Percussion drilling is similar to the early cable
drilling done before the 1920s. A hammer unit is
driven by compressed air giving a series of short,
rapid blows to the drill rods or bit, and at the
same time applies a rotating motion. These drills
may vary in size from small hand held units to large
truck mounted rigs capable of drilling large
diameter holes. Drill cuttings are flushed to the
surface by a stream of compressed air. There are two
types of this kind of drilling: down the hole or top
hammer percussion drilling rigs. The down the hole
hammer unit is lowered into a hole that is attached to the
lower end of the drill rods to operate a non coring
drill bit, which is sometimes known as a “button bit.” It can
drill depths of around 100 to 150 metres. Drill
cuttings are pushed to the surface by a stream of
compressed air. Track mounted drilling rigs, like
little tanks, can negotiate slopes of up to thirty
degrees and can drill on slopes in excess of
twenty-five
degrees, making the access to the exploration site
easier than taking larger vehicles up these
inclines.
The hammer unit for top drilling is also powered
by compressed air and uses non coring drill bits.
This type of system is usually smaller than
down-the-hole drills and they are used for holes up
to ten centimetres in diameter and drill up to depths
of 100 metres. This type of drilling rig is only
used to drill a maximum of twenty metres. Most use light
portable air compressors, which restricts the
drilling depth to a few metres above the water
table. This is due to the compressed air being too
light to blow back the heavy, wet, sludge-coated
rock chips to the surface. Mounted on either light
trucks or tractors, this type of drill is easy to air
lift into an exploration drilling site.
Evidence suggests that percussion drilling can
increase penetration in hard-rock formations, create
better and larger cuttings of the rock for analysis,
and lower drilling costs. However, the oil and gas
industry have not fully embraced this type of
drilling. It is because rock fragmentations
increase safety risks. In addition, further
research needs to be conducted on the actual
benefits of using percussion drilling versus other
methods.
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