Page 404 - Air and Gas Drilling Manual

9-8 Air and Gas Drilling Manual
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 compressed
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
incompressible drilling fluid injection and the compressible gas injection be
suspended 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 suspension
capabilities of the incompressible drilling mud is in general independent of the depth
of drilling. Conversely, the cleaning and lifting capabilities of compressed 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 bottomhole cleaning, lifting, and suspension of rock cuttings in the
annulus. The additional cleaning and lifting properties of the compressed gas to the
aerated drilling fluid should be considered as a bonus. This argument 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
There are a variety of minimum volumetric flow rate theories which can be used
to design incompressible drilling fluid properties and circulation characteristics for
direct circulation drilling operations [10 to 12]. In order to formulate a workable
procedure for determining the minimum volumetric flow rates of incompressible
drilling fluid and compressed gas to a borehole, the primary operational reasons for
using aerated fluid drilling should be reviewed. These reasons are; 1) to control loss
of circulation drilling situations, and 2) to facilitate underbalanced drilling
operations. Although aerated fluid drilling will generally improve some important
drilling parameters, for example penetration rate and openhole wall stability, it is
usually not possible to optimize all these parameters and still successfully
accomplish the primary mission of either loss of circulation control, or
underbalanced drilling.
For aerated drilling applications the minimum volumetric flow rate will be
determined using a rather simple, straight-forward procedure which requires that the
circulating incompressible fluid be capable, on its own, of maintaining a minimum
concentration of rock cuttings in the largest annulus section of the well [13]. This
requires that the average velocity of the fluid, V f, in the largest annulus section 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, V f, is

V V V (9-1)
f c t
where V f is the incompressible drilling fluid (ft/sec),