Page 261 - Air and gas Drilling Field Guide 3rd Edition

252 CHAPTER 10 Stable Foam Drilling

suspension when drilling operations are discontinued to make connections.
Stable foams have seven to eight times the rock cutting carrying capacity of
water.
n Rock cuttings retrieved from the foam at the surface are easy to analyze for
rock properties information.

Disadvantages
n Stable foam fluids injection cannot be continued when circulation is discon-
tinued during connections and tripping. Therefore, it can be difficult to
maintain underbalanced conditions during connections and trips.
n Because the injected gas is trapped under pressure inside the drill string by
the various string floats, time must be allowed for the pressure bleed down
when making connections and trips. Here again the bleed down makes it
difficult to maintain an underbalanced condition.
n The flow down the inside of the drill string is two-phase flow and, there-
fore, high pipe friction losses are present. The high friction losses result
in high pump and compressor pressures during injection.
n The gas phase in the stable foam attenuates the pulses of conventional
MWD systems. Therefore, conventional mud pulse telemetry MWD cannot
be used. Often EM MWD systems can be used.

10.4 MINIMUM VOLUMETRIC FLOW RATES
Most stable foam vertical performance drilling operations are drilled over a con-
stant cross-section interval with constant incompressible fluid volumetric flow
rates, constant compressible gas volumetric flow rates, and a constant back pres-
sure setting at the surface. Therefore, the minimum volumetric flow rates of
incompressible fluid and compressible gas to carry drill bit cuttings from the well
can be determined at the maximum true measured and/or vertical depth of the
constant cross-section well bore.
Because most potential producingformations are usually of the order ofa few hun-
dred feet or tens of meters thick, vertical drilling of these formations also requires lit-
tle changes in back pressure or flow rates as the section is drilled. Here again, the
maximum depth can be used to determine minimum volumetric flow rates to carry
the drill bit cuttings for the well. This will not be the case for drilling horizontal bore-
holes underbalanced. Horizontal drilling will be discussed in Chapter 12.
There are few minimum volumetric flow rate theories available for use in sta-
ble foam drilling modeling. Stable foam drilling fluids have high effective viscos-
ities and yield points. Early experiments have found that spherical rock
particles will fall at terminal velocities of the order of 10 to 20 ft/min [18]. These
terminal velocities are low when compared to terminal velocities in water. Early
experiments also indicated that terminal velocities tend to increase with