Page 26 - Air and gas Drilling Field Guide 3rd Edition

2.2 Circulation Systems 17

(or mud pump), through the standpipe on the mast, through the rotary hose,
through the swivel and down the inside of the kelly, down the inside of the drill
pipe and drill collars, and through the drill bit (at the bottom of the borehole) into
the annulus space between the outside of the drill string and the inside of the
borehole.
The drilling mud entrains the rock bit cuttings at the bottom of the annulus
and then flows with the cuttings up the annulus to the surface where the cuttings
are removed from the drilling mud by the shale shaker and the drilling mud is
returned to the mud tanks (where the slush pump suction side picks up the dril-
ling mud and recirculates the mud back into the well). The slush pumps used on
double (and triple) drilling rigs are positive displacement piston-type pumps.
For single drilling rigs, the drilling fluid is often treated fresh water in the mud
tank. A heavy-duty hose is run from the suction side of the onboard mud pump (see
Figure 2-4) to the mud tank. The drilling water is pumped from the tank, through
the pump, through an onboard pipe system, through the rotary hose, through the
hydraulic top-head drive, down the inside of the drill pipe, and through the drill bit
to the bottom of the well. The drilling water then entrains the rock cuttings from
the advance of the bit and carries the cuttings to the surface via the annulus
between the outside of the drill pipe and the inside of the borehole. At the surface,
the drilling fluid (water) from the annulus with entrained cuttings is returned to
a mud pit where the rock cuttings are allowed to settle out to the bottom.
The pumps on single drilling rigs are small positive displacement reciprocating
piston types.
Figure 2-7 shows a detailed schematic of a direct circulation compressed air
drilling system that would be used on a typical double or triple drilling rig.
Direct circulation requires that atmospheric air be compressed by the com-
pressor and then forced through the standpipe on the mast, through the rotary
hose, through the swivel and down the inside of the kelly, down the inside of
the drill pipe and drill collars, and through the drill bit (at the bottom of the bore-
hole) into the annulus space between the outside of the drill string and the inside
of the borehole. The compressed air entrains the rock bit cuttings and then flows
with the cuttings up the annulus to the surface where the compressed air and the
entrained cuttings exit the circulation system via the blooey line. The compressed
air and cuttings exit the blooey line into a large pit dug into the ground surface
(burn pit). These pits are lined with an impermeable plastic liner.
If compressed natural gas is to be used as a drilling fluid, a gas pipeline is run
from a main natural gas pipeline to the drilling rig. Often this line is fitted with a
booster compressor. This allows the pipeline natural gas pressure to be increased
(if higher pressure is needed) before the gas reaches the drilling rig standpipe.

2.2.2 Reverse Circulation
Rotary drilling reverse circulation (using either drilling mud and/or compressed
air or gas) can be a useful alternative to direct circulation methods. The reverse