Page 548 - Air and Gas Drilling Manual

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& 179 196 dP Chapter 10: Stable Foam drilling 10-73
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Substituting the values of H 1 into the right side of Equation 6-102 gives
6 650
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1 dh 6 650
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0
As can be seen in the above, the right and left hand sides of Equation 6-102 yield
the same answer. This shows that the lower limit pressure is correct.
The injection pressure while drilling at 10,000 ft of depth is approximately 229
psia. This is the approximate injection pressure for both the compressed air and the
incompressible fluid (water and surfactant) as they enter the surface flow lines that
lead to the top of the drill string. When drilling at a depth of 10,000 ft, the
corresponding for the injection pressure above is an air volumetric flow rate of 1,658
acfm (with an incompressible fluid volumetric flow rate of 105 gal/hr). This
compressed air injection pressure is the pressure the compressor output must match.
Figure 10-3 shows the stable foam pressures in the annulus and the aerated fluid
(both air and incompressible fluid) pressures inside the drill string as a function of
depth for this illustrative example (while drilling at the depth of 10,000 ft). The
figure shows the pressure at the bottom of the annulus is approximately 1,116 psia
(p a5 above). If a target oil or natural gas rock formation pore pressure at the bottom
of the borehole is above this value, the oil or natural gas will flow into the borehole
as the drill bit is advanced into the producing rock formation. This would constitute
an underbalanced drilling situation. If the pore pressure is less than this value, rock
cuttings from the advance of the drill bit will be forced into the exposed pores
around the bottom of the borehole resulting in formation damage. However, since
there are two controlling criteria (in the annulus) for stable foam drilling (namely,
maintaining foam quality in the annulus at 0.60 or greater and maintaining the
3
kinetic energy per unit volume at 3.00 ft-lb/ft or greater) the bottomhole pressure in
the annulus cannot be held constant while drilling an interval.
The anticipated drilling rate of penetration is estimated to be 60 ft/hr. The
vertical interval section to be drilled is from a depth of 7,000 ft to a depth of 10,000
ft, or a 3,000 ft length of borehole. Therefore, the estimated actual drilling time to
drill this interval is approximately 50 hours. Using similar calculations as those
given above, the injection pressures as a function of drilling depth and, thus, drilling
time can be obtained. Figure 10-4 shows the injection pressure as a function of
drilling time for the interval from 7,000 ft to 10,000 ft. The maximum injection
pressure is approximately 275 psia and occurs at the beginning of the interval at a