Page 76 - Air and Gas Drilling Manual

3-4 Air and Gas Drilling Manual
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. The bottom of the swivel has a threaded box
connection down. The swivel is split into two sections, a rotating section on the
bottom and a non-rotating section on the top (see Figure 1-5). The non-rotating
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 is held 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.
3.1.2 Dual Wall Pipe Drill String
Intermediate and shallow depth large diameter wells can be drilled with direct
circulation techniques. But reverse circulation techniques are more efficient and are
the preferred techniques. The drilling industry has developed some very unique
downhole tools for reverse circulation air drilling operations. Figure 3-3 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 often use standard drill string like that shown in
Figure 3-1. In the past two decades there has been a dramatic increase in the use of
air drilling reverse circulation techniques for drilling water wells, monitoring wells,
geotechnical boreholes, and other shallow (i.e., less than 3,000 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 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 3-3 is shown rotating a tri-cone drill bit. The top
drive system rotates the entire drill string. The tri-cone drill bits used in reverse
circulation operations have the same cutting structures as tri-cone bits used in direct
circulation operations. However, the reverse circulation bits are fabricated to allow
the compressed air to flow from the annulus between the two walls of the dual wall
pipe to the bit rock cutting face. At the bottom of the well the air flow entrains the
rock cuttings and flows to the surface through a large center orifice in the drill bit
that leads to the inside of the inner pipe of the dual wall pipe. The drill bit used in
a dual wall pipe reverse circulation operation is selected to have a diameter that is
slightly larger than the outside diameter of the dual wall pipe. Thus, the outside