Page 267 - Air and gas Drilling Field Guide 3rd Edition

258 CHAPTER 10 Stable Foam Drilling

In this special illustrative example, no friction due to flow is considered.
Therefore, the bottom hole pressure reflects only an effective static weight of
the foam column in the annulus. The nonfriction solution gives a bottom hole
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pressure of 386 psia (264 N/cm abs). The foam quality at the top of the annulus
is 0.98 (at the back pressure valve) and 0.93 at the bottom of the annulus.

10.5.2 Major and Minor Friction Losses
The governing equations for the direct circulation foam drilling friction solution
were presented in Chapter 6. These require complex trial and error solutions that
can be applied to deep wells with complicated borehole geometry. The adding of
major and minor friction losses to the foam drilling fluid theory allows analytic
solutions that simulate the actual drilling situation more closely.
In general, the adding of flow friction losses requires the addition of volumet-
ric flow rates of incompressible fluid and compressed gas to compensate for the
added energy losses in the system (relative to the nonfriction solution above).
This is reflected in a higher bottom hole pressure in the annulus and elsewhere
in the system.
Illustrative Example 10.2 In this illustrative example, data given in Illustra-
tive Example 10.1 are used for a solution that considers both major and minor
friction losses for direct circulation foam drilling. In this solution the pressure
and foam quality for annulus foam flow will be determined as a function of depth.
Also, the pressure inside the drill string as a function of depth will be determined
for this aerated flow region. The bottom hole cleaning characteristics of the foam
flow are also determined. For this example, it is assumed that a 100-ft (30.5 m)-
long, 5.625-in (143.9 mm) inside diameter return flow line allows foam from
the top of the annulus to flow through the back pressure valve to the separator.
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The back pressure at the end of the return line is 80 psig (55.2 N/cm gauge).
Also, it is assumed that the foam screening test run on the foam to be used in
the forthcoming drilling operation has been given a beaker height of 610 ml
and a half-life of 280 sec. It will be assumed that the constants C, a, and b for
Equation (10-4) are all 1.0.
Appendix E gives the Mathcad detailed solution of Illustrative Example 10.2.
Figure 10-3 shows the flowing foam pressures in the annulus and the pressures
in the aerated flow of the mixture of incompressible fluid and incompressible
gas inside of the drill string. The injected fresh water flow rate and compressed
air flow rate to the standpipe are 50 gpm (189 liters/min) and 2120 scfm (stan-
dard 1000 liters/sec).
It is clear from Figure 10-3 that the foam drilling fluid in this example can pro-
vide a significant side wall and bottom hole pressure in the open hole section.
The value of 1315 psia (907 N/cm 2 abs) bottom hole pressure at 10,000 ft
(3047 m) is higher than the bottom hole pressures calculated in the air drilling