Page 167 - Air and gas Drilling Field Guide 3rd Edition

158 CHAPTER 6 Direct Circulation Models

6.4 STABLE FOAM DRILLING MODEL
Stable foam drilling is a special case of the general derivation given in Section 6.2.
In essence, it is a special case of aerated drilling fluids. In stable foam drilling
operations, the mixture of gas (usually air or membrane generated nitrogen)
and water (with a surfactant) combine under the action of high shear flow
through the drill bit nozzles to form a foam.
Referring to Figure 6-1, foam gas volume fraction (or foam quality) G at any
position in the annulus is defined as

Q g
G ¼ ; (6-47)
Q g þ Q f
3
3
where Q g is the volumetric flow rate of gas (ft /sec, m /sec) and Q f is the volu-
3
3
metric flow rate of the incompressible fluid (ft /sec, m /sec).
The objective of the foam operation is to control the foam column inside
the annulus in such a manner as to maintain continuous unbroken (or stable)
foam throughout the annulus. In order to accomplish this, normal operational
practice is to place a back pressure valve on the return flow line at the surface.
The back pressure valve and appropriate instrumentation just upstream of
the valve allow monitoring of the foam gas volume fraction at that position
(i.e., G bp ). Knowing the foam gas volume fraction at this position, the foam gas
volume fraction at the bottom of the annulus G bh can be determined via model-
ing and operational experience. The gas volume fraction at the bottom of the
annulus must be maintained at approximately 0.60 or greater [10]. Thus, as
the drilling operation progresses and the drill bit is advanced to greater depths,
the foam gas volume fraction upstream of the back pressure valve must be
adjusted to allow the bottom hole gas volume fraction to be maintained at 0.60
or greater.
If the bottom hole foam gas volume fraction drops much below 0.60, the foam
will collapse to the three separate phases. To maintain the bottom hole foam gas
volume fraction in the annulus at 0.60 or greater, the foam gas volume fraction
upstream of the back pressure valve is usually maintained in the range of 0.90
to 0.99. This return line foam gas volume fraction is dependent on the geometry
of the well and of course on the quality of the additives (mainly the surfactant).
The flow up the annulus is a stable foam carrying the entraining cutting solids.
The exit pressure for stable foam drilling operations is the back pressure P bp
upstream of the valve in the return flow line from the annulus. Equation (6-26)
becomes

ð ð H
P bh
dP
¼ dh; (6-48)
B a ðPÞ 0
P bp
2
2
where P bp is the back pressure on the annulus (lb/ft abs, N/m abs) and