Reverse Circulation Drilling
RC drilling is similar to air core drilling, in that the drill cuttings are returned to surface inside the rods. The drilling mechanism is a pneumatic reciprocating piston known as a hammer driving a tungsten-steel drill bit. RC drilling utilizes much larger rigs and machinery and depths of up to 500 meters are routinely achieved. RC drilling ideally produces dry rock chips, as large air compressors dry the rock out ahead of the advancing drill bit. RC drilling is slower and costlier but achieves better penetration than RAB or air core drilling; it is cheaper than diamond coring and is thus preferred for most mineral exploration work. Reverse circulation is achieved by blowing air down the rods, the differential pressure creating air lift of the water and cutting up the inner tube which is inside each rod. It reaches the bell at the top of the hole, and then moves through a sample hose which is attached to the top of the cyclone.
The drill cuttings travel around the inside of the cyclone until they fall through an opening at the bottom and are collected in a sample bag. The most commonly used RC drill bits are 5-8 inches (12.7–20.32 cm) in diameter and have round metal ‘buttons’ that protrude from the bit, which are required to drill through shale and abrasive rock. As the buttons wear down, drilling becomes slower, and the rod string can potentially become bogged in the hole.
This is a problem as trying to recover the rods may take hours and in some cases weeks. The rods and drill bits themselves are very expensive, often resulting in great cost to drilling companies when equipment is lost down the bore hole. Most companies will regularly ‘sharpen’ the buttons on their drill bits in order to prevent this, and to speed up progress. Usually, when something is lost (breaks off) in the hole, it is not the drill string, but rather from the bit, hammer, or stabilizer to the bottom of the drill string (bit). This is usually caused by a blunt bit getting stuck in fresh rock, over-stressed metal, or a fresh drill bit getting stuck in a part of the hole that is too small, owing to having used a bit that has worn to smaller than the desired hole diameter.
Although RC drilling is air-powered, water is also used, to reduce dust, keep the drill bit cool, and assist in pushing cutting back upwards, but also when collaring a new hole. A mud called liqui-pol is mixed with water and pumped into the rod string, down the hole. This helps to bring up the sample to the surface by making the sand stick together. Occasionally, ‘super-foam’ (AKA ‘quick-foam’) is also used, to bring all the very fine cuttings to the surface, and to clean the hole. When the drill reaches hard rock, a collar is put down the hole around the rods which is normally PVC piping. Occasionally the collar may be made from metal casing. Collaring a hole is needed to stop the walls from caving in and bogging the rod string at the top of the hole.
Collars may be up to 60 meters deep, depending on the ground, although if drilling through hard rock a collar may not be necessary. Reverse circulation rig setups usually consist of a support vehicle, an auxiliary vehicle, as well as the rig itself. The support vehicle, normally a truck, holds diesel and water tanks for resupplying the rig. It also holds other supplies needed for maintenance on the rig. The auxiliary is a vehicle, carrying an auxiliary engine and or booster engine.
These engines are connected to the rig by high pressure air hoses. Compressors on an RC rig have an output of around 1000 cfm at 350 – 500 psi (500 L•s-1 at 3.4 MPa). Alternatively, stand-alone air compressors which have an output of 900-1150cfm at 300-350 psi each are used in sets of 2, 3, or 4, which are all routed to the rig through a multi-valve manifold.
There are two types of Reverse Circulation Drilling available today. Both of these methods use the exact same dual wall drill pipe. The only difference is the hammer that attaches to the pipe. The two types of Reverse Circulation Drilling are:
This is the newer of the two methods. This method uses a special Centre Sample Hammer with a hollow centre that allows the sample into the dual wall drill pipe right at the face of the drill bit.
This is the original RC with a conventional Down Hole Hammer and an “Interchange” (adaptor just above the hammer with a vertical slot to let the sample into the centre of the Dual wall drill pipe. The advantage of the Centre Sample Hammer is the sample goes immediately into the drill pipe without having to travel up the side of the conventional hammer and into the interchange (approximately 4 feet).
Inyati Drilling and Mining utilizes the Centre sample system on all our mineral exploration programs. (In the 4″ – 6″ sizes) The centre sample hammer is available up to a 24”-hole Diameter. Reverse circulation drilling is a method for obtaining chip samples of mineral occurrences.
Reverse circulation (RC) drilling utilizes a solid bit (usually tri-cone) to produce a hole and delivers rock chips to the surface for subsequent analysis. This type of drilling uses a large, rotary drill and a compressor to blow the rock chips produced by the bit to the surface. The air is blown down the annulus between the rock wall and the drill pipe or, more commonly, down a special annulus in the drill pipe.
The air and rock chips are then blown past the bit and up inside the drill pipe to the surface. Normally the air exhausts through a centrifugal classifier so that the rock chips and dust will be captured in a sampler. The reverse circulation drills are all truck mounted. The cost of drilling is much less than diamond drilling and the penetration rates are an order of magnitude greater than diamond drills. For this reason, many drill campaigns start with reverse circulation drills and move to diamond rigs if the mineralization warrants it.
Unlike diamond drills, RC drills do not produce a core which comes sequentially from the hole. The rock chips created by the drill bit are delivered to the surface generally in the order in which they are produced but not always. Nevertheless, the relatively higher speed and low cost of RC drilling makes it a popular method for obtaining mineral samples for assaying.