Page 154 - Air and gas Drilling Field Guide 3rd Edition

CHAPTER

6
Direct Circulation Models

In order to make reasonable predictions of the flow characteristics for direct cir-
culation air and gas drilling operations, aerated fluids drilling operations, and sta-
ble 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 aforementioned operations. All three basic drilling fluid circu-
lation models, air and gas, aerated, and stable foam, must utilize a combination of
mathematical theory and empirical correlations to develop a complete calculation
model for each.

6.1 BASIC ASSUMPTIONS
Direct circulation is defined as the injection of 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 circu-
lation 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, and nitrogen (or air stripped of oxygen). The incompressible
fluids used are treated freshwater, treated salt water (formation water), and water-
based drilling muds. Diesel oil, oil-based drilling muds, and crude oil (formation
oil) are somewhat compressible.
It is assumed that compressible gases can be approximated by the perfect gas
law. Further, it is assumed that the mixture of compressed gas and incompressible
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