Page 292 - Air and Gas Drilling Manual

Chapter
Seven

Reverse Circulation Models

In order to make reasonable predictions of the flow characteristics for reverse
circulation air and gas drilling operations and aerated fluids drilling operations, it is
necessary to derive consistent theory that can be used, with certain simplifying
limitations, to develop specific equations to model each of the above operations. It
should be noted that stable foam cannot be applied to reverse circulation operations.

7.1 Basic Assumptions
Reverse circulation is defined as the injection of the drilling fluid into the top of
the annulus, the flow of the fluid down the annulus to the bottom of the borehole,
the entraining of the rock cuttings into the drilling fluid at the bottom of the
borehole as the fluid sweeps past the bit cutting face, and then the flow of the
drilling fluid with the entrained cuttings up the inside of the drill string.
Figure 7-1 shows a simplified U-tube schematic representation of reverse
circulation flow. In general, in air and gas drilling operations two phase flow occurs
in the flow of fluids down the annulus to the bottom of the borehole. Three phase
flow occurs when the flow of the fluids at the bottom of the borehole entrains the
rock cuttings that are generated by the advance of the drill bit. This three phase flow
continues through the single large opening in the drill bit and then up the inside of
the drill string to the surface. The three phases are the circulating fluids, a
compressible gas and an incompressible fluid, and the solid rock cuttings. The
compressible gases that are used in reverse circulation drilling are air, natural gas,
and nitrogen (and air stripped of oxygen). The incompressible fluids that are used

7-1
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