Page 46 - Air and Gas Drilling Manual

Chapter 1: Introduction 1-23
3
the annulus (via the blooey line) with a specific weight of 0.0763 lb/ft . The surface
atmosphere for this example is assumed to be API Mechanical Equipment Standards
standard atmospheric conditions (dry air, pressure of 14.696 psia and a temperature
of 60˚F) [9]. This figure shows a typical friction resistance dominated drill string
flow (as opposed to hydrostatic column weight dominated). This type of flow has a
drill string injection pressure at the top that is higher than the pressure above the
drill bit at the bottom. Friction dominated flow results when the drill bit is run
with no nozzles.
Figure 1-22 is the concluding plot of these example calculations. This shows
the side-by-side comparison of the annulus velocities of the drilling mud and the
compressed air as they flow to the surface. It is the power of these return flows up
the annulus that keeps the rock cuttings entrained and moving to the surface at a rate
that allows the drill bit to be safely advanced.
The drilling mud flows in the annulus around the drill collars with an average
velocity of about 7.6 ft/sec. The drilling mud slows to an average velocity of about
3.0 ft/sec in the annulus around the drill pipe.
For the air drilling case, the compressed air flows in the annulus with an average
velocity of about 30 ft/sec around drill collars. The velocity increases up the
annulus to about 125 ft/sec at the exit to the annulus.
It is instructive to compare the power (per unit volume) of example flows at the
positions in the annulus where the power is likely the lowest. For both of these
examples the lowest power is just above the drill collars in the annulus around the
bottom of the drill pipe. The kinetic energy per unit volume, KE, is [1, 10]
KE = 1 ρ V 2 (1-1)
2

3
where KE is the kinetic energy per unit volume (ft-lb/ft ),
4
2
ρ is the specific weight of the fluid (lb-sec /ft ),
V is the average velocity of the fluid (ft/sec).
The density of the fluid, ρ , is
γ
ρ = (1-2)
g
3
where γ is the specific weight of the fluid (lb/ft ),
2
g is the acceleration of gravity (32.2 ft/sec ).
For the mud drilling example the specific weight of the drilling mud in the
3
annulus just above the drill collars is 75 lb/ft . Using these values in Equation 1-2
the density of the drilling mud is
.
ρ = 75 0
m
32 2
.
ρ = 233 lb − sec 2
.
m
ft 4