Page 221 - Air and Gas Drilling Manual

.
0 601
Q
.
= 4 552 ft /sec
Q ai = 0 132 3 Chapter 5: Shallow Well Drilling Applications 5-63
.
ai
The velocity of the gas above the drill bit inside the drill pipe, V ai, is
.
V = 4 552
π
  2
ai
0 389)
  (.
4
V ai = 38 3 ft/sec
.
The kinetic energy per unit volume of the gas above the drill bit inside the drill
pipe, KE ai, is (see Chapter 1 and Equations 1-1 and 1-2)
.
ρ = 0 132
ai
32 2
.
ρ = 0 00410 lb − sec 2
.
ai
ft 4
KE = 1 . 2
) 38 3 . )
ai (0 00410 (
2
ft − lb
KE ai = 3 007 3
.
ft
The trial and error process for the above illustrative example requires an iterative
selection of the value of q g until the kinetic energy above the drill bit inside the drill
3
pipe is equal to 3.0 ft-lb/ft . In this illustrative example the value of q g that will
3
give a kinetic energy value equal to 3.0 ft-lb/ft is 472.2 scfm.
This illustrative example shows the great advantage of reverse circulation
techniques over direct circulation techniques in the drilling of shallow large diameter
boreholes. The minimum volumetric flow rate of air required to drill this example
borehole with the direct circulation technique is 2,232 scfm. This is a factor of five
greater volumetric flow rate.
Using the calculation procedure as given above, plots can be prepared for a
variety of drilling depths and drilling rates (for API standard conditions). Figures 5-
16, 5-17 and 5-18 can be used to obtain the approximate minimum volumetric flow
rate for a variety of reverse circulation well geometric configurations and drilling
rates (i.e., solids weight rates of flow). Figure 5-16 is limited solids weight rates of
flow of 1.0 lb/sec or less. Figure 5-17 is limited solids weight rates of flow
between 1.0 lb/sec and 2.0 lb/sec. Figure 5-18 is limited solids weight rates of flow
of 2.0 lb/sec to 3.0 lb/sec. All these figures give the air minimum volumetric flow