STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Air and Gas Drilling 847
Drill Pipe Wear. Erosion can occur between the hard band and the tool joint
metal when the box end is hardbanded. This erosion is due to the high-velocity
flow of cuttings in the annulus section of the borehole.
Bits. Bits that offer the best gage protection should be selected for air (or gas)
drilling operations. Gage reduction always occurs in air (or gas) drilled boreholes,
particularly near the end of a bit run. Returning to an air-drilled borehole
with the same gage bit is dangerous unless care is taken in returning to the
bottom. It is nearly always necessary to ream the last third of the borehole length
(Le., the last bit run) to get to the bottom. Most manufacturers of bits make
tricone bits that are specially designed for air (or gas) drilling operations. These
bits have the same cutting structures as the mud bits. The differences between
air bits and mud bits are in the design of the internal passages for airflow and
in the cooling of the internal bearings of the bits. It is common practice in air
and gas drilling operations to operate the bits with no nozzle plates in the
orifice openings.
Air Hammer. This is a special downhole drilling tool for controlling severe
deviation problems and for drilling very hard formations. The air hammer is
an air percussion hammer system that operates from compressed air and the
rotary motion of the drill string.
Air (or Gas) Downhole Motors. Some positive displacement mud motors can
be operated on unstable foam. In general, these mud motors must be low-torque,
high-rotational-speed motors. Such motors have found limited use in air and gas
drilling operations where directional boreholes are required. Recently a downhole
turbine motor has been developed specifically for air and gas drilling
operations. This downhole pneumatic turbine motor is a high-torque, lowrotational-speed
motor.
Well Completion
In general, well completion procedures for an air- (or gas-) drilled borehole
are nearly the same as those for a mud-drilled borehole.
For mud drilling operations, the depth at which a casing is to be set is usually
dictated by the pore-pressure and fracture-pressure gradients. In air (or gas)
drilling operations, a casing is set to the depth at which significant formation
water will occur. If at all possible, casing should be set just after a significant
water zone has been penetrated so that the air drilling difficulties encountered
with all water influx to the annulus, can be minimized by sealing off the water
zones promptly after drilling through the zone. Thus, stand-by air compressors
and expensive foaming additives are used only for a short time.
In most air and gas drilling operations, open-hole well completions are
common. This type of completion is consistent with low pore pressure and the
desire to avoid formation damage. It is often used for gas wells where nitrogen
foam fracturing stimulation is necessary to provide production. In oil wells
drilled with natural gas as the drilling fluid, the well is often an open hole
completed with a screen set on a liner hanger to control sand influx to the well.
Liners are used a great deal in completion of wells drilled with air (or gas)
drilling techniques. The low pore-pressure subsurface limitations necessary to
allow air (or gas) drilling give rise to minimum casing design requirements.
Thus, liners can be used nearly throughout the casing program.