Page 399 - Air and Gas Drilling Manual

Chapter 9: Aerated Fluid Drilling 9-3
temperature gradient.
5. Establish the objective of the aerated drilling fluids operation:
• To drill through loss of circulation formations,
• To counter formation water entering the annulus,
• To maintain low bottom hole pressures to either preclude fracturing of
the rock formations, or to allow underbalanced drilling operations.
6. Determine whether direct or reverse circulation techniques will be used to
drill well.
7. If underbalanced drilling is the objective, determine the bottomhole
pressure limit that must be maintained in order to avoid formation damage.
8. For either of the above objectives, determine the required approximate
volumetric flow rate of incompressible fluid to be used in the aerated fluid
drilling operation. This is usually the minimum volumetric flow rate
required to clean the rock cuttings from the bottom of the well and transport
the cuttings to the surface. In most aerated drilling operations, the
incompressible fluid volumetric flow rate is held constant as drilling
progresses through the openhole interval.
9. Determine the approximate volumetric flow rate of air (or other gas) to be
injected with the flow of incompressible fluid into the top of the drill string
(or into the annulus) as a function of drilling depth in the openhole interval.
10. Using the incompressible fluid and air volumetric flow rates to be injected
into the well, determine the bottomhole pressure and the surface injection
pressure as a function of drilling depth (over the openhole interval to be
drilled).
11. Select the contractor compressor(s) that will provide the drilling operation
with the appropriate air or gas volumetric flow rate needed to properly aerate
the drilling fluid. Also, determine the maximum power required by the
compressor(s) and the available maximum derated power from the prime
mover(s).
12. Determine the approximate volume of fuel required by the compressor(s)
to drill the well.
In Chapter 6 the basic direct circulation drilling planning governing equations
have been derived and summarized. The equations in this chapter will be utilized in
the discussions and illustrative examples that follow. Also, in Chapter 7 reverse
circulation drilling planning governing equations have been derived and summarized.

9.2 Aerated Fluids Drilling Operations
There are several drill string and well configurations used for aerated fluid
drilling operations. 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
configuration. In this configuration both incompressible fluid 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 go 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