Page 480 - Air and Gas Drilling Manual

Chapter 10: Stable Foam drilling 10-5
9. Select the contractor compressor(s) that will provide the drilling operation
with the appropriate air or gas volumetric flow rate needed to create the
stable foam drilling fluid. Also, determine the maximum power required
by the compressor(s) and the available maximum derated power from the
prime mover(s).
10. Determine the approximate volume of fuel required by the compressor(s)
to drill the well.
In Chapter 6 the basic direct circulation drilling planning governing equations
have been derived and summarized. The equations in this chapter will be utilized in
the discussions and illustrative examples that follow.
10.2 Stable Foam Drilling Operations
Stable foam drilling operations are carried out using only drill string injection
configurations. Figure 10-1 shows a schematic of the drill pipe injection stable
foam drilling configuration. Both incompressible fluid (with surfactant) and
compressible 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. The stable foam is created when the fluids pass
through the drill bit nozzles. As the 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 (at P e) into a surface horizontal flow line. This horizontal flow line
flows to either, a burn pit, or to 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 used to contain contaminated fluids and gases, or hydrocarbons.
Sealed returns tanks are particularly useful in underbalanced drilling operations.
Given below are the advantages and disadvantages of the stable foam drill pipe
injection technique.
The advantages are as follows:
• The technique does not require any additional downhole equipment.
• Nearly the entire annulus is filled with the stable foam drilling fluid,
thus, low bottomhole pressures can be achieved.
• Since the bubble structures of stable foam drilling fluids have a high fluid
yield point, these structures can support rock cuttings in suspension
when drilling operations are discontinued to make connections. Stable
foams have seven to eight times the rock cutting carrying capacity of
water.
• Rock cuttings retrived from the foam at the surface are easy to analyze for
rock properties information.
The disadvantages are as follows:
• Stable foam fluids injection cannot be continued when circulation is
discontinued during connections and tripping. Therefore, it can be
difficult to maintain underbalanced conditions during connections
and trips.
• Since 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