a Mincon MR120 RC hammer and a 143 mm drill bit. Slow, methodical progress was made to ensure everything was safe and functioning as intended. After reaching a depth of 114 m the team retrieved the drill string from the hole, after which the first 4 m of the hole was reamed and widened to install a temporary casing. This was done to stabilise the soft ground near the surface, thus preventing hole collapse. When drilling resumed the team used a new, 140 mm RC drill bit, equipped with polycrystalline diamond-impregnated carbide buttons.
Over the next 4 days, drilling operations took place during daylight hours. Average progress was approximately 120 mpd, which included retrieving the drill string at 637 m in order to change sample tubes and swapping to a new, 137 mm RC drill bit - both being wear components.
The process of retrieving a drill string is straightforward, but time-consuming. After lifting each pipe out of the hole it needs to be unscrewed, removed, and the the remaining pipes reattached. This is repeated, six metres at a time, for the entire length of the drill string – more than 600 m at this point. Deep-hole drilling projects can become incredibly expensive if multiple drill-string retrievals are required, but this can be offset by using durable, reliable equipment that consistently delivers high levels of performance.
On day eight of the project, Torque Africa Exploration resumed drilling with the fresh 137 mm drill bit – only the third bit required for the project. In order to reduce time on site, a decision was also made to move to a 24 hour drilling schedule.
Rapid progress was made: in just two days the team powered through 382 m of hard granite and rushing groundwater to reach a depth of 1019 m and making this one of the deepest exploration holes drilled using air RC tools, putting the technology on par with diamond coring as a solution for deep-hole drilling.
With the target depth reached, the drill string was retrieved for its final inspection, which showed only expected wear and no failures.
Once retrieved, a post-drilling survey was conducted to verify the depth and straightness of the shaft, with the survey report indicating a hole deviation of only 0.83°. This exceeds the performance of competing technologies and proves yet another benefit of air RC systems.
A record-depth hole drilled in record time would be a pointless achievement if sample cuttings were unusable, though. Fortunately, this is where the air RC approach has yet another advantage: consistently returning uncontaminated sample cuttings to the surface. This is due to cuttings being flushed up through a sample tube rather than being exposed to the drilled hole.
Bezuidenhout noted that samples continued to be collected even after encountering extensive groundwater at 886 m. For every metre drilled, a sample was collected, bagged, and tagged, allowing geologists to analyse the ground conditions and mineral content.
Read the article online at: https://www.globalminingreview.com/exploration-development/14042020/case-study-torque-africa-exploration-drilling-south-africa/
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