Frictional pressure loss of drilling fluids in a fully eccentric annulus

Abstract

It is common practice when drilling oil and gas wells to assume that the drillstring is placed concentrically in the annular space with either the open hole or previous casing strings in order to predict annular frictional pressure losses. The assumption of such a concentric annulus is, however, a considerable simplification that may not properly reflect the majority of drilling applications in the field. In fact, with an increasing number of deviated/horizontal and extended reach wells being drilled, a fully eccentric annulus is actually present in a large section of the wellbore.In this study, we apply experimental, analytical, and numerical approaches to investigate the impact of drillpipe eccentricity on the annular pressure loss while circulating non-Newtonian drilling fluids. The length of the experimental section of a flow loop was 27.74 m (91′) and it consisted of 0.0245 m (1″) steel drillpipe and 0.0571 m (2.25″) acrylic casing with the inner diameter of 0.0508 m (2″). Drillpipe was placed at the bottom of the casing, thereby simulating a fully eccentric annulus. Four Yield Power Law (YPL) drilling fluids were tested in this flow loop. Annular pressure loss for a wide range of laminar flow rates was recorded for each fluid. A numerical model based on a finite difference approach was developed to estimate the annular pressure loss. Subsequently, the experimental data was compared with the proposed model and also with several other widely used analytical and numerical approaches previously reported in the literature. The obtained results show that in the laminar flow regime, the annular frictional pressure loss in a fully eccentric annulus is considerably less than a concentric annulus, on occasions by less than 50%. In general, all the applied models under-estimated the effect of eccentricity on pressure loss. However, the novel proposed model showed the least discrepancy with the experimental data. Furthermore, it was found that the difference between the estimated and experimental results increases with increasing fluid yield stress. This suggests that models and/or correlations that are developed to correct for the eccentricity effect for fluids with negligible yield stress (for instance Power Law fluids) are not suitable to estimate the pressure loss for YPL fluids with elevated yield stress.