Reverse circulation drilling, or RC drilling, is a form of percussion drilling that uses compressed air to flush material cuttings out of the drill hole safely and efficiently.
Thanks to the benefits enjoyed by this method, it’s become a popular technique in mining exploration and is one of the most common drilling methods across Australia.
Western Australians are proud to claim RC drilling as our invention, and it’s said to have originated in Kalgoorlie in the mid-20th Century when traditional open-hole drilling was proving difficult in soft iron ore and mineral sands.
As time moved on, so did technology, and 1972 saw the first specific RC drill pipes developed.
The first RC hammer was created in 1990, as a way to deliver cleaner cutting samples. The 1990s saw high-pressure boosters added to the drilling method as a way to boost performance and improve penetration rates.
This resulted in reduced costs for mining, which has seen RC drilling become one of the most cost-effective and accurate drilling methods available.
RC drilling uses a drill bit that features round, protruding tungsten-carbide buttons, to cut through hard rock and earth. The drill rods themselves are hollow and feature an inner tube with an outer barrel (similar to air core drilling).
A pneumatic reciprocating piston (also known as a downhole hammer) delivers rapid impacts to the drill stem, transferring this energy through the drill bit. This extra concussive force hammers the bit into the rock, pulverising it, while a rotational device ensures that the drill bit hits new sections of rock with each impact.
Inside the drill bit, a dual-walled drill pipe is used to achieve circulation, with compressed air getting injected between the tubes to flush the cuttings from the hole and up through the inner tube. The cuttings travel back up the tube to the surface, into a cyclone, where they go over a splitter that delivers the samples into a sample bag.
The exploration team then collects the samples, align them to the hole depth, and layout for their client to log. From here the sample is transported to their lab for assay.
RC drilling typically produces dry cuttings, as their compressors also pump air ahead of the drill bit, drying the rock in the process.
While the process itself is typically dry, water can be injected when collaring a new hole as a way to reduce dust and assist in lifting cuttings to the surface. When water is injected, an additive gets mixed into the water, known as Super Foam. This makes the cuttings bind to each other, increasing sample recovery.
RC drilling is commonly employed in open-cut mines and used for pit grade control and in delivering useful insight for mine planning and blasting. It has an optimal depth of between 300-500 metres.
RC grade control is extremely effective in defining boundaries between ore bodies and waste rock
RC Drilling is ideal for exploration, as it delivers contaminant-free samples, which are collected at the drill itself and can be sent immediately for assessment. It also reduces handling time, which in turn can serve to reduce overall program costs.
RC drill rigs with their considerably larger equipment can typically reach depths of 300 to 600m. Dependant on rig size and ground conditions, there has allegedly been holes drilled to a max depth of over 800m. The capacity to drill to such depths is negatively impacted by groundwater meaning drillers require more air pressure and also relies on the rig’s pullback capacity.
RC drilling has clear advantages over other drilling methods.
While there are many compelling advantages to RC drilling, it also comes with its own set of challenges that you should be aware of when planning your drill program.
RC drilling is an excellent drilling method for exploratory drilling. However, as with all drilling methods, RC drilling has specific safety and environmental considerations to take on board.
While larger than air core drill rigs, RC rigs have a smaller footprint than other methods, which eliminates the need for large earthworks. This results in a lowered environmental impact of your operations.
The percussive nature of the method—the consistent hammering of the drill bit into rock, the use of loud air compressors—can result in hearing damage, or loss, if the proper hearing safety equipment isn’t used.
Staff are required to retrieve cutting samples from the cyclone once drilling is complete. This can require manual handling of the downhole equipment, creating a potentially hazardous situation. These staff members open themselves up to hazards such as falling objects, their proximity to compressed air, potential crush injuries at pinch points around the rig, and physical strain from lifting equipment.
RC drilling is similar to air-core drilling, as the drill cuttings are returned to the surface through the drill rod’s inner tube using compressed air.
While Air-Core Drilling is better suited to situations where unconsolidated ground is present, and less force required, RC drilling features the added benefit of concussive force to power the drill bit, providing extra strength behind the drill. This makes RC drilling ideal for use on harder surfaces.
Thanks to the high speed and efficiency, and the reduced cost per metre, RC drilling is ideal for acquiring mineral samples in an exploration project’s initial stages. RC rigs also tend to have a greater capacity, and they’re designed to handle larger downhole equipment, so it’s better suited to larger drilling programs.
With experience in all aspects of Exploration, Geotechnical, Environmental drilling, and Hydro and Water Well Drilling, we are here to look after our clients in a professional, ethical, and reliable manner, while maintaining safe, environmentally responsible practices.
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