A Review of Cuttings Transport in Directional-Well Drilling
Authors: Bybee Karen 
Published: February 2015

A Review of Cuttings Transport in Directional-Well Drilling
This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 132372, “Review of Cuttings Transport in Directional-Well Drilling: Systematic Approach,” by T. Nazari, SPE, and G. Hareland, SPE, University of Calgary, and J.J. Azar, SPE, University of Tulsa, originally prepared for the 2010 SPE Western Regional Meeting, Anaheim, California, 27–29 May. The paper has not been peer reviewed.
Hole cleaning during directional-well drilling is a major concern in the oil field and must be monitored and properly controlled during the entire drilling operation. Inadequate drilled-cuttings removal can cause many costly problems. Low annular-fluid velocity, lack of drillpipe rotation, and the wrong mud properties are primary factors in ineffective hole cleaning. The full-length paper presents a review of previous hole-cleaning studies and discusses an approach that is better suited for monitoring and controlling hole-cleaning problems.
Introduction
The rotary-drilling process consists of a rock-cutting tool (drill bit) upon which a downward force is applied [weight on bit (WOB)] and rotation (rev/min) is imposed. The drilled cuttings generated by the drill bit are removed by circulating a drilling fluid from surface to bottomhole and back to surface (cuttings transport) (Fig. 1).
During directional-well drilling, cuttings removal becomes more difficult and if not controlled properly can result in serious problems such as mechanical pipe sticking (fishing or hole loss), excessive frictional torque (increase in rotary-power requirement) and frictional drag (inability to reach target), difficulty in landing casing, channeling problems during cementing, and difficulty in logging. The factors that are known to affect hole cleaning include annular eccentricity, inclination angle, drillpipe rotation, fluid flow rate (annular velocity and flow regime), rate of penetration (ROP), mud rheology and density, and cuttings (i.e., size, shape, and density). It is well documented that flow rate and drillpipe rotation have the most positive effect on hole cleaning. However, an increase in flow rate causes an increase in frictional pressure losses, which in turn causes an increase in equivalent circulating density (ECD), pump-pressure requirement, and potential hole erosion. An increase in pipe rotation can result in premature pipe fatigue failures caused by the induced cyclic stresses.
Cuttings-Transport Problem
In contrast to other industrial processes, the cuttings-transport problem in the drilling industry is more complex because of the many parameters that are interconnected nonlinearly. In simpler processes, some of the steps could be neglected.
Comparing this scheme to a drilling process results in the following correlations:
Process—drilling procedure
Reference—ECD limitation, pump hydraulic-horsepower limitation, hole erosion, ROP limitation, geological-target limitation, rotary-power limitation
Process inputs—flow rate, rev/min, hookload
Process outputs—pressures, ROP, torque
Internal states—WOB, eccentricity, hole cleaning
Monitoring/control—lost circulation, drag, torque, cuttings returns, ROP, circulating standpipe pressure
For a complete and reliable automated design process, the following steps must be taken:
Signal measurements
Monitoring of critical parameters
Modeling
Control