Page 224 - Air and gas Drilling Field Guide 3rd Edition

9.2 Aerated Fluids Drilling Operations 215

operation. This is usually the minimum incompressible volumetric flow rate
that would clean the rock cuttings from the bottom of the well and transport
the cuttings to the surface. In most aerated drilling operations, the incom-
pressible fluid volumetric flow rate is held constant as drilling progresses
through the open hole interval.
7. Using the incompressible fluid and gas volumetric flow rates to be injected
into the well, determine the bottom hole annulus pressures and the surface
injection pressures as a function of open hole interval drilling depth.
8. Select the contractor compressor(s) that will provide the drilling operation
with the appropriate air or gas volumetric flow rate needed to aerate the
drilling fluid properly.
Chapter 6 derived and summarized the basic direct circulation drilling
planning governing equations. The equations in that chapter are utilized in the
discussions and illustrative examples that follow.

9.2 AERATED FLUIDS DRILLING OPERATIONS
Several drill string and well configurations are used for aerated fluid drilling opera-
tions. These are divided into two general technique classes of air (or gas) injection
operations: drill pipe injection and annulus injection [4].

9.2.1 Drill Pipe Injection
Figure 9-1 shows a schematic of the drill pipe injection-aerated drilling configura-
tion. In this configuration, incompressible fluids and compressible air (or other
gas) are injected together into the top of the drill string (at P in ). These two fluid
streams mix as they flow down the inside of the drill string and pass through the
drill bit nozzles. When the mixture of these fluids flows into the bottom of the
annulus, the rock cuttings (from the advance of the drill bit) are entrained and
the resulting mixture flows to the surface in the annulus. The mixture exits the
annulus (at P e ) into a horizontal flow line. This horizontal flow line flows to either
conventional open mud tanks or sealed return tanks. Conventional open mud
tanks are used when the returning air is not mixed with contaminated fluids or
gases or mixed with produced hydrocarbons. Separators are used when hydrocar-
bon fluids are expected in the return flow from the well.
Figure 9-2 shows a schematic of the jet sub drill string injection drilling config-
uration. An alternative to flowing the mixture down the entire length of the inside
of the drill string to the drill bit is to place a jet sub above the drill collars to allow
most of the compressed air (or other gas) to pass from the inside of the drill string
into the annulus before the two-phase mixture flows through the inside of the drill
collars. The jet sub drill string injection technique is usually used for deep aerated
drilling operations where the bottom section of the well is usually drilled with