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Reverse Circulation (RC) Drilling Technique

Author: Zackery Brett
Reverse circulation (RC) drilling uses a dual-walled drill pipe (rod) with a solid drill bit to produce a hole in a formation and delivers rock chips (cuttings) to the surface for a subsequent analysis. The drilling mechanism is a pneumatic reciprocating piston (hammer) driving a tungsten-steel drill bit and a compressor to blow the cuttings, created by the drilling operation, to the surface. Reverse circulation is achieved by blowing air down the rods; the differential pressure creates air lift of the cuttings up the inner tube which is inside each rod. Normally the air exhausts through a centrifugal classifier so that the rock chips and dust will be captured in a sampler.

The most commonly used RC drill bits are 5-8 inches (12.7–20.32 cm) in diameter and have round metal “knobs” that protrude from the bit, which are required to drill through rock and shale. As the knobs wear down, drilling becomes slower and the rod string can potentially become bogged in the hole, therefore RC drill rigs must have a high pullback capacity and hydraulic power, otherwise trying to recover the rods may take hours and in some cases days.
While operating in remote sites or areas, Reverse Circulation drilling setup usually consists of several support vehicles, as well as the rig itself. The support vehicles hold the drilling rods (dual tube RC pipe), spare parts, diesel and water tanks, as well as any other tools and equipment required for resupplying and rig maintenance.

Most of the RC drill rigs have their own compressor to generate the air pressure needed to extract the cuttings. However, generally these compressors do not have the power for deeper drilling, due to lack of space on the rig itself for the larger, more powerful units. Alternatively, companies like Drilling Supply, are offering compressor free setup, which enables the driller to use a compressors with various power rated outputs to accommodate their particular project, while using the same RC Drilling Rig for all of them. As a consequence, even a broken air compressor will not cause disruption in the drilling operations. While a malfunctioning compressor is being repaired, a backup unit can be rapidly deployed to continue the drilling operation, saving the drilling contractor valuable time and money.

RC drilling routinely achieves depths of up to 500 metres. This depth is comparable to that of Diamond Core drilling, but unlike core drilling, RC drilling does not produce a solid core. The cuttings generated by the RC drill bit are generally delivered to the surface in the order in which they are produced but not always. Therefore, RC drilling is not as accurate as Core drilling. There is no way of knowing with a 100% certainty which depth the cuttings came from. Also there is a tendency for heavier minerals (e.g. gold) to fall to the bottom of the hole or be caught in elbows and curves in the drill pipe (Reverse Circulation Drill Pipe). As a result, assays from RC drilling may understate the amount of mineralization present in the formation; however this deficiency is largely offset by significantly reduced cost of drilling and rapid penetration rates.

For this reason, companies like Drilling Supply have developed a multipurpose hollow spindle rotary drive with variable/reversible hydraulic motor and automatic constant rotation power control. Drilling Supply can now offer drilling companies a capability to begin the drilling operation with Reverse Circulation drilling and once the mineralization warrants it and clean core samples are required, switch to Diamond Core drilling technique. This is done by using the same drill rig and simply changing from RC drilling tool set to Core drilling tools (coring rods and bit). This allows the driller to quickly and inexpensively achieve the mineralization depth and begin extracting quality cores for analysis.

About the author : Zackery Brett is a head mechanical engineer for a Reverse Circulation drilling rigs manufacturing company, Drilling Supply. Zackery Brett has been working in the drilling business for more then 15 years.


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