Factors Affecting Cuttings Removal During Rotary Drilling
Authors: Hopkin E.A. 
Published: December 2007

Abstract
Laboratory tests conducted by the author, together with actual field experience in Canada, have indicated the magnitude of some of the factors affecting ability for drilling mud to clean the hole. A correlation was observed between funnel viscosity and particle slip velocity. A relationship was also observed between the Bingham yield value of the mud and the particle slip velocity. Laboratory tests indicated little change in slip velocity with funnel viscosities up to about 80 sec/qt or to yield values of about 15 lb/100 sq ft . The slip velocity of the particles studied varied from 160 ft/min with water to nearly zero for high mud viscosities. Increasing the mud density, creating laminar annular mud flow or rotating the drill pipe may also improve the carrying capacity of mud. Field tests indicated that during fast upper-hole drilling, the rate of cutting removal must be sufficient to maintain the concentration of cuttings in the annulus at less than 5 percent by volume to prevent balled-up drill collars and stuck drill pipe.

Introduction
In the Mid-Continent area, hole cleaning during rotary drilling normally involves the use of low-density drilling muds with funnel viscosities varying from 26 to 80 sec/qt. However, frequently it is necessary to raise the mud viscosity in excess of 200 sec/qt to clean the hole. In addition, field experience has indicated that during very fast upper-hole drilling, annular mud velocities considerably in excess of the maximum slip velocity of the cuttings (normally considered to be 120 to 150 ft/min) are necessary to prevent balling the bit and drill collars. In view of these variations in mud properties and circulation rates necessary to clean the wellbore, a study of factors affecting hole cleaning and the relative magnitude of each was undertaken. The study was also prompted by the knowledge that a drilling mud with properties close to those of water would drill at rates several times that of a viscous mud. The carrying capacity of these near-water muds was not clearly understood, however, and many drillers were reluctant to use them.

Discussion: EFFECT OF MUD PROPERTIES ON DRILLING RATE
The effect of mud properties on drilling rate has been studied by a number of investigators. It is clear from their work that mud properties close to those of water will significantly improve drilling rate and bit life. If the water-type mud will also effectively clean the hole and cause no logging or formation impairment problems, the choice of an appropriate drilling fluid is clear-use water. Often, however, a water-type drilling fluid is not capable of effectively cleaning the hole.

THEORY OF PARTICLE SETTLING
The forces acting on a particle that is allowed to settle through a fluid quickly reach equilibrium, and the particle settles at some constant velocity referred to as the slip velocity. The slip velocity of any particle depends upon a number of factors such as density and viscosity of the fluid, the volume, specific gravity, shape and roughness of the particle, and upon the shape and area of the projected face of the particle at right angles to the direction of relative fluid solid movement. Brown' expressed this relationship for spheres in Newtonian fluids in the following equation.
vs= ........................................(1)
where vs= slip velocityDs= diameter of sphereps= density of spherept= density of liquidfD= friction factorg= constant.
The friction factor fD varies as the size, shape. surface roughness and slip velocity of the body, and as the density and viscosity of the fluid varies. Waddel found experimentally that where dynamic similarity exists, f, was a function of the particle Reynolds number (Nrc) of the sphere (Fig. 1). Fig. 1 shows that f, for spheres varies from over 10 for low NRe. numbers to 0.44 for NRe over 600.