Page 229 - Air and gas Drilling Field Guide 3rd Edition

220 CHAPTER 9 Aerated Fluids Drilling

incompressible drilling fluid injection and the compressible gas injection be sus-
pended when connections and trips are made. Similarly, the annulus injection
technique requires that the incompressible drilling fluid injection be suspended
when connections and trips are made. Further, the cleaning, lifting, and suspen-
sion capabilities of the incompressible drilling mud is generally independent of
the depth of drilling. Conversely, the cleaning and lifting capabilities of com-
pressed gas are dependent of the depth of drilling. Also, it must be noted that
compressed gas drilling fluids have little or no suspension capabilities. Therefore,
when designing an aerated drilling fluid, the injected compressed gas should not
be assumed to contribute to bottom hole cleaning, lifting, and suspension of rock
cuttings in the annulus. The additional cleaning and lifting properties of the com-
pressed gas to the aerated drilling fluid should be considered bonuses. This argu-
ment requires that the incompressible drilling fluid properties and circulation
characteristics be designed to provide the aerated drilling operations with
stand-alone cleaning, lifting, and suspension capabilities of the rock cuttings in
the annulus.

9.3.1 Discussion of Theories
A variety of minimum volumetric flow rate theories can be used to design incom-
pressible drilling fluid properties and circulation characteristics for direct circula-
tion drilling operations [10–12]. In order to formulate a workable simple
procedure for determining the minimum volumetric flow rates of incompressible
fluid and ultimately the gas, this chapter will confine its attention to performance
drilling operations.
The average annulus velocity of the fluid, V f , in the largest annulus cross section
will be the sum of the critical concentration velocity, V c , and the terminal velocity,
V t , of the average size rock cutting particle in the incompressible drilling fluid.
Thus, the average fluid velocity in the annulus is
V f ¼ V c þ V t ; (9-1)
where V f is the incompressible drilling fluid (ft/sec, m/sec), V c is the critical con-
centration velocity (ft/sec, m/sec), and V t is the terminal velocity of the rock cut-
tings particle (ft/sec, m/sec).
The critical concentration velocity is the additional velocity needed to distrib-
ute the rock cuttings through the incompressible drilling fluid at a predetermined
concentration factor. The usual concentration factor is 0.04 or lower [13]. There-
fore, the critical concentration velocity V c is
k
V c ¼ ; (9-2)
3; 600 C
where k is the instantaneous drilling rate (ft/hr, m/hr) and C is the concentration
factor (usually assumed to be 0.04).
Equations (9-1) and (9-2) can be used with any consistent set of units.