Model Aids Cuttings Transport Prediction
Authors: Gavignet Alain A. , Sobey Ian J. 
Published: December 2007

Summary
Drilling of highly deviated wells can be complicated by the formation of a thick bed of cuttings at low flow rates. The model proposed shows what mechanisms control the thickness of such a bed, and the model predictions are compared with experimental results.

Introduction
One function of drilling fluids in rotary drillig is the lifting of cuttings up the annulus. Cuttings transport in vertical wells has been studied by many investigators.1–13 While it was found that pipe eccentricity, velocity profile, and drillpipe rotation had some effect on the transport of cuttings up the hole, Sample and Bourgoyne5,6 showed that the most relevant parameter is the settling velocity of the cuttings in stagnant mud and proposed a device to measure the settling velocity of the cuttings at the wellsite because the existing correlations for predicting settling velocities were not always reliable. Gavignet and Wick13 recently showed that the existence of a yield stress in the rheogram of the drilling fluid strongly influences the settling velocity and proposed a general method for calculating settling velocities from the polynomial fitting of multispeed rheograms, thus eliminating some of the uncertainties in the previous methods for designing cuttings transport in drilling.
The problem of cuttings transport is significantly different in deviated wells than in vertical wells. Refs. 14 through 17 report the results of laboratory experiments carried out with various drilling fluids and real cuttings in plastic pipes at inclination angles ranging from 0 to 90° from vertical. These studies used an annular size of 5×1.9 in. [12.7×4.8 cm] and flow rates up to 200 gal/min [0.013 m3/s] and were able to rotate the inner pipe in the annulus.
Tomren et al.16 observed thatwhen deviation from vertical is &lt;10°, cuttings transport is essentially similar to the vertical situation;when deviation increases, a cuttings bed develops at low flow rates;for a given flow rate, the bed thickness increases with deviation up to an angle where it becomes independent of the deviation angle; andin given conditions of deviation and flow rate, the bed thickness is strongly influenced by drillpipe eccentricity, but only moderately influenced by fluid viscosity.
This paper looks at the hydrodynamic mechanism of the transport of a layer of solids by a liquid in highly inclined pipes and develops a model for the cuttings transport in deviated wells.
If a bed of solids is formed while liquid two mechanisms can be expected to cause solids displacement: saltation (i.e., when the drag force exerted by the liquid on a particle resting at the interface causes it to be lifted into the stream of liquid) and sliding (i.e., when the viscos forces exerted by the liquid over the rough surface of the bed of solids cause the entire bed to overcome the solids/solids friction at the pipe wall and slide up the well). In real life, bed transport in solids/liquid flow may be a mixture of saltation and sliding. However, it is interesting to find out whether one of the two mechanisms dominates in drilling.
Wicks18 studied solid transport at low concentrations in horizontal pipes. He reported experimental results carried out with fine sand (average particle size=0.15 mm) and Newtonian liquids (viscosities of 0.9 to 27 cp [0.9 to 27 mPa·s] in pipes of 1- and 5–5 in. [2.54- and 13.4-cm] diameter). Wicks developed a model based on saltation that accounted for his experimental results. His model can be used to predict bed thickness, assuming that it will be determined by the liquid velocity in the upper layer, where particle lift occurs.
To calculate the drag force on the particles lying on top of the bed, Wicks assumed that they were small enough to be embedded in the laminar film at the liquid/solids interface, even if the flow in the liquid layer happened to be turbulent. As a consequence, the bed thicknesses predicted by his model depend very strong on the viscosity of the transporting liquid.