34
|
World Coal
|
June 2016
C
oal exploration often presents
significant drilling challenges
due to the variety of ground
conditions encountered. By
virtue of its formation process, coal deposits
are found in sedimentary rock, which also
typically contains swelling clays and shales,
porous sandstones, broken zones,
conglomerates, gravel and bands of quartz.
All of these pose their own unique challenges
for core drilling and diamond bits.
Clay minerals all have a strong attraction
for water; they swell easily and may double
in volume when wet. Similarly, shale is a
general name for clay-rich sedimentary
rocks. Clay minerals account for about
50 – 60% by weight of most shales, causing
them to swell in a fashion similar to clay
when exposed to water. More importantly,
shales are the most abundant type of
sedimentary rocks constituting over half of
the rock in a stratigraphic column. If not
controlled through mud programmes and
oversize bits, swelling often leads to stuck
rods. Diamond bit performance is impacted
as well: the cuttings swell and tend to
accumulate on the face of the bit, known as
‘balling up’. Larger diamonds or cutters are
better able to maintain penetration rates
under these conditions, explaining the
continued use of surface sets instead of
impregnated bits. That is, until SSUMX™
series bits became available.
Porous sandstones, friable
conglomerates, gravel and broken zones
offer a different set of challenges from clays
and shales. First, they often lead to loss of
fluid circulation. The lack of water increases
rod friction and vibration, as well as
reducing cooling flow over the bit and
leaving cuttings at the bit face. Second, the
broken nature of the rock makes it much
more difficult to cut. Imagine trying to cut a
tomato without holding it in place. It is
much the same for diamond bits. To be
really effective, the rock needs to be
stationary as the diamond cuts it. If it
moves, it becomes more of a crushing
process, causing a lot more wear on the bit.
These conditions would favour tough
Chris Lambert,
Boart Longyear, USA,
explains how to increase
the productivity of diamond
drilling in coal mining.
KNOW THE
N H
