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DRILLING
METHODS
To prevent caving of the hole, the fluid level
must be kept at ground level at all times, even
when drilling is suspended temporarily, to
prevent a loss of hydrostatic pressure in the
borehole. The hydrostatic pressure of the
water column plus the velocity head of the
downward moving water outside the drill pipe
are what support the borehole wall.
Erosion of the wall is usually not a problem
because velocity in the annular space is low.
A considerable quantity of make up water is
usually required and must be immediately
available at all times when drilling in permeable
sand and gravel. Under these conditions,
water loss can increase suddenly, and if this
causes the fluid level in the hole to drop
significantly below the ground surface, caving
usually results. Water loss can be addressed
by the addition of clay additives, but this action
is only taken as a last resort.
Often to aid the upward movement of water
through the drill string, air is injected, lifting the
contents to the surface. Another reason to use
air is the fact that the suction pump lift is
limited in its capacity to create enough vacuum
to start up the water movement after a rod
change. When air lifting is used to assist in
reverse mud drilling this method becomes
similar to the reverse air rotary drilling method
discussed below.
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