Page 260 - Air and gas Drilling Field Guide 3rd Edition

10.3 Stable Foam Drilling Operations 251

Chapter 6 derived and summarized the basic direct circulation drilling
planning governing equations. The equations in this chapter will be utilized in
the discussions and illustrative examples that follow.

10.3 STABLE FOAM DRILLING OPERATIONS
Stable foam drilling operations are carried out using only drill string injection con-
figurations. Figure 10-1 shows a schematic of the drill pipe injection stable foam
drilling configuration. Both incompressible fluid (with surfactant) and compress-
ible air (or other gas) are injected together into the top of the drill string (at P in ).
These fluid streams mix as they go down the inside of the drill string and pass
through the drill bit nozzles. Stable foam is created when the fluids pass through
the drill bit nozzles. As stable foam is generated in the bottom of the annulus, the
rock cuttings (from the advance of the drill bit) are entrained in the foam and the
resulting mixture flows to the surface in the annulus.
The mixture exits the annulus into a horizontal surface return flow line just
upstream of the back pressure valve and gauge (upstream of P s ). The pressure read-
ings just upstream of the back pressure valve can be converted to foam quality values.
In order to assure that there is a continuous column of stable foam from the bottom of
the annulus to the return line back pressure valve, the foam quality upstream of the
valve should be held at 0.98 or less. As conditions change in the well (e.g., an influx
of formation water, wall caving), the back pressure valve can be adjusted by rig per-
sonnel to give pressure readings required to give the needed foam quality at the sur-
face. Some operators design their return line systems so that the system itself provides
sufficient back pressure resistance to the foam flowing from the top of the annulus. In
these return line systems, the stable foam transitions to an unstable foam as the flow
reaches the top of the annulus (or in the horizontal return line itself) and the
flow from the return line at its exit (at P e ) is in slugs of unstable foam.
This horizontal return flow line flows to either a burn pit or sealed returns
tanks. The burn pit is used when the returning air is mixed with hydrocarbons
that can be burned off as they enter the pit. Sealed returns tanks are also
used to contain contaminated fluids and gases, or hydrocarbons. The use of
sealed tanks can only be carried out where the stable foam drilling operation is
utilizing membrane-generated inert nitrogen gas as the foaming gas.
Advantages and disadvantages of the stable foam direct circulation drill are as
follow:
Advantages
n The technique does not require any additional downhole equipment.
n Nearly the entire annulus is filled with the stable foam drilling fluid, thus a
variety of low bottom hole pressures can be achieved.
n Since the bubble structures of stable foam drilling fluids have a high fluid
yield point, these structures can support the small rock cuttings in