STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

848 Drilling and Well Completions
In many air and gas drilling operations when casing or a liner is set, the
casing or liner is lowered into the dry borehole and once the bottom has been
reached, the casing or liner is landed (with little or no compression on the lower
part of the casing or liner string). After landing the casing or liner, the borehole
is then filled with water (with appropriate additives), and cement is pumped to
the annulus around the casing or liner and the water in the borehole is displaced
to the surface. The cement is followed by water and the cement is allowed to
set. After the cement has set, there is water inside the casing (or liner) that
must be removed before air (or gas) drilling can proceed.
The following procedure is recommended for unloading and drying a borehole
prior to drilling ahead on air [73]:
1. Run the drill string complete with desired bottomhole assembly and bit
to bottom.
2. Start the mud pump, running as slowly as possible, to pump fluid at a
rate of 1.5 to 2.0 bbl/min. This reduces fluid friction resistance pressures
to a minimum and pumps at minimum standpipe pressure for circulation.
The standpipe pressure (for 1.5 to 2.0 bbl/min) can be found from
standard fluid hydraulic calculations.
3. Bring one compressor and booster on line to aerate the fluid pumped
downhole; approximately 100 to 150 scfm/bbl of fluid should be sufficient
for aeration.
If the air volume used is too high, standpipe pressure will exceed the
pressure rating of the compressor (and/or booster). Therefore, the
compressor must be slowed down until air is mixed with the fluid going
downhole.
The mist pump should inject water at a rate of about 12 bbl/hr; the
foam injection pump should inject about 3 gal/hr of surfactant; this binds
fluid and air together for more efficient aeration.
As the fluid column in the annulus is aerated, standpipe pressure will
drop. Additional compressors (i.e., increased air volume) can then be
added to further lighten the fluid column and unload the hole.
Compared to the slug method of unloading the hole, the aeration
method is more efficient. The slug method consists of alternately pumping
first air (injected up to an arbitrary maximum pressure) and then
water (to reduce the pressure to an arbitrary minimum); this procedure
is repeated until air can be injected continuously. The aeration method
takes less time, causes no damage to pit walls from surges (as can happen
with alternate slugs of air and water) and can generally be done at lower
operating pressures.
5. After the hole has been unloaded, keep mist and foam injection pumps
in operation to clean the hole of sloughing formations, providing a mist
of 1.5 barrels of water per hour per inch of hole diameter and 0.5 to 4
gal of surfactant per hour, respectively.
6. At this point, begin air or mist drilling. Drill 20 to 100 ft to allow any
sloughing hole to be cleaned up.
7. Once the hole has been stabilized (Le., after sloughing), stop drilling and
blow the hole with air mist to eliminate cuttings. Continue this procedure
for 15 to 20 min or until the air mist is clean (i.e., shows a fine spray
and white color).
8. Replace the kelly and set the bit on bottom. Since the hole is now full
of air, surfactant and water will run to the bottom. Unless mixed with
air and pumped up the annulus (which cannot be done if the drill bit is