Coiled Tubing Drilling, grown significantly in recent years, is normally associated with high angle to horizontal and extended reach wells. It is, however, in these applications that hole problems become more troublesome because of inefficient cuttings removal. Among the many parameters affecting efficient cuttings transport in Coiled Tubing Drilling are pump rates, well dimensions, fluid properties, solids sizes, solids loading and hole inclination. Several attempts have been made to determine the optimum operating range of these parameters but complete and satisfactory models have yet to be developed.
The purpose of this paper is to provide a critical review of the state of the art on efficient cuttings transport during Coiled Tubing Drilling, present the critical parameters involved, establish their range according to what is observed in practice and propose a different approach for predicting the minimum suspension velocity. Finally the laboratory system that has already been set up is presented. Its primary purpose is to allow the gathering of good quality data, missing from the literature, which could enhance our understanding of the flow of solid - liquid mixtures in annuli.
The advantages of Coiled Tubing Drilling (CTD) are numerous and have been indicated and proved in practice by a large number of investigators. A significant drawback is the difficulty for efficient cuttings transport primarily because the pipe is not rotated.
Cuttings transport during drilling (either conventionally or with Coiled Tubing) has a major impact on the economics of the drilling process. Inefficient hole cleaning from the cuttings can lead to numerous problems such as stuck pipe, reduced weight on bit leading to reduced rate of penetration (ROP), transient hole blockage leading to lost circulation conditions, extra pipe wear, extra cost due to additives in the drilling fluid and wasted time by wiper tripping.
These many problems have prompted significant research into cuttings transport during the past 50 years. Excellent reviews on the subject have been given in the past1–3. Pilehvari et al.1–2 state that fluid velocities should be maximized to achieve turbulent flow and mud rheology should be optimized to enhance turbulence in inclined / horizontal sections of the wellbore. Turbulent flow of non-Newtonian fluids needs much more work and should be extended to include pipe rotation and dynamics for conventional drilling. Future work should focus on getting more experimental data, validation of fluid models, cuttings transport mechanistic models verified by comprehensive experimental data. Azar & Sanchez3 conclude that a combination of appropriate theoretical analyses (complete free body diagrams, accurate rheological models, accurate annular flow models), experimental studies (extensive testing concentrating on individual variables or phenomena), statistical modeling (rheological models, unstable cuttings transport conditions), and high - tech research facilities (accurate measurement of pertinent variables, analysis of video to develop flow pattern maps) will be necessary for further progress.
While many cuttings transport problems were addressed quite successfully for conventional drilling in vertical, inclined and horizontal wells in the past, the increase in activity of CTD has called for renewed interest into cuttings transport problems in horizontal and highly inclined annular geometries with no rotation of the inner pipe.
In recent years there have been several theoretical, semi-theoretical and experimental investigations for assessing the important parameters for efficient cuttings transport in highly inclined and horizontal geometries during CTD4–10 or conventional drilling but not taking into account the rotation of the inner pipe11–15.