What is Reverse Circulation Drilling?

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 in a safe and efficient manner.

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 and South America.

Short history of Reverse Circulation Drilling

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.

How does Reverse Circulation Drilling Work?

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.

How is reverse Drilling Applied?

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.

How deep does Reverse Circulation Drill?

RC drill rigs with their considerably larger equipment can typically reach depths of 300 to 600m. Depending 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.

The benefits and advantages of RC Drilling

RC drilling has clear advantages over other drilling methods.

1. Achieves greater penetration in hard rock than air core drilling, thanks to the percussive force and the strength of the drill bit.
2. Provides greater sample accuracy, and cuttings are easy to catalogue. The collected samples have an exact location and depth where they were acquired, making it easier to more precisely locate mineral deposits.
3. Ideal for harsh environments. RC drilling uses approximately 40% less water than diamond drilling, making it a better option for remote area drilling.
4. Reduced risk of cross-contamination. As the cuttings travel through the drill bit’s inner tube and aren’t introduced to other areas of the drill hole, the samples remain uncontaminated.
5. Faster and more efficient, reducing costs and providing greater returns. RC drilling achieves higher production rates and can achieve up to 200/300 metres per day. This means that results can be delivered to clients faster, as the drilling itself takes less time. The speed and efficiency involved also reduces manhours on the drill rig, meaning savings on overall staff hours. Thanks to the power and efficiency of the equipment, RC drilling is more resilient in harsh environments, which makes it a cheaper option overall. In fact, RC drilling has been shown to reduce costs by 25-40%.

The Disadvantages Of RC Drilling

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.

1. It’s more expensive than air core drilling. RC drill rigs are considerably larger than air core drill rigs, and they require more equipment and more accurate driving skills.
2. Unable to operate on poorly maintained surfaces. Due to the size of the drill rigs, well-maintained roads are essential for RC drilling.
3. Less geological information is delivered through samples, due to the pulverisation of the rock by the drill bit.
4. Holes can deviate due to the nature of the drilling technique, which can provide less precise sampling. Ensure you’re using a Harlsan Spiral Stabiliser Sub

Safety And Environmental Considerations

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.

Lower environmental impact

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.

Increased personnel risk

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.

What’s The Difference Between RC Drilling And Air Core Drilling?

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.

Get Support From The Experts

At Harlsan’s, we’re experts in supporting drilling companies in getting the most value from your RC drilling programs.

Ready to start your next venture? Get in contact with us today for advice, support, and assistance in setting up for your next RC drilling project. We’ll help you choose the right equipment so you can get the best result from your program, every time.

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