Page 244 - Air and gas Drilling Field Guide 3rd Edition

9.4 Nonfriction and Friction Illustrative Examples 235

inside of the drill string while drilling at a depth of 10,000 ft (3048 m). This is to
be done while maintaining a bottom hole annulus pressure of 3360 psig (2317 N/
2
cm gauge).
The detailed Mathcad solution for this example is given in Appendix D.
Figure 9-6 shows multiphase flow pressures throughout the circulation system
in the project well while drilling at 10,000 ft (3048 m). In Figure 9-6, the bottom
2
hole annulus pressure is 3373 psia (2326 N/cm abs) in the annulus and the
2
injection pressure at the standpipe is 637 psia (439 N/cm abs).
The solution requires a trial and error selection of the volumetric flow rate of
gas injected into the top of the drill string. This gas injection rate is approximately
1225 scfm (596 standard liters/sec). This injection rate, together with the incom-
pressible volumetric flow rate of 191 gpm (723 lpm), is required to maintain the
2
bottom hole annulus pressure of 3360 psig (3214 N/cm gauge) at a drilling
depth of 10,000 ft (3048 m). The relatively high injection pressure (for both
the liquid and the gas) reflects the homogeneous hydrostatic weight of the fluid
column inside the drill string above the bit nozzle chokes. This homogeneity of
the multiphase flows in the annulus and inside the drill string are also reflected
by the nearly equal slopes of the annulus and inside the drill string pressure plots
shown in Figure 9-6.

Pressure (psia)
0 1000 2000 3000 4000
0 0
Exit Injection
–500
–2000

Inside drill string –1000
–4000
Depth (ft) –1500 Depth (m)

–6000
–2000
Annulus
Bit
–8000 –2500

–10000 –3000
0 500 1000 1500 2000 2500 3000
Pressure (N/cm 2 abs)

FIGURE 9-6. Illustrative Example 9.3 pressure versus depth.