Drillpipe Eccentricity Effect on Drilled Cuttings Behavior in Vertical Wellbores
Authors: Thomas R.P. , Azar J.J. , Becker T.E. 
Published: December 2007

Effects of drill pipe eccentricity and rotation of the eccentric drill pipe on the carrying capacity of drilling muds in a vertical wellbore have been observed on a simulated wellbore test apparatus under steady-state conditions at laminar fluid flow. The applicability of the semiempirical cuttings transport model developed by Zeidler to the experimental conditions also was tested. Experimental parameters included two velocity values 30.48 and 45.72 cm/s, four eccentric positions, and four rotary speeds, with a transparent simulated drilling fluid whose rheological properties were kept constant.

Results show that drill pipe eccentricity has a minor influence on annular cuttings volumetric concentration and that the effect appears to be oscillatory in nature. Although increasing rotary speed generally improves particle transport, it is more pronounced at lower annular fluid velocities and appears to be negligible at high velocities. Also, there is a threshold value of rotary speed above which there is little change in transport behavior.

Inspection of the equation developed by Zeidler I to predict the volumetric cuttings concentration in a vertical wellbore during drilling will show that the predicted concentration may be erroneously high at low fluid velocities. Experimental results indicate that the equation will yield accurate values of cuttings concentration, provided that the annular fluid velocity is at least twice the particle settling velocity. Otherwise, the equation should not be used.

Introduction

In the past, extensive work has been carried out in pursuit of a complete understanding of drilled cuttings behavior in vertical wellbores during drilling and during the hole-cleaning process after drilling has stopped. Perhaps the earliest study conducted on the problem of cuttings removal was that of Pigott in 1941, which considered fluid flow in mud pits, pipes, and wellbores. He discussed the application of Stokes' law for laminar flow and Rittinger's formula for turbulent flow to drilled particle settling velocity calculations. He concluded that high fluid viscosity was not necessary and suggested that laminar flow in the annulus would lead to more efficient cleaning. For trouble-free operation, he also recommended that the volumetric cuttings concentration in the annulus be kept less than 5%.

Both field and laboratory tests were conducted in 1950 by Hall et al. with drilled cuttings settling velocities computed through estimates of cuttings circulation time from bottom. Among their results, the observed discontinuity in transport at the transition from laminar to turbulent flow was significant. A similar study conducted the following year by Williams and Bruce led to the conclusion that low viscosity and low get strength were advantageous in removing cuttings. However, their results indicated that turbulent flow in the annulus was preferable for cuttings transport, in apparent disagreement with Pigott's conclusions. Hopkin devoted considerable attention to particle slip velocity and presented correlations between slip velocity and funnel viscosity and Bingham yield value. Among his conclusions were that inducing laminar annular mud flow and rotating the drill string will improve drilling mud carrying capacity. Later, analytical relationships involving annular velocity, penetration rate, hole size, and drilling fluid properties with the problem of hole cleaning were developed by Chien, which included the influence of non-Newtonian viscosity on settling velocity. He found that there is an optimal annular velocity that varies with penetration rate, for any given pipe and hole size, and fluid properties.

Results and conclusions from particle transport tests conducted in full-scale vertical annuli were reported by Sifferman et al., who studied various types of fluids.

JPT

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