Page 256 - Air and Gas Drilling Manual

Chapter
Six

Direct Circulation Models

In order to make reasonable predictions of the flow characteristics for direct
circulation air and gas drilling operations, aerated fluids drilling operations, and
stable foam drilling operations it is necessary to derive a consistent theory that can
be used, with certain simplifying limitations, to develop specific equations to model
each of the above operations.

6.1 Basic Assumptions
Direct circulation is defined as the injection of the drilling fluid into the inside
of the top of the drill string, the flow of the fluid down the inside of the drill
string, through the bit orifices or nozzles, the entraining of the rock cuttings into the
drilling fluid at the bottom of the borehole, and then the flow of the drilling fluid
with the entrained cuttings up the annulus between the outside of the drill string and
the inside of the borehole.
Figure 6-1 shows a simplified U-tube schematic representation of direct
circulation flow. In general, in air and gas drilling operations two phase flow occurs
in the inside of the drill string and through the orifices or nozzles in the drill bit.
Three phase flow occurs when the fluids with entrained rock cuttings move up the
annulus from the bottom of the well to the surface. The three phases are a
compressible gas, an incompressible fluid, and the solid rock cuttings from the
advance of the drill bit. The compressible gases that are used most in drilling are
air, natural gas, nitrogen ( or air stripped of oxygen). The incompressible fluids

6-1
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