Page 27 - Air and gas Drilling Field Guide 3rd Edition

18 CHAPTER 2 Air and Gas Versus Mud

ROTARY HOSE
STANDPIPE

SWIVEL
COMPRESSOR
ROTATING
HEAD

KELLY

BLOOEY
LINE

DRILL PIPE

DRILL COLLAR

BIT

FIGURE 2-7. Direct circulation air system.

circulation technique is particularly useful for drilling relatively shallow large
diameter boreholes (e.g., conductor and surface casing boreholes).
In a typical reverse circulation operation utilizing drilling mud, the drilling mud
flows from the mud pump to the top of the annulus between the outside of the drill
string and the inside the borehole, down the annulus space to the bottom of the
borehole. At the bottom of the borehole the drilling mud entrains the rock bit cut-
tings and flows through the large center opening in the drill bit and then upward
to the surface through the inside of the drill string. At the surface, the cuttings are
removed from the drilling mud by the shale shaker and the drilling mud is returned
to the mud tanks (where the pump suction side picks up the drilling mud and
recirculates the mud back to the well).
Reverse circulation can also be carried out using air and gas drilling tech-
niques. Figure 2-8 shows a typical application of reverse circulation using com-
pressed air as the drilling fluid (or mist, unstable foam) [7]. This example is a