Page 119 - Petroleum Geology

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Fig. 5-2. Bailer operation.
parts of the world, any hole drilled in the ground will be found to contain
some water within about 10 m of the surface. If there is no ground water,
then water must be put into the hole for it cannot be drilled far without.
It was part of the traditional knowledge of cable tool drilling that if the
hole contained mud rather than water, drilling performance and bailing were
improved because of improved suspension of the cuttings in the mud. Often
the sedimentary rocks drilled contained sufficient clay to turn the ground
water in the hole into mud. If not, then clay could be added with advantage,
so increasing the viscosity.
The static water Eevel is always well known to cable tool drillers, and this
level is watched and noted. The new bit can be heard splashing into the water
when running in, and the water level can be felt on the sand line with the
bailer. If the hole is straight and vertical, daylight (or sunlight from a mirror)
is reflected back from the surface of the water. This is a standard check on
straightness and verticality. Any change in the static water level during drilling
is significant, for a sudden rise or fall indicates that an aquifer has been pene-
trated that has different hydraulic properties from those already drilled. If
normal bailing results in a lowering of the level in the borehole, then clearly
the rocks penetrated so far have little capacity to yield water. Rates up to
about 1 l/s (800 gallons/h) can be achieved with a bailer at shallow depth.
The drawdown (Fig. 5-3) is thus a measure of the permeability of the rocks
to water.
In hard rocks, the bit drills a hole only marginally larger than the bit, and
the margin can be so slight that a bit pulled undergauge may have been drill-