Abstract

The effects of cuttings bed at intermediate deviation of wellbore (30 – 60°) from vertical are quite complicated in cuttings transport because the movement of cuttings bed with its direction has not been well understood. The effect of cuttings bed movement in this region has also been neglected in most other mathematical models. The reason for this might be the instability of cuttings bed. This study shows that neglecting the cuttings bed characteristics may result in improper selection of coiled tubing drilling parameters, which may frequently lead to costly problems such as stuck pipe. However, the existing models, which can handle from vertical to horizontal wells with only a function of the wellbore deviation, cannot properly characterize the effects of cuttings bed movement in cuttings transport mechanism for the intermediate deviation of wellbores.

A mathematical model is proposed based on momentum and force balances in this region. All parameters, which involve cuttings bed movement and its direction, build up and erosion of cuttings bed, and wellbore deviation, are analyzed quantitatively. The analysis of the data obtained from the simulation allows interpretation of the relationship between major parameters affecting characterization of the cuttings bed in this region. The concept of minimum anti-sliding velocity of cuttings bed (MASV) is developed based on the inter-relationship between parameters, which involves fluid rheology, wellbore deviation angle, interfacial friction between suspension layer and cuttings bed, and in-situ fluid velocity in suspension layer. This study also provides the selection guide of drilling parameters affecting wellbore cleaning by a compromise process for optimum combination of the parameters.

Introduction

Velocity at the bottom part of wellbore, at which coiled tubing is located in intermediate wellbore deviation (here-in-after called as transient segment), is lower than top part in annulus. As cuttings settle in the drilling fluid, cuttings bed is formed at the low-side annular wall under normal drilling fluid pumping rate (within nominal annular velocities of 1 to 5 ft/s)1. Wellbore cleaning has been a major concern for years in the drilling industry. An investigation by Amoco2 showed that 70% of lost time due to unscheduled events was associated with stuck pipe. A case study by Hopkins3 revealed that a third of all stuck pipe problems are due to insufficient wellbore cleaning.

In an attempt to avoid such problems in drilling operations often include such practices as washing and reaming, wherein the drilling fluid is circulated as the bit is inserted into the wellbore, and back reaming, wherein the drilling fluid is circulated as the bit is withdrawn from the wellbore 4. Other operations such as wiper trips or pumping out of the hole are often performed to attempt to control the amount of cuttings accumulated in the wellbore.

The detailed review of the published experimental data reveals that the cuttings transport characteristics are changed with the change in the wellbore deviation. Researchers5–7 reported from their experimental results that the cuttings bed in annuli is unstable under a certain range of wellbore deviation. Most unstable and difficult region for cuttings transport in deviated well is reported as 30 – 60° from vertical.8–10 In addition, the cuttings bed is unstable and sometimes it slides down toward 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 of intermediate wellbore deviation. One set of conventional mathematical model, which can handle from vertical to horizontal wells with only a function of the deviated angle of the well, cannot properly characterize the cutting transport mechanism in horizontal and highly deviated wells.

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