Cuttings Transport Mechanism in a Large-Diameter HDD Borehole

Abstract

Drilling mud (or drilling fluid) is a vital component in a horizontal directional drilling (HDD) crossing project, for which a key function is the removal of the cuttings from the borehole. The rheological properties of drilling mud, including flow index, consistency index, effective viscosity, and flow patterns, are briefly reviewed. A model of the drilling mud behavior in the annular space is thereby established, based upon which the cuttings transport mechanism is analyzed. In this model, the critical factors for the efficient removal of cuttings are considered to be the rheological properties and return velocity of the drilling mud in the annular space surrounding the drill rod within the hole. The rheological properties should therefore be carefully controlled so the cuttings can be suspended or sink very slowly in the mud, and an appropriate return velocity must be selected so the flow pattern is suitable for effectively transporting the cuttings without compromising the stability of the borehole. Finally, a decision procedure is created by which a drilling mud engineer can select the proper mud rheological properties and return velocity. The model and decision process have been applied to a large-diameter HDD river crossing project to demonstrate their practicality.