Page 77 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-5
annulus between the inside of the borehole and the outside of the dual wall pipe is
kept quite small. This outer micro annulus together with a skirt structure on the
drill bit itself (see Figure 1-9) restrict the return flow to the surface of air and
entrained rock cuttings to the inside of the inner pipe [3].

Figure 3-3: Dual wall pipe drill string for reverse circulation operations (courtesy of
Foremost Industries Incorporated).

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 threaded box
connection up. The bottom threaded pin connection (of the bottom 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 a special side inlet sub (shown in Figure 3-4). This sub allows
compressed air to be injected into the dual wall pipe annulus through the non-
rotating 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