Page 174 - Air and gas Drilling Field Guide 3rd Edition

CHAPTER

7
Reverse 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 aforementioned operations. All three basic drilling fluid cir-
culation models, air and gas, aerated, and stable foam, must utilize a combination
of mathematical theory and empirical correlations to develop a complete calcula-
tion model for each.

7.1 BASIC ASSUMPTIONS
Reverse circulation is defined as the injection of the drilling fluid into the top of
the annulus space, the flow of the fluid down the inside of the annulus (between
the inside of the casing or open hole), entrain the rock cuttings as the drilling
fluid flows into the large opening in the drill bit, and then flow with the cuttings
to the surface through the inside of the drill string.
Figure 7-1 shows a simplified U-tube schematic representation of reverse circu-
lation flow. In general, in air and gas drilling operations, two-phase flow occurs in
the inside of the annulus. Three-phase flow occurs when fluids with entrained
rock cuttings pass through the large opening drill bit and then move up the
inside of the drill string from the bottom of the well to the surface. The three
phases are a compressible gas, an incompressible fluid, and the solid rock cut-
tings from the advance of the drill bit. The compressible gases used most in dril-
ling are air, membrane generated nitrogen, nitrogen, or natural gas. The
incompressible fluids used are treated fresh water, treated salt water (formation
water), and water-based drilling muds. Diesel oil, oil-based drilling muds, and
crude oil (formation oil) are somewhat compressible.

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