Direct Circulation Models

Publisher Summary This chapter explores direct-circulation models, 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 and 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. This chapter derives the basic direct circulation drilling planning governing equations. In order to make reasonable predictions of the flow characteristics for direct circulation air, gas, aerated fluids, and stable foam drilling operations, it is necessary to derive a consistent theory that can be used to develop specific equations to model these operations. To carry out the derivation of the governing equations for direct circulation, the weight rate of flow of air (or gas) to the well must be determined. All three basic drilling fluid circulation models must utilize a combination of mathematical theory and empirical correlations to develop a complete calculation model for each. To emulate the stable foam characteristics, two basic empirical correlation methodologies are viscosity correlation and friction factor correlation. The air and gas drilling model requires no special empirical correlations to adjust the results to provide results that agree more closely to field data.