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.
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 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...
