A Three-Segment Hydraulic Model for Cuttings Transport in Coiled Tubing Horizontal and Deviated Drilling
Authors: Cho H. , Shah S.N. , Osisanya S.O. 
Published: March 2010

Abstract
This study proposes a new mathematical model to overcome the limitations in existing hydraulic models used to predict cuttings transport in coiled tubing horizontal or deviated well drilling. A new three-segment (a horizontal and near horizontal segment, a vertical and near vertical segment, and a transit segment) hydraulic model under two-phase (solid-liquid) flow in an annulus was developed to predict and interpret cuttings transport mechanisms. In particular, the model developed in this study advances a three-layer (a stationary bed layer of drilled cuttings at the bottom, a moving bed layer above it, and a heterogeneous suspension layer at the top) hydraulic model for the horizontal and near horizontal segment. An existing two-layer model was modified for a transit segment, and a one-layer model was used for the vertical and near vertical segment.
This paper specifically describes model development for each segment, solution, and the simulation results of the integrated three-segment model. The simulation results show how to obtain a reasonable pumping velocity and optimize the rheology of drilling fluid with the possible lowest pressure gradient that might serve as an operational guideline during drilling. Moreover, the effects of various parameters that affect the efficiency of cuttings transport are discussed. These results were compared with published experimental data. The observed agreement and discrepancies are discussed.

Introduction
Considerable efforts have been made for several decades to better understand cuttings transport in the drilling industry. There are numerous mathematical and empirical models for the prediction and interpretation of the hydraulics of cuttings transport mechanism. Common problems with most of these cuttings transport models include inaccurate predictions, when compared with the experimental results or in situ drilling results, and discrepancy between the models(1). There seem to be two main reasons for these problems. First, researchers make ambitious attempts to develop comprehensive models that cover a wide range of conditions (from vertical to horizontal) simply as a function of wellbore deviation. Second, researchers make too many assumptions, or neglect certain observed phenomena, as pointed out by Azar and Sanchez(1).
A detailed review of the published experimental data reveals that the cuttings transport characteristics are changed with an a increase in wellbore angles. Researchers(2–4) reported that the cuttings bed in annuli is unstable, under a certain range of well deviations. The most unstable and difficult region for cuttings transport in a deviated well is reported as 30 – 60 ° from the vertical(5–7). In addition, the cuttings bed is unstable, and sometimes slides down towards the bottom hole. However, the existing models, which can handle cuttings transport in highly deviated to horizontal wells, do not consider these characteristics for this region. One mathematical model, which can handle from vertical to horizontal wells as only a function of the deviated angle of the well, cannot properly characterize the cutting transport mechanism in horizontal and highly deviated wells.
Therefore, a new three-segment mathematical model considering the characteristics of deviated wellbores on cuttings transport in annuli is proposed to predict and interpret the cuttings transport mechanism.