Page 380 - Air and Gas Drilling Manual

bottomhole pressure in the annulus, and the weight rate of flow of air being injected
into the top of the drill string. Thus, the flow rate of injected water, q iw, is
determined from [15]
 p Chapter 8: Air, Gas, and Unstable Foam Drilling 8-63


q iw =  sat   3 600,  w ˙ g (8-6)
 p bh − p sat   833.
where q iw is the volumetric flow rate of injected water (gal/hr or bbl/hr).
Illustrative Examples 8.5 describes the implementation of the basic planning
step No. 11 given in Section 8.1.
Illustrative Example 8.5 Determine the volumetric flow rate of injected water
needed to saturate the volumetric flow rate of air at bottomhole conditions in the
Illustrative Example 8.3 series (drilling at a depth of 10,000 ft). Assume the
injected water has a specific gravity of 1.0.
In Illustrative Example 8.3a compressors were selected for drilling the 10,000 ft
deep borehole. The total volumetric flow rate of air delivered by these compressors
was 2,400 acfm. In Illustrative Example 8.3b it was found that while drilling at
10,000 ft the bottomhole annulus pressure was 204.5 psia and the bottomhole
annulus temperature was 604.41 R (or 144.74˚F). In Illustrative Example 8.3b the
weight rate of flow of air through the circulation system for the example was 2.635
lb/sec.
Using the bottomhole temperature given above, Equation 8-5 becomes
  1 750 286  
,
.
6 39416 −   
.
  217 23 + 0 555 144 74)   
(
.
.
.
p sat = 10 
.
p sat = 3 251 psia
Using the bottomhole pressure, the weight rate of flow and the above saturation
pressure, Equation 8-6 becomes
 3 251  , 
.
2 635)
q iw =    3 600  (.
.
 204 5 . − 3 251   833
.
.
q = 18 5 gal/hr
iw
The above water injection volumetric rate is slightly less than half a barrel per hour.
The illustrative example above demonstrates that very little injected water is
needed to saturate an air flow at bottomhole conditions. This is also the case for the
other two important gases used for drilling operations, natural gas and inert air. In
essence, it requires a small volumetric flow rate of injected water to create a more