Page 228 - Air and gas Drilling Field Guide 3rd Edition

9.3 Minimum Volumetric Flow Rates 219

difficult to maintain constant bottom hole annulus pressure throughout the
entire drilling operations.
n Because the injected gas is trapped under pressure inside the drill string by
the various string floats, time must be allowed for the pressure bleed down
when making connections and trips. Here again, the bleed down makes it
difficult to maintain a constant bottom hole pressure.
n The flow down the inside of the drill string is two-phase flow; therefore,
higher pipe wall friction losses are present. The high friction losses result
in high pump and compressor pressures during injection. This is somewhat
relieved when a jet sub is used.
n The gas phase in the aerated flow attenuates the pulses of conventional
measure-while-drilling (MWD) systems. Therefore, conventional mud pulse
telemetry MWD cannot be used.

The Annulus Injection Technique
Advantages to the annulus injection technique are as follow:
n Aeration of the incompressible drilling fluid in the annulus above the gas
annulus entrance position can continue during connections and trips.
n Flow down the inside of the drill string is single phase; therefore, conventional
mud pulse MWD can be used in aerated directional drilling operations.
n The compressor pressure used to maintain gas injection will usually be low
when compared to the pressure required for direct circulation (to the drill
bit) aerated drilling operations.

Disadvantages to the annulus injection technique are as follow:
n Because the parasite tubing string or the temporary casing is placed at a par-
ticular fixed location in the well, the aeration technique is more inflexible
than the drill pipe injection techniques.
n Initiating (kickoff) gas flow to the annulus requires very high compressor
pressures.
n Because the gas is injected into the annulus at fixed depths that are well above
the bottom of the annulus, higher gas volumetric flow rates are required to
maintain constant bottom hole pressures than in the drill pipe injection
technique.

9.3 MINIMUM VOLUMETRIC FLOW RATES
Most aerated drilling operations are planned with a constant volumetric flow rate
of incompressible drilling fluid and only the volumetric flow rate of the com-
pressed gas is allowed to vary. The volumetric flow rate of gas is usually increased
as the depth is increased in order to maintain the same aerated fluid properties in
the annulus column. The drill pipe injection technique requires that both the