An Introduction to Reverse Circulation Drilling
When choosing a drilling method, especially for exploration and grade control, one must consider many factors such as cost, time, environmental impact, depth of drilling and sample quality. Reverse Circulation drilling offers many advantages over other drilling methods, such as Rotary Air Blast drilling or diamond drilling. This article will explain what Reverse Circulation drilling is, its benefits, its health and safety considerations, and why it is the best method of grade control.
1. What is Reverse Circulation drilling?
Reverse Circulation drilling, or RC drilling, is a method of drilling which uses dual wall drill rods that consist of an outer drill rod with an inner tube. These hollow inner tubes allow the drill cuttings to be transported back to the surface in a continuous, steady flow.
Unlike diamond drilling, it compiles sample rock cuttings instead of rock core. The drilling mechanism is most often a pneumatic reciprocating piston called a hammer, which in turn is driving a tungsten-steel drill bit, specifically made to be able to crush hard rock.
The hammer is used to remove rock samples which are pushed through the machine with compressed air. When air is blown down the annulus (ring-shaped structure) of the rod, the pressure shift creates a reverse circulation, bringing the cuttings up the inner tube. When the cuttings reach a deflector box at the top of the rig, the matter is moved through a hose attached to the top of the cyclone.
The drill cuttings will travel around the cyclone until they fall through the bottom opening into a sample bag. These bags are marked with the location and depth of the place where the sample was collected and can be transported directly to the assay lab for analysis.
2. The benefits of RC drilling
Reliable and uncontaminated samples:
When the cutting travels through the bit into the inner tube towards the cyclone, it is not introduced to other areas of the hole, keeping it free of cross-contamination. This creates the possibility of producing large quantities of high quality, reliable samples.
Sample cuttings are easier to catalogue and keep track of. Because the samples collected will have an exact location and depth at which they were found, surveyors and interested parties will be able to locate mineral deposits more precisely.
High drilling penetration rates:
RC penetration rates are comparable to open drilling methods and are often faster at greater depths. Sample velocity through the inner tube can reach speeds of 250 m/sec, making the retrieval of drill cuttings a rapid but safe method requiring fewer man-hours.
Production rates are superior to diamond drilling; rates of 200-300 m/day are routine at speeds faster than 10 m/h. Therefore, results from RC drilling can be revealed to clients much faster because the drilling process takes less time.
Versatile in tough conditions:
RC drilling is fairly straightforward and requires far less water than diamond drilling, making it ideal in places such as Australia, where water may be scarce or costly. RC drilling is also a more cost-effective method than diamond drilling, with operating costs reduced by up to 40% compared to diamond drilling.
Cost reduction is especially noticeable in geologically challenging locations, which would increase the cost of more conventional drilling methods even more. RC drilling will keep those costs down since it is more resilient in harsh environments.
3. Why is RC drilling best for grade control?
Grade control is used to define the ore grades and blocks in the pit. For an exploration program, the quality of the samples is imperative for mine planning and blasting—the samples must be reliably accurate.
RC drilling is the superior method used for grade control in open pit mining operations because:
- samples obtained via RC drilling are free of contaminants
- the samples are collected at the drill and sent directly to the lab to be assessed
- RC sampling requires less handling than other methods
- less handling results in cost reduction and faster turnaround times
The reliability of the cuttings produced during RC drilling is an industry asset. RC grade control is the most cost effective and efficient method to differentiate between waste rock and minerals.
4. Key safety and environmental considerations
Exploration drilling is strictly controlled and regulated when an exploration licence is approved. Most drilling requires further governmental approval, which will demand a thorough environmental survey, proposed methodology, administrative plan, and environmental mitigation.
RC drilling safety regulations will vary depending on location, but there are general precautions to observe. RC drilling has some inherent risks to be aware of.
The sounds of constant hammering of drill bits into rock combined with the use of loud air compressors may cause hearing loss. That’s why occupational health and safety laws require workers to wear some sort of hearing loss protection as well as all other appropriate personal protective equipment. This includes not only ear plugs or ear muffs but also respiratory masks, safety footwear, hard hats and industrial work gloves.
RC drilling requires ‘Samplers’ or ‘Offsiders’ to process the cutting samples from the cyclone. They may have to manually handle some of the downhole equipment, putting them in a more hazardous situation than in a standard drilling site such as well drilling. They must remain vigilant about a number of hazards such as:
- the physical strain they could receive from manually handling samples,
- the potential of falling objects,
- proximity to compressed air, and
- pinch points or areas around the rig that could be the source of crush injuries.
However, adopted control measures make it more difficult to access potential pinch points, and added interlocks prohibit operation under unsafe conditions.
Because RC drilling has a small rig footprint, it eliminates the need for extensive earthworks. This means RC drilling has a minimal environmental impact.
5. RC drilling best practices
There are many procedures and techniques required to achieve the best possible results from RC drilling. Holes are set up similarly to conventional holes, and the most important element of RC drilling is the sample. The goal is to capture as much sample as possible from the hole, preferably through the inner tube.
High inside sample circulation is achieved by having the appropriate clearance between the bit shroud and the hole wall. This should forge a seal, thereby forcing the sample inside. If there is water in the hole, allow the hole to collar off as this helps to keep the samples dry, making them easier to transport and process.
Since its inception in the early 1970s in Australia, RC drilling has become a preferred method for initial exploration and grade control due to its many advantages:
- the samples extracted by RC drilling are reliable and free of contaminants,
- the method is timely and cost-efficient,
- it is well-suited for difficult terrains,
- it has high penetration rates, and
- it has minimal environmental impact.
Taken together, these positive aspects of RC drilling make it an excellent choice for mining exploration and grade control.