Abstract

Cuttings transport in highly deviated wellbores is more challenging and critical than in vertical wells. In inclined wells, the fluid velocity has a reduced vertical component that may not be sufficient to transport all the cuttings to the surface. When cuttings returns do not appear to be sufficient for the drilling rate, hole cleaning sweeps are applied to clean the borehole or reduce cuttings bed thickness. Fiber-containing sweeps have been very effective in cleaning highly deviated and extended reach wells. In addition, substantial torque and drag reduction has been reported when fiber-containing drilling fluids are used in the field. Although field observations are encouraging, currently very little is known about flow behavior, hydraulics and cuttings transport efficiency of fiber sweeps.

There is a great need for understanding how fiber particles enhance the cleaning capabilities of fiber-containing sweeps. The interaction between fiber particles and drilling fluid is still not fully understood, although the improvement in cuttings and solids transport is attributed to the formation of a fiber mat/network that enhances the carrying capacity of drilling fluids.

This article presents results of experimental investigations conducted to study hole-cleaning performance of a fiber sweep. Flow loop experiments have been carried out to evaluate and compare sweep efficiencies of the fiber sweep (0.47% Xanthan Gum and 0.04% synthetic fiber) and the base fluid (0.47% Xanthan Gum). Equilibrium bed heights were measured at different sweep flow rates in horizontal and inclined configurations. Results from this study indicate that a fiber-containing sweep has better hole cleaning capabilities than the base fluid, even though these two sweep fluids have very similar rheological properties. Moreover, adding fiber slightly reduces friction pressure loss under turbulent flow conditions.

1. Introduction

Inadequate hole cleaning can lead to costly drilling problems such as stuck pipe, premature bit wear, slow drilling rate, formation fracturing, and high torque and drag. A number of previous studies indicated that cuttings transport in directional wells is strongly dependent on wellbore inclination angle, rotary speed of the drill pipe, fluid rheology, flow rate, wellbore geometry and other drilling parameters. Over the years, various field procedures have been introduced to control the formation of cuttings beds. Most of these procedures involve addition of drilling fluid additives such as viscosifiers and weighting agents that enhance the cuttings transport ability of the drilling fluid. Unfortunately, these methods are inefficient in completely preventing the formation of a cuttings bed. At best they delay the buildup of cuttings beds but often cause additional problems. As a result, corrective methods such as drilling fluid sweeps are often applied in the field. In highly inclined and horizontal wells, drilling fluid sweeps can be applied to reduce cuttings bed thickness. Conventional drilling fluid sweeps are classified into:

  • high-viscosity sweeps;

  • high-density sweeps;

  • low-viscosity sweeps;

  • combination sweeps; and

  • tandem sweeps.1

A recent experimental study 2 with conventional sweeps indicates that in the absence of drill pipe rotation, high viscosity and high-density sweeps are found to be ineffective in a horizontal configuration. However, field observations show the effectiveness of fiber sweeps in cleaning highly deviated and extended reach wells. Investigating hole-cleaning performance of fiber sweeps can be useful to increase the understanding of these fluids and to establish a procedure for designing and application of fiber sweeps. In this study, flow loop tests were carried out to evaluate and compare the performance of fiber-containing sweeps using the experimental facility shown in Fig. 1. Experiments were performed in horizontal and inclined configurations.

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