Drill Pipe Rotation Effects on Frictional Pressure Losses in Slim Annuli. SPE-member Abstract The effect of rotation on the frictional pressure loss in an annulus has been studied experimentally. A direct comparison with results obtained with a 3D CFD (computational fluid dynamics) code has been performed for laminar flow. Experimental results for three fluids are presented. In this paper we will focus on two effects of rotation in a slim hole annulus. The increase in the frictional pressure loss with high rotation rates for low viscosity fluids which is caused by the onset of centrifugal instabilities. The reduction of the frictional pressure loss with rotation for high viscosity shear thinning fluids. Introduction Within the last decades there has been done a significant amount of work with the objective of predicting the frictional pressure loss for axial flow in annuli. In the oil and gas well drilling industry, especially when drilling slim hole wells, this topic is of the greatest importance as described by Marken et al. There are several works where the frictional pressure loss has been studied using real wells, thus restricting the controlled conditions of their experiments. These studies concluded that applying rotation to the inner cylinder in the annulus yielded an increase in the frictional pressure loss of the flow. This is in contradiction to the results obtained by Walker et. al. which found experimentally that flow is enhanced with drill string rotation. The reduced frictional pressure loss resulting from helical flow in annuli has also been discussed for a wider range of non-Newtonian fluids. A reason why the studies by Walker et. al. found enhanced flow is that their shear thinning fluids were quite viscous. Furthermore, they studied fairly narrow annuli. Any centrifugal instabilities were therefore suppressed and enhanced flow was observed. McCann et. al. used an especially designed slim hole flow loop and found that the frictional pressure loss for power law fluids increases with increased pipe rotation in turbulent flow and decreases with increasing pipe rotation in laminar flow. They also found a significant decree in the frictional pressure loss for power law fluids with increasing eccentricity. This decrease for power law fluids in laminar flow has been calculated numerically by Haciislamoglu et al. where a correlation for the calculation of Recc, the ratio between the frictional pressure loss with eccentricity and without eccentricity, is provided. Several other authors have studied the axial flow of non-Newtonian fluids in eccentric annuli. Chandrasekar has reviewed centrifugal instabilities. Appearance of centrifugal instabilities on axial flow results in increased frictional pressure losses. This was studied in detail by Simmers et. al. where the axial flow without inner cylinder rotation was laminar. Numerical simulations of centrifugal instabilities have been performed by Majumdar et al., Lockett et al. and a feasibility study was performed in [18]. In the study by Majumdar et al. the fluid was Newtonian and no axial flow was present. In [11] and other publications by the same authors, axial flow is present and effects of eccentricity are commented. They focused on the effect of centrifugal instabilities on cutting transport.

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This paper presents an experimental study of the displacement process in a 60 0 deviated laboratory well with a 55 % eccentric annulus. The inner pipe was lying near the bottom of the simulated wellbore. In this geometry the displacing fluid have a tendency to flow in the upper, wide part of the annulus, and to bypass mud in the narrow part of the annulus. The results from the tests were applied to practical field operations.

A model to predict the pressure loss for slim hole drilling has been constructed. The model, which is constructed by theoretical and numerical analysis and experimental measurements, incorporates the effect of eccentricity, drillstring rotation and rheology. Introduction Slim-hole drilling has been pursued in the last decade as a mean to drill exploration wells with a minimum of logistic support. Slimming down the well design will also give a cheaper well. However, the technique will bring new technical challenges, such as the correct estimation of ECD and kick detection. In a conventional well 90 % of the standpipe pressure originates from friction in drillstring and drill bit. In a Slim-hole well 90 % of the standpipe pressure is due to friction in the annulus. Correct estimation of the frictional pressure loss is crucial for estimating ECD, and hence also an important issue for well control.

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