Drillpipe eccentricity is very important factor in analyzing directional wells. In this paper we will predict the drill pipe eccentricity as a function of hole inclination angle, weight on bit, and hole size. The equation used for drill pipe eccentricity prediction was modified to give accurate prediction because the old equation was found to give some errors. In this paper we represent the results of prediction in the form of graphs and tables, which can be used easily in drilling applications. The prediction of drillpipe eccentricity enables us to find the value of drillpipe eccentricity at any depth of drilling and for any hole inclination angle. The predicted value of the drillpipe eccentricity can be used to determine the effective flow rate of the drilling fluid and the requirements of hole cleaning. Also, the prediction of drillpipe eccentricity is important in calculating the carrying capacity of the drilling fluid in the low side of the eccentric annulus.
Many previous investigators have neglected the the relation between the hole inclination angle and the drillpipe eccentricity [1]. A number of studies have been reported for the flow of non- Newtonian fluids through the eccentric annuli [2,3,4]. They approached the problem by using bipolar co-ordinates to define the eccentric annular geometry and developed methods for the calculation of the velocity profile which involve extensive numerical iterative computation. Vaughn [5] and Iyoho and Azar [6], treated an eccentric annulus as a slot with variable height and developed the analytical model of the velocity profile for powerlaw fluids. However, because their models are in essence the modified models for flow between parallel plates, unrealistic symmetric velocity and linear shear stress profile results. Luo and Peden [7], analyzed the annular flow of drilling fluids in eccentric annuli based on the eccentric annular geometry using a new method which treats an eccentric annulus as an infinite concentric annuli with variable outer radii. The two most commonly used rheological models, i.e. power law and Bingham plastic models, are used in the analysis to define the rheological behavior of the drilling fluids. Then based on the above analysis, the effect of the pipe/hole eccentricity on cuttings transport is evaluated. Luo and Peden [7], derived an equation for calculating the radius at which the velocity is maximum and shear stress is zero in eccentric annulus for non- Newtonian fluids based on power law and Bingham plastic fluids which can be represented as follows: (Equation) (Available in full paper)
Effect of drill pipe eccentricity on the carrying capacity of drilling fluid 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 semi empirical cuttings transport model developed by Zeidler [8] to the experimental conditions also was tested. Experimental parameters included two-velocity values 60 and 90 ft/min., four eccentric positions, and four rotary speeds, with a transparent simulated drilling fluid whose rheological properties were kept constant.
