TABLE OF CONTENTS 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 circulation 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.
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 circulation 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 cuttings from the advance of the drill bit. The compressible gases used...
