Development Of Improved Laboratory And Field Procedures For Determining The Carrying Capacity Of Drilling Fluids
Authors: Sample Kenneth John , Bourgoyne Adam T. 
Published: December 2007

A new simplified technique has been developed for establishing the carrying capacity of drilling fluids. The procedure involves the use of cutting slip velocity measurements taken in static, or near static, drilling fluids. This value of slip velocity is then used to predict cutting transport for flowing conditions. predict cutting transport for flowing conditions. Equipment was developed which can be used to routinely apply the new technique to field muds. In the new device, the position of the cuttings is detected using piezoelectric sonic transducers. This detection procedure piezoelectric sonic transducers. This detection procedure was found to be the most suitable for application with opaque field muds.

The application of cutting transport data in the determination of optimum drilling practices was also examined through the use of a computer model. Use of the computer model in conjunction with experimentally obtained slip velocities allows the determination of the optimum annular velocity. It can also be used to determine if insufficient cutting transport should be enhanced by increasing drilling fluid viscosity or by increasing the circulation rate. The computer model also indicated that the current drilling practice of maximizing jet impact force almost always results practice of maximizing jet impact force almost always results in the use of a near optimal annular velocity when modern rig equipment is available.

Introduction

The study of cutting transport has been undertaken by numerous investigators since the inception of rotary drilling in 1900. These previous investigations have attempted to determine the affect of variables that influence the ability of a drilling fluid to transport rock cuttings from the bottom of the borehole to the surface. The term carrying capacity is often loosely used to refer to this ability to transport cuttings. More definitive terms also in common use include cutting transport velocity and transport ratio. The transport velocity, vt, is simply the velocity of the cuttings relative to the surface and is equal to the difference between the Annular fluid velocity, va, and the cutting slip velocity relative to the fluid, vs.

(1)

The transport ratio is defined as the transport velocity divided by the annular fluid velocity.

(2)

For positive transport ratios, the cuttings will have a net upward velocity and they will be transported to the surface.

Many different correlations have been developed for predicting cutting slip velocity while drilling. In each predicting cutting slip velocity while drilling. In each case, the correlation is based upon settling velocity measurements obtained in static Newtonian fluids. In applying the Newtonian settling velocity correlations to the rotary drilling situation, it is necessary to define an apparent Newtonian viscosity for the prevailing downhole conditions. Unfortunately, the prevailing conditions for cuttings transport during rotary drilling operations are quite complex. The geometry of the cuttings is difficult to describe and varies with formation characteristics and drilling conditions. Drilling fluids are generally non-Newtonian in nature and it is difficult to describe the effective shear rate in the fluid surrounding the cuttings. Shear results both from the movement of the fluid in the annulus and from the relative movement of the cuttings through the fluid. Drill pipe rotation further complicates the situation.