Page 73 - Air and gas Drilling Field Guide 3rd Edition

64 CHAPTER 4 Downhole Equipment

The drill string for a dual wall pipe reverse circulation air drilling operations is
rather simple. These specially fabricated reverse circulation drill bits have a
threaded box connection. The bit sub has a threaded pin connection down and
a threaded box connection up. The bottom-threaded pin connection (of the bot-
tom dual wall pipe joint in the drill string) is made up to the top connection of
the bit sub. Additional dual wall pipe are made up to each other, and the top-
threaded box connection of the top pipe is made up to a special side inlet sub
(shown in Figure 4-4). This sub allows compressed air to be injected into the dual
wall pipe annulus through the nonrotating outer section of the sub. The return
flow of air and entrained rock cuttings from the inside of the inside pipe flows
up through the rotating inner section of the sub to and through the rotary top
drive.
The side inlet sub acts as the “first swivel” for the dual wall pipe drill string.
This first swivel accommodates the injection of air into the circulation system
while allowing the air and entrained rock cuttings to pass through. The “second
swivel,” which is the rotary top drive, is above the side inlet sub. This second
swivel provides torque and the rotation of the drill string while allowing the air
and entrained rock cuttings to return to the surface.

FIGURE 4-4. Side inlet sub for dual wall pipe reverse circulation operations (courtesy of Holte
Manufacturing Company).