STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

846 Drilling and Well Completions
equipment to air drilling operations. The rotating head packs off the annulus
return flow from the rig floor (i.e., seals against the rotating kelly) and diverts
the upward flowing air (or gas) and cuttings to the blooey line. Little pressure
(a few psig) exists in the annulus flow at the rotating head.
Kelly. Because of its greater seal effectiveness within the rotating head, a hexagonal
rather than a square kelly should be used in air (or gas) drilling operations.
Scrubber. This removes excess water from the injected air (or gas) stream to
ensure that a minimum of moisture is circulated (if dry air for drilling is
required) and to protect the booster.
Sample Catchers. A small-diameter pipe (about 2 in.) is fixed to the bottom
of the blooey line to facilitate the catching and retaining of downhole cutting
samples for geologic examination.
De-Duster. This provides a spray of water at the end of the blooey line to wet
down the dust particles exiting the blooey line.
Gas Sniffer. This instrument can be hooked into the blooey line to detect very
small amounts of natural gas entering the return flow from the annulus.
Pilot Light. This is a small continuously operated flame at the end of the blooey
line, and it will ignite any natural gas encountered while drilling.
Burn Pit. This pit is at the end of the blooey line and provides a location for
the cutting returns, foam and for natural gas or oil products from the subsurface
to be ignited by the pilot light and burned off. The burn pit should be located
away from the standard mud drilling reserve pit.
Meter for Measurlng Air (or Gas) Volume. A standard orifice meter is generally
used to measure air (or gas) injection volume rates.
Downhole Equipment
Special pieces of downhole equipment and special concerns must be considered
during downhole air (or gas) drilling operations.
Float-valve Subs. These subs are at the bottom and near the top of the drill
string. The bottom float-valve sub prevents the backflow of cuttings into the
drill string during connections or other air (or gas) flow shutdowns that would
otherwise plug the bit. The bottom float-valve sub also aids in preventing
extensive damage to the drill string in the event of a downhole fire. The top
(or upper) float-valve subs aid in retaining high pressure air (or gas) within a
long drill string while making connections or other shutdowns.
Bottomhole Assemblies. In general, the drill pipe, drill collars and, in
particular, bottomhole assemblies for air (or gas) drilling operations are the same
as those in mud drilling. However, because the penetration rate is much greater
in air (or gas) drilling operations due to the lack of confining pressure on the
bit cutting surface, care must be taken to control unwanted deviation of the
borehole. Thus, for air (or gas) drilled boreholes, a packed-hole or stiff bottomhole
assembly is recommended.