Page 241 - Introduction to Mineral Exploration

224 M.K.G. WHATELEY & B. SCOTT

TABLE 10.7 A selection of core,
Size Hole diameter Core diameter Core area drillhole and drill bit sizes.
(mm) (mm) (% of hole area)

METRIC SIZES
Conventional drilling
36T 36.3 21.7 36
56T2 56.3 41.7 55
66T2 66.3 51.7 61
Wire-line drilling
56ST 56.3 35.3 39

AMERICAN SIZES
Conventional drilling
AX 48 30.1 39
BX 59.9 42.0 49
NX 75.7 54.7 52
Wire-line drilling
AQ 48 27 31
BQ 60 36.5 36
NQ 75.8 47.6 39
HQ 96.1 63.5 44
The hole diameter is the diameter of rock cut by the drill bit. Part of this is
removed by the cutting and grinding action of the bit and water flushed to the
surface, while the central part is retained as core in the core barrel.

(Australian Drilling Industry Training Com- (this can be adjusted using hydraulic rams at
mittee (1997). the surface), the right number of revolutions
4 Casing. Cylindrical casing is used to seal the per minute, and the correct choice of drill bit.
rock face of the hole. It provides a steel tube in Until recently there was no direct method of
which the drill string can operate in safety and knowing what operating conditions existed at
prevents loss of drill strings caused by rock col- the drill bit, which are the critical factors if
lapse and either loss or influx of water. Casing drilling efficiency is to be optimized. Recent
and drill bits are sized so that the next lower developments in “measurements while drill-
size (i.e. smaller diameter) will pass through ing (MWD)” have overcome this and allow a
the larger size in which the hole had been previ- variety of direct measurements such as tem-
ously drilled. perature, pressure at the bit face, and rate of
5 Speed and cost of drilling. Drilling machines water flow, to be made (Dickinson et al. 1986,
used in mineral exploration usually have a cap- Prain 1989, Schunnesson & Holme 1997).
acity of up to 2000 m, exceptionally to 6000 m Another technique being developed is a re-
(see section 14.4) and may be inclined from tractable bit that can be raised, replaced, and
horizontal to vertical. The rate of advance lowered through the string of drill rods. At pre-
depends upon the type of drill rig, the bit, the sent when a worn bit requires replacement
hole diameter (usually the larger the hole the complete drill string has to be withdrawn,
diameter the slower the advance), the depth which is a time-consuming process (Jenner
of hole (the deeper the hole the slower the 1986, Morris 1986). However, modern rigs with
advance), the rock type being drilled (hard or automated rod handling can pull the string and
soft, friable, well jointed, etc.), and the skill of unscrew the rods at very high speed.
the driller. Drilling advance rates of up to 10 m an hour
The driller must decide on the best volume are possible but depend a great deal upon the
of drilling fluid to be flushed over the drill bit, skill of the driller and rock conditions. Costs
the right pressure to apply at its cutting surface vary from $US50 to 80 a meter for holes up to