Page 71 - Air and gas Drilling Field Guide 3rd Edition

62 CHAPTER 4 Downhole Equipment

surfaces of the kelly, the left-hand thread at the top of the kelly is tightened by
the inertial drag of the nonrotating components above the kelly. All of the com-
ponents above the kelly are left-hand thread connections. Above the kelly is a
kelly cock sub (optional). The kelly cock is a special valve that allows sealing
off of the inside of the drill string in a blowout event during oil or natural gas dril-
ling operations. The kelly cock sub has a threaded pin connection down and a
threaded box connection up. Above the kelly cock sub is a swivel sub. The
swivel sub protects the swivel and has a threaded pin connection down and a
threaded pin up. Above the swivel sub is the swivel itself. The bottom of the
swivel has a threaded box connection pointed down. The swivel is split into
two sections: a rotating section on the bottom and a nonrotating section on
the top. The nonrotating section of the swivel is held in the mast by the traveling
block and hoisting system. A sealed bearing allows the bottom section of the
swivel to rotate while the top section can be held in position by the traveling
block. The swivel allows the circulation fluid (drilling mud or compressed air
or natural gas) to flow through the swivel to the rotating drill string.
For direct circulation, the circulation fluid flows down the inside of the drill
string to the drill bit, flows through the drill bit orifices (or nozzles), entrains
the rock cuttings from the drill bit, and flows up the annulus between the outside
surface of the drill string and the inside surface of the borehole.

4.1.2 Dual Wall Pipe Drill String
Conductor and surface casing wellbores are drilled to shallow depth with large
diameter boreholes. These boreholes can be drilled with direct circulation techni-
ques. However, reverse circulation techniques are often more efficient. The dril-
ling industry has developed some very unique downhole tools for reverse
circulation air drilling operations. Figure 4-3 shows a schematic configuration of
a rotary reverse circulation operation using dual wall drill pipe.
Reverse circulation techniques are not restricted to air drilling operations.
Reverse circulation techniques can also use standard drill string like that shown
in Figure 4-1. In the past two decades there has been a dramatic increase in the
use of air drilling reverse circulation techniques for drilling water of deep wells,
monitoring wells, geotechnical boreholes, and other shallow (i.e., less than
3000 ft) wells. The increased use of reverse circulation techniques has been
encouraged by the development of new technologies. One of these innovations
is the development of dual wall drill pipe.
Rotary dual wall pipe reverse circulation operations must be used on drilling
rigs equipped with hydraulic rotary top drive systems (for single drilling rigs) or
with hydraulic or electric power swivel systems (for double and triple drilling
rigs) to rotate the drill string. Dual wall pipe is quite rigid and has a much higher
weight per unit length than standard single wall drill pipe. Thus, dual wall pipe
can be used like drill collars (the lower portion of the drill string can be placed
in compression). The dual wall drill string in Figure 4-3 is shown rotating a