Page 476 - Air and Gas Drilling Manual

Chapter
Ten

Stable Foam Drilling

The term stable foam describes a special class of aerated drilling fluids. This
class of drilling fluid is made up of a special mixture incompressible fluids injected
with compressed air or other gases. To create a stable foam drilling fluid, the
incompressible component is usually made up of treated fresh water with a surfactant
foaming agent. The term stiff foam refers to the use of viscosified water instead of
fresh non-viscosified water in the incompressible fluid component (typical viscosity
additives are polyanionic cellulose, xanthan gum polymers, and carboxymethyl
cellulose). The surfactant foaming agent usually comprises about two to five percent
by volume of the treated water being injected (depending on the surfactant product).
The mixture of the incompressible fluid (with surfactant) and compressed air (or
other gase) flows as an aerated fluid as the mixture flows down the inside of the drill
string. Nozzles in the drill bit are required in order to allow the foam to be
generated at the bottom of the annulus as the aerated fluid mixture passes through
the drill bit. There are some shallow drilling situations where the foam can be
preformed at the surface and injected into the inside of the drill string. But in deep
drilling operations, it is not possible to maintain the mixture as a foam as it flows
inside the drill string. The pressures inside the drill string are too high to allow
foam properties to be maintained. Thus, for this discussion of stable foam drilling
fluids, it will be assumed that the mixture flows as an aerated drilling fluid down
the inside of the drill string, is transformed into a foam as the mixture passes
through the drill bit nozzles, and then flows up the annulus as a stable foam.
A new term foam quality, , must be introduced to the discussion so that the
physical characteristics and the operational parameters of stable foam drilling

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