Flow Patterns and Minimum Suspension Velocity for Efficient Cuttings Transport in Horizontal and Deviated Wells in Coiled-Tubing Drilling
- V.C. Kelessidis (Technical U. of Crete) | G.E. Bandelis (Technical U. of Crete)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2004
- Document Type
- Journal Paper
- 213 - 227
- 2004. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 1.6.6 Directional Drilling, 5.3.3 Particle Transportation, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.3.2 Multiphase Flow, 1.11 Drilling Fluids and Materials, 5.2.2 Fluid Modeling, Equations of State, 1.7.7 Cuttings Transport, 1.10 Drilling Equipment, 1.7.1 Underbalanced Drilling, 4.3.4 Scale, 1.6 Drilling Operations
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Coiled-tubing drilling (CTD), which has grown significantly in recent years, is normally associated with high-angle to horizontal and extended-reach wells. In these applications, however, hole problems become more troublesome because of inefficient cuttings removal. Among the many parameters affecting efficient cuttings transport in CTD are pump rates, well dimensions, fluid properties, solid sizes, solid 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 as follows:
To provide a critical review of the state-of-the-art modeling for efficient cuttings transport during CTD.
To present the critical parameters involved.
To establish their range according to what is observed in practice.
To propose a different approach for predicting the minimum suspension velocity.
To describe the laboratory system that has been set up.
The primary purpose of the flow system is to enable the gathering and publication of good-quality data that, together with previously published data, could further enhance our understanding of the flow of solid/liquid mixtures in annuli.
The advantages of CTD are numerous and have been indicated and proved in practice by a large number of investigators. One of the significant drawbacks of CTD is the difficulty of efficient cuttings transport primarily because the inner 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 because of additives in the drilling fluid, and wasted time for wiper tripping.
These many problems have prompted significant research into cuttings transport during the past 50 years. Excellent reviews on the subject have already been given. Pilehvari, Azar, and Sanchez1,2 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.1 Turbulent flow of non-Newtonian fluids needs much more investigation and should be extended to include pipe rotation and dynamics for conventional drilling. Future works should focus on getting more experimental data and validating fluid models and cuttings-transport mechanistic models that are verified by comprehensive experimental data. Azar and Sanchez2 conclude that a combination of appropriate theoretical analyses (complete free-body diagrams, accurate rheological models, and accurate annular-flow models), experimental studies (extensive testing concentrating on individual variables or phenomena), statistical modeling (rheological models and 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 previously addressed quite successfully for conventional drilling in vertical, inclined, and horizontal wells, the increase in activity of CTD has renewed interest in 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, semitheoretical, and experimental investigations for assessing the important parameters for efficient cuttings transport in highly inclined and horizontal geometries during CTD3-9 or conventional drilling, not taking into account the rotation of the inner pipe.10-14 Practical recommendations were also given for optimizing hole cleaning.15,16
Despite these efforts, there is still a need for good-quality published data against which current and future models can be compared. Many existing models use data for validation that are not applicable (e.g., using data with inner-pipe rotation)3,12 or do not use any data at all.10 And yet, some models compare their results only with the results of other models.13 Some authors present the data together with their own theoretical analysis, but they give very limited information on experimental parameters (e.g., rheology of fluids and cuttings concentrations).5,6,8
The approach taken by many investigators in modeling cuttings transport for highly inclined and horizontal annuli with no inner-pipe rotation is that of a two- or three-layer model. The basic model proposed for annuli is adopted from the one first proposed for solids transport in pipes by Doron et al.17 and later extended by the same authors.18-20
The steady-state models are based on mass-balance equations for solids and liquid together with momentum balance equations for the two or three layers, resulting in a system of coupled algebraic equations. Closure relationships describing the interaction of the two phases are needed to solve these equations, which are taken from published correlations.
In what follows, we present the state-of-the-art understanding of cuttings transport in horizontal and inclined wellbores. We analyze the main theoretical models that have been proposed: the two- and three-layer models. The general framework of the models is similar and is followed by most of the investigators who addressed the problem. What differs among the various approaches is the use of the closure relationships.
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