Effect of two-phase gas-liquid flow patterns on cuttings transport efficiency

Effective cuttings transport and accurate drilling hydraulics prediction remain issues of concern during drilling operations of horizontal, extended reach and multilateral wells. While several studies have adopted a two- or three-layered modelling approach to evaluate cuttings transport efficiency, they have neglected the effect of the gas-liquid fluid flow pattern within the annulus on cuttings transport. An experimental and theoretical study was carried out to evaluate the interplay between the two-phase gas-liquid flow patterns and the major drilling parameters and investigate its influence on the cuttings and fluid flow dynamics in a horizontal and inclined drilling wellbore. Several mathematical flow pattern dependent multi-layered models valid for any level of wellbore eccentricity were developed for the different cuttings transport mechanisms in the bubble, dispersed bubble, stratified and slug gas-liquid flow patterns, thereby providing a method to evaluate cuttings transport efficiency and perform wellbore hydraulics calculations for underbalanced drilling operations. Experimental results show that both fluid flow pattern and the drilling fluid flowrate are the most influential controllable parameters that affect the cuttings transport efficiency. Moreover, the hole cleaning requirements for an eccentric annulus is higher than that required for the concentric annulus of both single-phase and two-phase Newtonian or non-Newtonian fluids. Inclination angle was also found to influence hole cleaning and the degree of its effect is highly dependent on the fluid properties, the cutting transport mechanism and prevailing gas-liquid flow pattern. In the horizontal and inclined eccentric annuli, drillpipe rotation can improve cuttings transport for both single-phase and two-phase flows, but generally the effect of the drillpipe rotation on two-phase flow for cutting transport is much less than that of the single-phase flow. Overall, a good match was found between the mathematical flow pattern dependent multi-layered models and the experimental data. The findings of this study serve as a guide in the prediction of the wellbore dynamics for underbalanced drilling operations and provides a tool that can be applied for wellbore pressure management and the evaluation of hole cleaning based upon the specified flow conditions.