Page 381 - Air and Gas Drilling Manual

8-64 Air and Gas Drilling Manual
effective formation water carrying circulation gas. Nearly all injected water is
injected with additives (see Table 8-2).
After water has been injected into the circulation gas, it is necessary to
determine how much formation water can be carried from the openhole section of a
borehole. Since the injected water saturates the circulation gas, it can be assumed
that the formation water will be carried from the well as droplets. Since the gas
volumetric flow rate to the well is known and is greater than the minimum
volumetric flow rate needed to clean the well (carry the anticipated drilling rate of
rock cuttings), then the extra volumetric flow rate of gas (beyond the minimum) will
allow a much greater drilling rate. This additional drilling rate that the gas
volumetric flow rate can support can also be droplets of formation water. Thus, the
additional drilling rate can be converted to a weight rate of flow or a volumetric flow
rate of formation water that can be carried from the well.
Illustrative Examples 8.6 describes the implementation of the basic planning
step No. 12 given in Section 8.1.
Illustrative Example 8.6 Determine the influx of formation water into the
annulus that can be carried from the example well in the Illustrative Example 8.3
series (drilling at a depth of 10,000 ft). Assume a formation water specific gravity
of 1.07.
The Illustrative Example 8.3b the 10,000 ft deep well was drilled with an air
circulation volumetric flow rate of 2,400 acfm. In Illustrative Example 8.2 the
minimum volumetric flow rate required to clean the example well was found to be
1,840 acfm. Using a actual volumetric flow rate of 2,400 acfm (from the
compressors) with either the problem setups in Illustrative Examples 8.1 and 8.2, or
the more detailed problem set-up in Illustrative Example 8.3b, trial and error
calculations can be made to determine the drilling rate value κ that can be supported
by this volumetric flow rate. Using the more detailed problem setup in Illustrative
Example 8.3b the minimum kinetic energy position in the annulus of the well is
found to be at the bottom of the drill pipe body inside the cased section of the well
(i.e., at the bottom of H 1). Using the minimum kinetic energy criteria of 3.0 ft-lb/ft 3
at the bottom of H 1, the calculated maximum drilling rate that can be supported by
the volumetric flow rate of 2,400 acfm is found to be approximately 400 ft/hr.
Using a 20 percent factor of safety reduction of calculated maximum drilling rate, the
actual sustainable maximum drilling rate is
κ max = 400 (1 − . 0 20 )

κ max = 320 ft/hr

The approximate volumetric flow rate of formation water, q fw, that can be carried is

π − ) (62 4 . ) S s
2
q fw = D ( κ max κ (8-7)
h
.
4 (833 ) S fw
where q fw is the volumetric flow rate of formation water (gal/hr or bbl/hr),