Page 401 - Air and Gas Drilling Manual

Chapter 9: Aerated Fluid Drilling 9-5
the jet sub is placed in the drill string in the drill pipe section above the drill collars.
Usually the jet sub is placed several drill pipe joints above the drill pipe to drill
collar transition. There are usually two to three jet nozzles in the jet sub. The jet
sub orifices can be sized to allow the compressed gas to be vented to the annulus
before the gas aerated fluid can flow to the drill collars. The orifices in the jet subs
can be sized using Equations 6-35 to 6-39.
When the aerated drill string injection technique is used to drill through loss of
circulation zones, a constant volumetric flow rate of incompressible drilling fluid is
circulated through the top of the drill string. The actual volumetric flow rate of the
incompressible drilling fluid is the flow rate that will adequately clean and carry the
rock cuttings from the bottom of the annulus. The volumetric flow rate of injected
compressible gas is determined by trial and error in the field. The flow rate of gas is
increased until the desired reduction or elimination of drilling fluid loss is obtained.
Underbalanced drilling operations require that a constant bottomhole pressure be
maintained as the drill bit is advanced and the well is deepened. To accomplish this
a constant volumetric flow rate of incompressible drilling fluid is injected into the
inside of the drill string. The actual volumetric flow rate of the incompressible
drilling fluid is the flow rate that will adequately clean and carry the rock cuttings
from the bottom of the annulus. In order to maintain a nearly constant bottomhole
annulus pressure, the volumetric flow rate of injected compressed gas must be
increased as the well is deepened.

Figure 9-2: Schematic of jet sub drill pipe injection aerated flow.
9.2.2 Annulus Injection
Figure 9-3 shows a schematic of the annulus injection aerated drilling
configuration (can be accomplished as parasite tubing string drilling, parasite casing
drilling, or through completion drilling) [4]. In this configuration the