Effects of drill string eccentricity on frictional pressure losses in annuli

Abstract

It has been shown that drill pipe eccentricity can significantly reduce the annular pressure losses and must be taken in to account for accurate prediction of equivalent circulating density (ECD). This study investigates effects of axial flow of Yield-Power-Law (YPL) drilling fluid on frictional pressure losses in eccentric annuli. A numerical model is developed to simulate the laminar flow of yield-power-law fluids in eccentric annular geometries. The governing equations are solved numerically using an implicit finite difference method and employing a non-uniform grid distribution system. The numerical solution yields velocity profile in the annular space, which is integrated to obtain the volumetric flow rate. The frictional pressure losses are calculated using an iterative scheme that is based on convergence of the calculated volumetric flow rate to the given flow rate. Comparing the frictional pressure loss predictions of the model with three previous numerical studies indicates an excellent agreement for the case of non-Newtonian fluids. The numerical model is validated through extensive comparisons between the predicted frictional pressure losses and experimental data derived from the literature. Our analysis indicates excellent agreement between the model prediction and the experimental data. The numerical results conclude that the reduction in frictional pressure losses due to eccentricity is more prominent at high radii ratios. Results of extensive simulation scenarios for yield-power-law fluids are fitted using non-linear regression analysis, which provides a new correlation for prediction of frictional pressure losses of yield-power-law fluids in eccentric annuli. Comparing the predictions of the developed correlation with the published correlation in the literature reveals more accurate estimation of frictional pressure losses for the proposed correlation and hence safer controlled drilling operations.