Hole cleaning (cuttings transport) is a major concern and challenge while drilling inclined wells. Insufficient hole cleaning can cause several costly problems, such as an increase in ECD, excessive torque and drag, pack-off, cementing problems, stuck pipe, etc.

The physics of particle cuttings flow is very complicated, and it is impossible to accurately predict the cuttings flow with many different particles, differing in size, shape and density, in non-Newtonian fluids with multiple flow regimes.

The Critical Transport Fluid Velocity (CTFV) is the minimum fluid flow velocity in a pipe or annular region required to prevent the formation of a stationary cuttings bed. This paper will show that the determination of the CTFV for inclined wellbores is, for the most part, dependent on simple geometrical parameters and on fluid and cuttings parameters. This paper is the result of a study to determine the most significant physical phenomena involved in establishing the CTFV.

The governing equations discussed in this paper have been tested against the measurements of Thor Inge F. Larsen published in his Master's Thesis at the University of Tulsa in 1990. The results presented in this paper will show that physics based equations can replace many of the correlations suggested by Larsen.

The paper also shows that the same geometrical considerations are governing the changes in CTFV as the inner and outer diameters of the wellbore changes. We will show that one consequence of this is that using only the pipe hydraulic diameter, i.e. the difference in the inside and outside diameters in the annular region, is not sufficient in determining the change in CTFV as the pipe diameters change.

Suggestions for further experiments and necessary measurements in order to improve the accuracy are presented.

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