Study on the Flow Model of Annulus Drilling Fluid in the Large Diameter Horizontal Directional Drilling

The huge annular area between borehole wall and drill rod leads to a low velocity of drilling fluid in the reaming procedure of large diameter Horizontal Directional Drilling (HDD) crossing project. In order to ensure efficiency of cuttings transmission, the viscosity of the drilling fluid is always increased. The drilling fluid filled in annular area is always mixed by several fluid models, so it's difficult to analyze the flow field of the high viscosity drilling fluid in the annular space of borehole using a single fluid model. The simulation herein classifies drilling fluid into two parts: (1) the fluid close to the drill rod and borehole wall, and (2) a gel which is in the middle of the first part. This paper uses the N-S equation to determine the relationship between the velocity and the viscosity of drilling fluid. Furthermore, the equation obtained from the N-S equation shows that boundary thickness depends on the drilling fluid's viscosity. The author used the Herschel-Bulkle model to describe the drilling fluid and obtain the shear stress equation.